Geology, basin analysis, and hydrocarbon potential of Mozambique and the Mozambique Channel

Earth-Science Reviews, 30 (1991) 81-124 81 Elsevier Science Publishers B.V., Amsterdam

Geology, basin analysis, and hydrocarbon potential of Mozambique and the Mozambique Channel

Alan E.M. Nairn, Ian Lerche and James E. Iliffe ESRI and Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA

(Received October 25, 1989; revised and accepted January 25, 1990)

ABSTRACT

Nairn, A.E.M., Lerche, I. and Iliffe, J.E., 1991. Geology, basin analysis and hydrocarbon potential of Mozambique and the Mozambique Channel. Earth-Sci. Rev., 30: 81-124.

The Phanerozoic structural and stratigraphic history of Mozambique can be separated into a "Karoo" and a post-Karoo phase. Outcrops of Karoo rocks, limited to the Zambezi valley and small outliers in the Mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates. The wide coastal plain and immediate offshore of southern Mozambique is marked by extensional horsts and grabens with a Mesozoic fill covered by Cenozoic sediments. Little is known of lower stratigraphic horizons. In this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east. Upwards the succession passes into the Cenozoic without a noticeable break.

In the offshore in the north seismic lines help define two basins, the Zambezi Delta basin and the Mozambique Channel basin separated by the Beira High. In the south several offshore grabens have been defined, one of which, the South Mozambique graben, has been examined here by an integrated basin analysis technique. The technique, which uses available depositional, structural and thermal data, provides information on maturation and the timing of hydrocarbon maturation.

The analysis showed variation in the extension rate with time, and two phases of rifting. The greatest rate of extension, accumulation and subsidence, between 96 and 76 Ma, depends upon interpretational assumptions. During this time interval fault blocks were systematically tilted "domino" fashion. This event broadly coincides with the rifting between Madagascar and Antarctica, a period of plate reorganization. The earlier rifting phase was probably related to the strike-slip separation of Madagascar from Africa.

Modelling heat flow suggests that the later event is the more important from the maturation-hydrocarbon standpoint. The offshore, and potentially the deeper, onshore post-Karoo basins have traps, reservoirs and seals with the presumed source rock within Lower Cretaceous or Jurassic strata. There is, at the present time, no means to evaluate the potential of the Karoo rocks.

I N T R O D U C T I O N

Many striking parallels exist between the east and west coasts of Africa. Both are pas- sive margins, both show a phase of extensional rifting during mid-Cretaceous time, linked with the opening of the South Atlantic in the west and the late stage of separation of East from West Gondwana in the east. The two regions are even linked through the Central African shear zone. The principal difference resides in the fact that the west coast basins have proved to be rich hydrocarbon

province; even the interior basins in the Sudan show a currently established potential of 400 million barrels, whereas, apart from some gas, the east coast grabens have proven singularly barren. This difference can scarcely be attributed to the fact that movement in the west was dominantly extensional whereas in the east shear motion predominated, nor can it be attributed to distinctive differences in sedimentary history. The question is, therefore, to what extent does the difference reflect the extent of, or degree of sophistication of, the exploration methods? We present here the

0012-8252/91/$15.40 © 1991 - Elsevier Science Publishers B.V.

82 A.E.M. NAIRN ETAL.

suggestion that, with the application of more sophisticated techniques currently being developed, the East African margin may be found to possess an unrealized hydrocarbon potential. This view is supported by a review of currently available data on the geology of Mozambique and an illustration of what potential plays may be derived from the use of more sophisticated analysis of existing data for a single offshore graben near Maputo (Iliffe et al., 1989). In this analysis we go beyond the position adopted by Kihle (1983)

and DeBuyl and Flores (1986) following the acquisition of over 25,000 km of offshore seismic lines, and show that the currently existing data can be used to evaluate what is a speculative basin. We further indicate what additional data are required to reduce significant uncertainties. Our procedure and evaluation method for Mozambique can be applied equally to offshore Tanzania, Kenya or Somalia.

The geology of Mozambique should be viewed in historical perspective. Until inde-

Alan E.M. Nairn received his B. Sc. from the University of Durham in 1951. Following graduation from the University of Glasgow in 1954 with a thesis on Carboniferous stratigraphy, Dr. Nairn worked in paleomagnetism, concentrating first on the establishment of polar wander curves to validate continental drift; this led into paleoclimatic studies. Subsequently, following the general acceptance of drift/plate tectonics, smaller scale geologic applications of paleomagnetism were studied: microplate rotation (e.g., Corsica, Bohemian platform), rotation of thrust sheets (e.g., central France, Sicily), and early magneto-stratigraphy (Permian sediments of the western United States and Tertiary volcanic rocks primarily in Europe, and carbonates in Tunisia). More recently, Dr. Nairu has turned his attention to the history of tectonic belts, with a variety of results from the Eastern Desert, southern France, and the Appalachians. Currently, his research efforts include integration of regional geology and plate tectonics. Dr. Nairn is Senior Associate Director and Professor at the Earth Sciences and Resources Institute (ESRI) of the University of South Carolina, Columbia. Dr. Nairu is internationally recognized as an editor: two symposia volumes on paleoclimate; several volumes in Elsevier's Phanerozoic Geology of the Worm series, and Plenum's eight volume series on Ocean Basins and Margins. He is also Editor-in-Chief of all ESRI publications.He has published numerous papers in a variety of professional journals and has been responsible for many reports resulting from industry-sponsored projects. He also directs ESRI's training programs.

Ian Lerche received a B.Sc. in 1962 and his Ph.D. in astronomy from the University of Manchester, England. During the period 1965-1981 he was a member of the Physics Department faculty of the University of Chicago. From 1981 to 1984 he was employed by Gulf Research and Development Company (GRDC), as a Research Associate in Geophysics. Since 1984 he is a Professor of Geology at the University of South Carolina. Lerche is a recipient of the Alfred P. Sloan Foundation Fellowship (1966-1968), Alexander yon Humboldt Senior U.S. Scientist Award (1974-75), Visiting Professor at M.I.T. (1975), visiting principal research officer at the Division of Radiophysics at C.S.I.R.O., Sydney, Australia (1977-1979), visiting research scholar (GRDC) (1980-1981), and a Fellow of the Royal Astronomical Society.

James E. Iliffe received his B.Sc. (Hons.) from University College, Swansea, Wales (1982), his M.S. from the University of South Carolina (1985), and submitted his Ph.D for 1990 graduation, again from the University South Carolina, where he is currently a Research Assistant. His M.S. was a structural and stratigraphic field study of part of the western margin of the Gulf of Suez Rift, Egypt. His Ph.D. research investigated the mechanical, thermal and economic aspects of the development of extensional regimes at both the individual fault block level using forward numerical modelling, and at the basinwide scale in the Northern North Sea Viking Graben and the South Mozambique Graben. In addition to Ph.D. studies, James has also worked as an exploration geologist for Westmont Mining Inc. (U.S.A.) and D.H.E. (Ghana) investigating gold mineralization in the transpressive terrains of the South Carolina Slate Belt, and the Birrimian of Ghana, West Africa. Present address; Department of Geological Sciences, University of South Carolina, Columbia, S.C. 29208, U.S.A.

GEOLOGY. BASIN ANALYSIS AND HYDROCARBON POTENTIAL OF MOZAMBIQUE 83

pendence in 1975, the major part of the literature resulted from work under the colonial administration, was published in Portuguese and not readily available. Most of the generally available information was derived from summary reports at international meetings (e.g. Borges, 1952) or from regional surveys (e.g. Flores, 1973), or from papers published in South Africa (e.g. Flores, 1964; 1970), although the latter tended to concern specific regional problems (e.g. King, 1957). The situation was aggravated by several geological problems, in addition to the perennial problem of lack of outcrop in the coastal plain. In particular the failure to recognize the inter- ference pattern of more than one period of rifting and, in the off-shore, the delay in recognizing until very recently the pattern of movement in the Mozambique Channel, have hindered basic understanding of the geology of Mozambique. Although the northern de- rivation of Madagascar in the opening of the

Mozambique Channel was indicated by Du Toit (1937), and by Heirtzler and Burroughs (1971), Flores (1970) still followed Wegener's original juxtaposition of Madagascar against Mozambique, and Kamen-Kaye (1982) main- tained that the Mozambique Channel was an open-ended, non-compressional "geosyncline" which existed since at least Carbonifer- ous time.

First we summarize the basic stratigraphic and structural history of Mozambique as a means of delineating areas of potential exploration interest. Following this review we then apply quantitative techniques to an offshore basin, an area with the greatest potential to delineate high-risk and lower-risk areas in this frontier basin.

SUMMARY OF MOZAMBIQUE GEOLOGY

The principal area covered in this paper extends from the Zimbabwe-Mozambique

TABLE 1

Stratigraphic succession in the Zambezi Karoo

1 2 Between Lower Chire Moatize (Tete) and the Zambezi Basin

3 Upper Zambezi (Zimbabwe)

Basalts

Rhyolites

Red standstone with dinosaurs

Upper sandstone with Dadoxylon sp. and Rhexoxylon africanus

Batonga sandstone

Guenga sandstone

Red marls with Mpiusa shale Cyzicus and with vertebrates Palaeanodonta and Glossopteris fish scales

Middle sandstone Tete sandstone

Batoka basalts

Forest sandstone

Escarpment grit (and Somabula beds)

Madumabisa shale

Upper Wankie sandstone

Stormberg

Beaufort, Upper and Middle

Beaufort, Lower

Locations for the successions are shown on Figure 2.

84

border in the west to the Davie Ridge, which traverses the Mozambique Channel, in the east (Fig. 1) and northwards to about the

A.E.M. NAiRN ET AL.

mouth of the Zambezi. The northern half of the country, between the Malawi border and the ocean, is predominantly the domain of

1 t t 30" 32" 34

ZAMBIA

Z a m b e z l

K a r o o

B a s i n

Z I M B A B W E

t

I.~ALA W

÷ * + ÷ + + + ÷

.•+++++• + + +

~ + + +

[ I 36" 38"

T A N Z A N I A Tundu ru

Bas in

+ + + + + + + + + + + + + + + + + + + + + + + : + + + .-:=. .~- + + + + + + + + +

+ + + + + + + + + + ~ + + + + + + + + + + + + + + + + + + + 4 ++ + + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

*MOZAMBIQUE ' ~+ + + + + + + . + ~ -, ~ ÷ + % + ¢ . . . . . . . . . . . .

\ ÷ d . . . . . . . . . . . . . + + + + + + + + + + + + + ~+ + + + + + + + + + + + * + + + + + + ÷ + + + + +

÷ * * B ~ ' L T+ . . . . . . + + + + + . + + + M o c a m b l q u e • + + + + + + +

/ " + + + + + + + + + + + + + + + + + + + + + + + + + +

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Porto A m e l i a

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SWAZILAND ~ . ~ _

-- =~'U L U L A N D /

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- - = , . . . ~ o i 1 ~ O u t c r o p o f M e s o z o i c

a n d C e n o z o i c B a s i n s

W e l l s w i t h s o m a

s t r e t l g r a p h l c i n f o r m a t i o n

o W e l l s w i t h o u t s t r s t l g r e p h i c

i n f o r m a t i o n

A+ 'A ' C r o s s S e c t i o n

0 1 0 0 2 0 0 3 0 0 k i n

2 6 * - -

~ ° 38- 4 o - 4 2 -

I I | I J Fig. 1. Mozambique and adjoining areas showing principal sedimentary basins outlined in this paper. Approximate well locations are shown, and wells are named where some stratigraphic information is available. Line of section (Fig. 3) A-A'.

GEOLOGY, BASIN ANALYSIS AND HYDROCARBON POTENTIAL OF MOZAMBIQUE 85

3'0"E

i I ~ m A w I

~3" 3f6"[

/

Fig. 2. The distribution of Karoo System rocks in the Zambezi Valley simplified after Borges (1952). No attempt has been made to subdivide the Karoo as in many areas the sequence is undifferentiated (volcanics excluded). The numbers refer to local successions listed in Table 1.

the Precambrian Mozambique belt. In this northern region, Phanerozoic, Karoo, sediments occur in a small basin along the Litule River, and in the graben-like Tunduru depression, which extends northeastwards from Lake Nyassa parallel to the Ruhuhu graben of Tanzania. The section is of interest because Flores (1973) reported the occurrence of a middle limestone section in the Ecca Songea (or Lunho) Series. Along the coast of the northern area, to about the latitude of Porto Amelia, Phanerozoic sediments are restricted to a narrow strip. North of Porto Amelia a small basin gradually widens to a maximum of 120 km along the Rovuma River continuing north into Tanzania to approximately the latitude of Lindi.

The Zimbabwe-Mozambique border, which strikes approximately north-south from the Zambezi River, follows the line of the Lebombo monocline, a major crustal fracture through which enormous volumes of lava were extruded. These lavas can be traced under the Mozambique coastal plain out into the Mozambique Channel (F~Srster, 1975). The Lebombo monoclinal axis is parallel to the Davie fracture zone, which forms the eastern

margin of the southern Mozambique basin. The southern limit close to Maputo (Lauren~o Marques) is marked by the northeast-striking line of a gravity high which appears to trun- cate or offset the line of the Mozambique Ridge (the location of the hinge line of De Buyl and Flores, 1986).

The junction between the northern and southern provinces occurs where the older Zambezi graben is intersected by the younger Chire-Urema graben (about 18°S). West of the Lupata gorge where the Zambezi cuts through Cretaceous volcanics, along the line of the Zambezi River, lies the best development of Karoo Supergroup rocks in Mozam- bique.

The areas of potential exploration interest are thus restricted to southern Mozambique and concern primarily Mesozoic rocks. How- ever, some consideration must be given to the Karoo sequence because of the possible source rock potential of the Ecca shales, and to the Cenozoic sediments of the Zambezi River basin. Sources of information on this vast area are unfortunately few. Although the pace of exploration increased since World War II, with independence in 1975 there has been a

8 6 A.E.M. NAIRN ET AL.

Potent ia l Sou rce Rocks G e n e r a l i z e d St ra t ig raphy Potent ia l Reservo i r Rocks

PLIOCENE " ' • ' • ' . ' . • • • - : M O R R U M B E N E I r • • , • • • • ' l I . • • • . : ~ . ~ • . . . . . . . ! r . : • : • : • : ! Tc

~ J,,~ J,.J,,z.,,e¢.

:~. MIOCENE " ' • • . ' . ' • ' . •o ] M A Z A M B A / .-.-.-,-.• Basa l Tb sands, basal part rr . . . . . . . . INHARRIME w-~-.,-~ of a t r ansg ress i ve cyc le '4: " : ; ; ; ~ • ' , , , . ~ ._ , Ti0 P o t e n t i a l p a y t h i c k n e s s V-

OUGOCENE . : . : . : ~ . '" " " " " 100-135 feet . . . . . . . . • . • . : ~.~'~...'-," Po ten t ia l poros i ty 25-30%

I ~ ~ " " I I 7_._._ - - ~ : i Ta l imestones Ta sha les EOCENE I

I I I ;.-.•...................~_.L--7. Ta Poten t ia l poros i ty 10-15% I J C H E R I N G O M A . : .,-.,-.,-.,-.,-.,-?~..r:. . Ta sands , coa rsen ing sands ,

PALEOCENE ° 7 _ _ Poten t ia l po ros i t y 30% ':"~". " " ' • " " 7 ~ i Poten t ia l pay th ickness 150 feet

• • . . • ,-,-, ~ -.+.." T~ " Kb2 sands, rese rvo i r Ior Pande,

i ! i ~ ...,-,-,.-"'a'~"" T e m a n e and Buzi wi l ls onshore Kb2 sha les SENONIAN Z_ ~ (b; Upward coa rsen ing sands,

• . un i ts usua l l y 100-150 feet • :." .~...~-- Z_ GRUDJA .---..-...-" w Poten t ia l po ros i t y 20 -30%

¢,0 C.c'- = ~ --- o 7__ K b l sands , w ide ly sp read tu :.:.:...,;-,-,"~.;. " " " Poten t ia l pay th ickness

Kb l sha les o< 7_ 7-- . . . . . . ~ Kb 130-140 feet

TU•ONIAN 7--- ~ - - ~ Poten t ia l poros i ty 20 -25% CC 7__ . z . . . ~ . . - ; - .

Ka2 sha les o - - ~ " " Ka sands, va lue reduced " " ~ by presence of f e l l dspar CENOMANIAN SENA 7__ 7__DOM 0 = ~ :.

..':" " ". '. ca; and vo lcan ic las t i cs ALBIAN Z - 7--. - - . . ~ , , . Po ten t ia l poros i ty 10-17%

Z_MAPUTO ' • "~-"~': ,PT,,N Z

V V V V V V " ~ V V V V v v v v v BELO v v v VVVVVV VYVV

Fig. 3. Generalized stratigraphic index of the post-Karoo beds of Mozambique.

downturn in the volume of published information, although through the works of Kihle (1983) and De Buyl and Flores (1986) the government of Mozambique has tried to stimulate interest in exploration and commissioned the shooting of 25,000 km of offshore seismic lines. As a result more is known of the offshore than of the onshore•

The Karoo Sequence of the Zambezi

Although Karoo rocks crop out in the Limpopo embayment (the Zoutspanberg re- entrant) the Zambezi graben holds the best studied sequence. The general distribution is shown on Figure 1. In the Zambezi valley, Borges (1952) provided the most comprehen-

sive account of the region with the map reproduced here on Figure 2. The beds found range in age from Ecca to Beaufort and Stormberg and can be correlated with outcrops in Zimbabwe and South Africa (Table 1). Some important facies differences occur and the amount of shale present is higher than in either of the other two areas, the presence of limestone is reported, and the faunal and floral content seems to be higher. Downstream (i.e. eastwards) the section thickens. All of these features seem to indicate a basinal environment more to the east and the possibility of not too distant marine conditions, suggesting that if the Karoo exists in the lower Zambezi it may possess enhanced source-rock potential.

(IEOLOGY, BASIN ANALYSIS AND HYDROCARBON POTENTIAL OF MOZAMBIQUE 8 7

Two volcanic sequences are known, the Batonga basalts which are the local equivalents of the Stormberg basalts of South Africa, and the Lupata volcanics which have a Cretaceous age. Basalts of older sequences have yielded radiometric ages consistent with a Liassic age, as suggested by the flora in intercalated and immediately underlying beds. Of the same age as the younger Lupata volcanics are the Movene basalts, known in southermost Mozambique. The faults of the Zambezi graben were active in post-Karoo, Early Cretaceous time for they contain Lower Cretaceous sandstones. No record of the Mid- dle to Late Jurassic transgression in the Zambezi exists.

Basic stratigraphy of the southern basin

The Mozambique and Rovuma Basins took on their present configuration in the Early Cretaceous when a marine transgression flooded the eastern margin. In both basins, a belt of Lower-Middle Cretaceous continental or transitional sediments parallel Karoo or Precambrian rocks to the west. To the east and upward, they grade into marine rocks which extend through Upper Cretaceous- Paleocene-Eocene-Oligocene. The Creta- ceous sediments in Mozambique may rest directly on Karoo or post-Karoo igneous rocks or even on Precambrian basement. Whether there was a pre-existing Jurassic or older de- pocenter is speculative. The generalized stratigraphy is shown in Figure 3 (from Kihle, 1983).

The sedimentary history reflects, and is in part controlled by, rifting and associated faulting. Compressive structures are absent; folding is represented only by broad warping which itself may be related to early stages of a rifting cycle. Two major fault trends are seen at the surface to as far as the Buzi River; these have N W - S E and NNE-SSW trends. South of the Buzi River, the fault continua- tions are inferred from magnetic, gravity, and airphoto studies. As far as is known (Flores, 1973), the rifting was not associated with the

formation of evaporites, although in the Western Geophysical report (1982) existence of evaporites is postulated.

The combined Cretaceous and Cenozoic history is relatively simple: a transgression which reached a maximum in the Eocene, with regression setting in during Late Eocene. Examination of the foraminifera in cores from pockets of sediment on top of the Davie Ridge have been interpreted as evidence that water depths of about 2 km have persisted since Eocene time. Onshore, Oligocene is either absent or represented by continental facies and Flores (1973) associated this interval with the deepening of the Mozambique channel. In the Lower Save River there was an incipient evaporitic phase during the Oligocene, whereas in the offshore the Oligo- cene is persitently marine. Major transgression resumed in Late Miocene and Pliocene.

In contrast to the thick Karoo of the Zambezi-Chire region, the sequence is nota- bly thin west and southwest of Beira. There are scattered outcrops with thicknesses of the order of 600 feet maximum for the subvolcanic sediments. Similar thicknesses were recorded in exploration well Nhamura-1, about 25 km north of a gas seep. After pass- ing through a 100-foot (30-m) basalt cap, the drill penetrated about 650 feet (200 m) of a red bed, quartzitic, sandstone lithology resting on metamorphic basement. An apparent age of the basalt is 200-300 Ma (the unpub- lished date given by Kamen-Kaye (1982) is presumably a K / A r minimum age, see also DeBuyl and Flores, 1986).

The age range of the basalt in Nhamura-1 (see Fig. 1) is just wide enough to frustrate any attempt to assess Karoo potential there. The only source rocks in the Karoo, based on what is known of the lithologies or outcrop in southeastern Africa and Antarctica (which is presumed adjacent at the time), lie in the Ecca. At 200 Ma for the basalt, Ecca would be presumed absent; a date of 300 Ma would be equivalent to basal or sub-basal Dwyka. Kamen-Kaye (1982) assumed the former. In Malawi, Tanzania and Zimbabwe, Ecca does

8 8 A.E.M. NAIRN ET A L

occur and contains coal, but nowhere is the coal-bearing sequence thick and, in Tanzania at least, lies within a predominantly red bed sequence. Coal occurs in the Ecca section in the Zambezi graben, but source-rock potential cannot be highly rated.

In southernmost Mozambique, west of Maputo, the outcrops mostly consist of volcanics, although some Stormberg sandstones may crop out. More detailed study shows a disconformity between the lower and upper volcanics. The latter, known as the Movene volcanics, yield a radiometric age of 137 + 10 Ma (Flores, 1970); they are therefore Early Cretaceous-Late Jurassic in age and equivalent to the Lupata volcanics further north. The volcanics, which appear to be eruptions from fissures, young from west to east. Du Toit (1937) reported that vents and pipes are inclined, interpreted as a result of tilting contemporaneous with extrusion. The structure of the Lebombo Range, where the volcanics are best exposed, is monoclinal with dips of 18-19 ° (exceptionally rising to 25- 30°).

In places the volcanics are directly overlain by the marine Cretaceous (Albian-Aptian) Domo shales, in others by the Maputo sandstone. The volcanics appear to underlie the plain of Mozambique and, according to their age, were extruded during the southern movement of Madagascar. The basement on which

the volcanics lie is not known, but is assumed to be associated with the thinning of the crust in the Mozambique Channel and restricted extension. The presence of a Karoo section below these lavas has been inferred, with the estimated thickness in excess of 1,000 m from the dip of the pyroclastics. The volcanics here have never been penetrated; of the nine wells with TD's in excess of 4,000 m, only five reached basalt. No presumption can be made of the age of the volcanics (Cretaceous or Karoo?) which are subaerially altered.

Above the volcanics lie shales of the Domo series, regarded by Kamen-Kaye (1983) as potential source rocks, which grade westward to continental and eastward to open marine conditions (found in Turonian-Senonian).

The post-Karoo sequence The relationship of the Karoo to post-

Karoo beds is best seen in the eastem part Zambezi graben (Fig. 3) where the continental sediments (Sena Formation) and intercalated volcanics (Lupata volcanics) of the Lupata Group rest upon the Early Jurassic lavas which terminate the Karoo sequence. The Sena Formation is usually assigned to Cenomanian-Turonian to Senonian; however, if the dating of the igneous intercalations is accurate the formation is somewhat older. The K / A r date given is 115 + 10 Ma (F~3rster, 1975).

Mt Gorongosa Horizontal Scale 1:250,000 W / Vertical Exaggeration x 10

E ,,.,¢~. ** .%~2~,w.-,...~ Inhaminga

. + + + + + , . . ~ : : . ~ ' f : : " -

MAZAMBA SANDSTONE

CHERINGOMA LIMESTONE ~ STORMBERGSERIES

I.*,-'..'.'~,'.1 GRUDJAFORMAT~ON P'+~'~;';] GORONGOSAGRANITE

[ ~ : ' l BELO FORMATION AND I * + + 1 BASEMENT SENA SANDSTONE (WITH IGNEOUS MEMBERS)

Fig. 4. A cross-section across the Urema graben which cuts the Zambezi grahen (see Fig. 12) with only relicts of Stormberg volcanics shown (simplified from Flores, 1973). Line of section shown on Fig. 1 of A-A'.


The continental sandstones of the Sena Formation fill the Zambezi graben and can be traced east across the Urema graben, where they are downfaulted to the Inhaminga high, and southward to the Save River. The maximum thickness reached in the Inhaminga region (Fig. 4, from Flores, 1973), 8,030 feet (2675 m), diminishes toward the coast, and in the Beira area only 600-1,500 feet (180-450 m) are recorded in the subsurface. In two wells offshore from Beira, the Sena Forma- tion is not present; nor is it found north of the Zambezi delta, nor south of 23 ° S.

In the vicinity of the mouth of the Save River, the Sena Formation is replaced by a shaly, glauconitic, marine sandstone lithology that has an assigned Albian-Aptian age. The

38"

Quel imane

Gas Seep / 18"

19*

/// 21 ~

23 °

2g'l- . o / 1 / / / R ' f J24"

_ _ ~ / o 40 sokm

25" ~ 5"

34 ° 35 ° 36 ° 37*

Fig. 5. Structure contour of the D o m o Format ion (Flores, 1973). Contour interval 500 feet.

contact is said to be transitional, reflecting the deepening of the Mozambique basin. This 250-foot (75-m)-thick sequence, called the Maputo Formation, crops out west of Maputo (Laurenco Marques) and can be traced into Zululand. FOrster (1975) attempted to give possible coastal limits for the area immediately north and south of Maputo.

The Maputo is succeeded by the Domo Formation. This formation, which is known only in subsurface, consists of dark grey to black, thinly bedded, marly shale with rare sand streaks. It extends from Beira in the north, south to beyond Inhamabane, parallel- ing the coast. Figure 5 illustrates Domo thickness and distribution. The age range of the Domo is from Albian--Aptian up to Cenoma- nian-Turonian, and, off the mouth of the Save River, the lithology reaches well up into the Turonian. Flores (1973) suggested, on the basis of the poorly oxygenated sediments, that this may be an initial graben fill.

Above the Domo lies the Grudja Forma- tion, which is found in all coastal wells and in two deep offshore tests according to Flores (1973), with thicknesses of the order of 5,000 feet (1500 m). Where exposed on the Imham- inga and Buzi highs, however, the thickness is reduced to 60 (18 m) and 300 feet (90 m), respectively. Within the Grudja there are ten relatively well defined sand bodies traceable along strike parallel to the coast. The general depositional environment indicated is of littoral to neritic conditions. The isopach map (Fig. 6) gives an indication of distribution of the Grudja Formation.

To the south in Zululand, an unconformable relation has been reported, with the Grudja Formation possibly unconformable upon the Cenomanian in the onshore. How- ever, in subsurface, no break is obvious and the formation appears to span from Conia- cian to Paleocene.

The interesting relationship during the Up- per Cre taceous is that while poor ly oxygenated conditions existed in the Beira-In- hambane offshore, open marine conditions reigned northeastward in the northern basin

90 A . E . M . N A I R N E T A L

34 ° 35 ° 36 ~ 37 ° 38 °

Ouellmllne Gas Seep

' Q O

II/ ~ ~Ooo / I /7~-Oo~~_ "

> ~ I I / 3~_uO

<> <~i /~do~°_o ~ uO 0 i : o

,14/1 B l i f l

,( t

q ,","0/% ) ~'o~o;o°o v

t \ \ . . _ ,ooo \ "--,.,.,- ~ ~' 3 0 0 0

- ~ " 2000

~ 2 0 0 0

24 ° / ~ h a m b a n e

25" / A ' ~ I 34 ° 35 ° 36 ° 317 o

Fig. 6. Isopach map of the Grudja Formation 1973). Contour interval 1000 feet.

40 8 0 k m I

18 °

19 °

20 °

21 °

>2 °

23 =

o 24

25 ° ~°

Flores,

and to the southeast. The Beira Ridge seen offshore may represent the other margin of a proposed graben.

There is no significant break at the Meso- zoic-Cenozoic boundary, because where the Grudja Formation is complete the same pattern of sedimentation continues through the Paleocene.

The Grudja is followed by the Paleocene- Eocene Cheringorna Format ion, a dominantly carbonate unit 400-500 feet (120-150 m) thick near the mouth of the Save River, but only half that thickness in the region of the Inhaminga high (the coastal area on the southeast continuation of the Zambezi graben) and up to 1,500 feet (450 m) thick north of the Zambezi delta (Fig. 7).

Lithologically, the Cheringoma Formation is typified by its development, near the mouth of the Save River, as an algal-oolitic limestone which persisted through the Eocene and represents the peak of the transgression be- gun during the Cretaceous. In the west, the Cheringoma may rest directly upon the A1- bian-Aptian Maputo Formation in the area south of Lauren~o Marques; eastward, the Cheringoma grades to, and intercalates with, marly and sandy facies which are likely derived from a region such as the Beira high.

North of the Zambezi delta, Lower Eocene is absent and the Cheringoma Formation is present in a marly, sandy facies resting unconformably upon beds of the Grudja For- mation. A thickness of 1,500 feet (450 m) is

\÷ 36 °

t " Q u e l l m l n e

2500 i~>l - / l l l / /J l " : .o ~' / ~ -li9"

J,, I \

n h l m b a n e

24 °

,o . . ,m

25 ° 25"

34" 3.5" 36 ° 37 °

Fig. 7. Isopach map of the Cheringoma Formation (Flores, 1973). Contour interval 100 feet.


recorded in Micaune-1 well and even greater thicknesses, in excess of 2,000 (600 m) feet and more complete Cheringoma Formation equivalents (Lower Eocene is present), are reported for the so-called "inclined beds" interval seen in seismic data between Beira and the mouth of the Zambezi. The explanation lies in tectonic events--the activity of the Inhaminga fault system which resulted in the deflection of the clastic sediments of the Zambezi River by the active tilting of the lower part of the Zambezi graben.

The regression which made itself felt at the end of the Eocene continued through the Oligocene. The marine Inharrime Formation, known both in outcrop and subsurface, consists of shale with some sand inclusions. The lower part of the Inharrime is equivalent to the top of the Cheringoma Formation in the area of the Save River. North of the Zambezi delta, the facies equivalent, a sand-shale se- q u e n c e - t h e Buzi Formation, represents a continuation of the deposition of Zambezi River sediment.

The continuing regression, however, left a local development of evaporites, dark grey claystone, with intercalations of gypsum, anhydrite, and stringers of gypsiferous limestone and sands, all of which are referred to the Temane Formation. The localization of this formation is clearly seen on the isopach map (Fig. 8).

The Miocene in Mozambique is referred to as the Mazamba Formation and consists of continental sandstone and conglomerates that may contain a brackish water fauna at the base. The Mazamba has an unconformable, erosional contact with the underlying rocks, which may be as old as Senonian in the Umfolozi River areas in Zululand. Lithologi- cally, the formation has many similarities to the Sena Formation, which it oversteps in the western part of the basin.

In the eastern part of the area, and in the coastal wells, marine conditions persist in Middle Miocene, with sandy limestones and dolomites (the Jofane Formation), which also have a basal unconformity. Coral debris and

~ o 36*

G I I Seep / Q u e l ~ m a n e

19"

Begr l l

21"- ~'~ \ 2~" \ \

~ - I 22" "200 ~ . / /

nhmmblne

24

o ,,0 ,0,km J 25 ° ~ 25

34* 35 ° 36 ° 37 ° 38 °

Fig. 8. Isopach map of the Temane Formation (Flores, 1973). Contour interval 100 feet.

oolites common in the Jofane Formation are indicative of high-energy conditions. The thickness may reach 500-600 feet (150-180 m), but the formation seems restricted to the coastal areas south of the Save River.

The Pliocene-Holocene consists primarily of sands resting disconformably on the Mazamba deposits; however, in the southern part of the basin, in coastal areas around Inharrime, there is a marine Pliocene facies and data from Zululand indicate a not too distant shoreline or shallow offshore environment.

Basic stratigraphy of the northern Roouma basin

The small Rovuma basin is poorly known in Mozambique (see Fig. 14) for the greater

92 A.E.M. N A I R N Err AL.

part of the basin lies in Tanzania. Flores (1973) suggested that it may be the western segment of a NNW-ESE-trending coastal graben. The oldest sedimentary unit, the Makonda Beds, consists of up to 1,500 (450 m) feet of sandstone and conglomerate in fault contact with Precambrian. The beds which thin toward Tanzania are characteristi- cally cross-bedded and suggest deposition in a fluviatile environment. On the basis of fossil wood found in the Makonda beds, they are regarded as Neocomian and equivalent to the Dinosaur Beds of Malawi.

Eastward, the Makonda Beds grade into finer lithologies with evidence of a marine littoral character, though conglomeratic inter- vals remain. The thickness, however, is reduced to 180 feet (54 m).

In the southern part of the Rovuma Basin there are more open marine facies, of Neocomian to Albian-Aptian age. The beds referred to as the Porto Amelia Beds are dated by ammonites and consist of about 100 feet (30 m) of marls, limestones, and silt- stones. A sequence of about 700 feet (210 m) of Globotruncana marls is also found, of Campanian to Maestrichtian age. The other Cretaceous horizons are missing.

The Cenozoic section is likewise incom- plete, with Paleocene sandy limestones followed by middle Lower Eocene nummulitic limestones (similar to the Cheringoma limestone in the southern basin). Possible Lower Eocene biohermal limestones around Porto Amelia continue up into Oligocene and are followed by silty marls and shales. Lower Miocene in the same area shows about 100 feet (30 m) of calcarenites which rest unconformably on Oligocene. The Mio-Pliocene beds are the cross-bedded, reddish, fluvio-deltaic Mikindani Beds.

It is impossible to tell from literature whether the gaps may be due to faulting or to non-deposition. The impression, however, is of being near a depositional margin, with the principal depositional area presumably east and northeast--i .e, in Tanzania.

OFFSHORE MOZAMBIQUE

General considerations

The Mozambique Channel is floored by thinned continental or intermediate crust which is isostatically compensated (Darracott, 1944). Oceanic crust exists to the south in the Mozambique basin. The line of separation, approximately 24°S, is not marked by any physical discontinuity but does coincide with a gravity anomaly axis and, as the line also marks the southern limit of a zone of weak seismicity, it can reasonably be taken as marking one limit of the Mozambique Chan- nel. In the north, oceanic crust underlies the Comores Islands.

The current consensus supposes that Madagascar was derived from a northerly location against Somalia and Tanzania and was displaced southward by activity along a ridge and transform system which was oriented east-west. The anomalies associated with this system have been identified as the M anomaly sequence and are found in both the Mozam- bique basin and the Somali basin. The anomalies indicate the displacement of Madagascar, presumably as part of East Gondwana, occurred from early Upper Jurassic to Hauterivian-Barremian time although marine horizons of this age have not been recorded in Mozambique up to the present time. No further movement of Mada- gascar relative to Africa has occurred since. These magnetic anomaly data rule out a de- rivation from a position adjacent to Mozam- bique. A result consistent with paleomagnetic data, which of itself is not conclusive.

The Davie Ridge and associated fractures mark the line of displacement of Madagascar. The ridge, which strikes obliquely across the Mozambique Channel, is an asymmetric east-facing scarp. From sediments in pockets on top of the ridge, it has been established that the ridge has been at its present depth, the crest 2 km below the surface, since Eocene time. The parallel Mozambique Ridge to the


south, which forms the offshore margin of the Natal Valley we regard as a similar structure.

The few islands in the Mozambique Chan- nel (Europa Island and Bassas da India) are either volcanic or built around extinct volcanoes.

Offshore seismic data

The earliest seismic lines are the 1969 Gulfrex lines, and include two lines, MC7 and MC9, which cross the Mozambique Channel, and shorter fines examining the coastal zone off Maputo. Only provisional interpretations from the Cruise Report are

available (Fig. 9a, b). Those interpretations are biased by the assumption of a "Mozam- bique geosyncline" following Dixey (1956) and continued by Kamen-Kaye (1982). The pre-Cretaceous surface cannot be identified with any certainty. However, a thick Tertiary section (4,000-5,000 feet) (1200-1500 m) and a Cretaceous section of at least 3,000-7,500 feet (900-2250 m), seem to be present. On MC9 there appear to be three basins: the Limpopo Proximal Natal Valley basin, the Zambezi Delta basin, and the Mozambique Channel basin. In this section, the Davie Ridge fracture zone is close to the Madagas- car coast. The sections cannot easily be corre-

L I N E M C - 9 East Horizontal

tlary == M o z a m b i q u e Channe l Vertical ~ Gul f rex Cru ise 42

- - ~ "t'~s ca r p m e n I lie Europa

~%~.~7 ~.~, Cretaceous Rough sea bottom

SEA LEVEL

-500OFT

i ~ Morondava Basin IO000FT

- - -15000FT

-20000FT.

scarpment L I N E M C - 7 E a s t Mozambique Channel Horizontal t

v Vertical ~ 90- / ~ . ~ e r t i a r y Gulfrex Cruise 42

- - ~ Cret . . . . . . , 1~ /

- - ~ E ~ e n t ~

Natal Basin

SEA LEVEL

-5000 FT.

-tO000 FT.

-15000 FT

-20000 FT

-25000 FT

Fig. 9. Provisional interpretation of the Gulfrex seismic lines MC7 and MC9 which cross the Mozambique Channel.

94 A.E.M. N A I R N E T AL.

30"E 315" 1 0*5

30°E

5. OOkm

M O Z A M B I Q U E

tl / ; ~ . / , I / &1) _ f

( \ x 'x,s It W" --,

40"

i~ c ......

45" 5~ 'E I I0 "S

ha,,..3"',/ ) / / ~ " " ,~/F,~ \ -'r -'~ 3 ~ / "'s"

A: Zambezi Del la Basin

B: Cretaceous Mozambique Basin

4b. 4~. s~,E

Fig. 10. As isopach map of the northern Mozambique Channel (from Lort et al., 1979) showing the location of the Zambezi Mouth basin and the Mozambique Chan- nel basin, separated by the Beira High. It should be observed that the Beira high is not a well defined feature; it does not impede the Zambezi submarine channel. To the south of the Sul do Salve province the isopach pattern appears to contradict the isopachs shown in Figs. 7-9.

lated with the lines published by Lort et al. (1979) without reprocessing the data; inclu- sion here is by way of information.

The new information they provide is of the possible existence of as much as 1,000-1,500 feet (300-450 m) of Tertiary volcanics east of lie Europe. Although Islas Europe and Bassas da India are of Cenozoic volcanic origin, this is the first report of extensive Tertiary volcanicity south of the northern tip of Madagascar.

The results of seismic reflection and refrac- tion data collected from the Mozambique Channel between 1971 and 1973 (Lort et al., 1979) shows two offshore basins, the Mozambique Channel basin, roughly triangu- lar in shape, and the Zambezi Mouth (Delta) basin. The greater sediment thickness in the latter is in the upper part of the section and consists of clastic sediments from the Zambezi. The two basins are separated by a high, referred to as the Beira High (Fig. 10). As with the Davie Ridge, the Beira High is asymmetrical, with the steeper, 1,500 meter

scarp facing east. Behind the scarp is a west- dipping wedge of sediments off the port of Beira.

The Mozambique Channel basin pinches out to the north where the Davie Ridge comes close to the African coast. To the south, the basin appears to merge into the Mozambique Channel.

The acoustic basement is irregular and faulted (where seen) and "is probably different in the east and west of the basin" (Lort et al., 1979). Sills are observed in some profiles. In the west, near the African coast, the margin of the basin is marked by a series of basement high features.

The velocity data on the sediments is consistent with Mesozoic-Cenozoic sediments. The basement velocities are in the range for Karoo sediments, or weathered basalt. A continuous reflector (B) marking the base of velocity 1.8 k m / s may be correlated with the base of the Tertiary in AGIP's Mariarano well and is consistent with DSDP Tertiary velocities at sites 241 and 249 (the latter on the Mozambique Ridge). A second reflector (C) (possibly an intra-Cretaceous discontinuity?) separates sediments with velocities below 3 k m / s from those with higher velocities. The DSDP wells do not penetrate deeply enough to identify the reflector, but the Mariarano well indicates a potential boundary with the Upper Cretaceous volcanics [at least in the northeastern part of the channel (Lort et al., 1979)]. This interpretation is questiona- ble since Mariarano is in the Madagascar block on which relative motion ended only in the Early Cretaceous, and synchroneity of the Upper Cretaceous igneous rocks would have to be established, although ages of about 80 Ma have been estimated for both basaltic lavas, and gabbroic intrusions known onshore in Madagascar.

More recently Geco and Western Geo- physical have shot a sequence of near shore lines as a consequence of the Mozambique government's attempt to generate exploration interest. Petroleum exploration activity is summarized in Tables 2 and 3. Table 4 sum-

G E O L O G Y , BASIN A N A L Y S I S A N D H Y D R O C A R B O N P O T E N T I A L O F M O Z A M B I Q U E 95

marizes gas production in the three onshore fields (see Hydrocarbon Prospects).

STRUCTURAL MODEL FOR MOZAMBIQUE

Structure model

There have been relatively few attempts to develop a structural model for the origin of the Mozambique Channel, which should include the broad plain now forming the greater part of Mozambique. The only structural model of distinction is that of Flores (1973) which was generated before the M anomaly sequence was recognized. The old geosyncli- nal model of Dixey (1956) is tenacious and was espoused by Kamen-Kaye (1982); presumably the model is related to the con- troversy surrounding the location of Mada- gascar and reflects the difficulties which arise when considering a problem in isolation.

To date there does not appear to have been assembled the variety of geological and geophysical data essential to provide a compre- hensive assessment. The structural patterns in Kihle (1983), Kamen-Kaye (1982), Western Geophysical (1982), and Salman (1982) are perfunctory and inadequate and, for the most part, are repetitions without any improve- ment over the figure given by Flores (1973). Figure 11 has been drawn to emphasize the principal structural features. Some features, such as the Davie Ridge or fracture zone, are well known, others are less well known and their significance is in doubt, still others have seldom been included in the context of the origin of the Mozambique Channel, such as the Lebombo monocline and volcanics, and yet others have not been suggested.

Ridges The Davie Ridge or fracture zone is the

best known; it is an asymmetrical structure with an east facing scarp, and is regarded as marking the line of southerly movement of Madagascar. This motion is apparently con- firmed by the M anomaly pattern and is consistent with the paleomagnetic data. When

Section Ga 6-1 l from Loft et al. Bassa$

NW da Zambezi SE India Canyon ~ ~ -

I "> I ' ( Mozambique Channel Basin

Zambezi Delta Basin

p(qB ~ Quel imane

Mozambique

Inhamin9~ ~ - ~ - - ~ ' ' - - . Channel

/ u ~ H ~ Basin ~lra ",Basin

su, Do

Salve

M

aputo Proximal

Natat Valley Basra

.X""%.. ",\ oi".

la ~°'°);o; ,, ', :\

lnhambane

MMR- Mobote (Mazengo) Rift FR- Funhalouro Rift SR- Southern Rift UCG- Urema Chire Graben BH- Beira High CG- Chissenga Graben PG- Palmeira Graben

32"E 36* 40 °

Fig. 11. The principal structural features of Mozam- bique.

16"

20"

traced to the north, the Ridge appears to trend into the Lamu Embayment, and offshore a parallel series of fractures has been identified (e.g., Dhow, VLCC).

Parallel to the Davie Ridge lies the Mozambique Ridge, which appears to be structurally similar. At its southern extremity, the Mozambique Ridge appears to fit well with the tip of the Falkland Plateau extension. The northern extension is much less clear; Dingle and Scrutton (1974) seem to show the extension offset but, continuing northward, the extension would trend toward the Inhaminga high and the western side of the Zambezi graben.

The Beira Ridge is the least distinct structure with the same trend, it has been recognized on some of the seismic profiles of Lort

96 A.E.M. N A I R N ET AL.

et al. (1979), but no structural features are known.

Although not obviously a ridge, the Lebombo monocline has the same trend and the same steep easterly face; it certainly indicates a line or zone of deep crustal fractures which provided egress for Jurassic and Creta- ceous volcanics and coincides with a large positive isostatic anomaly. A lesser known north-south feature is a line of weak seismicity with which the Zambezi canyon coincides.

Basins Between these ridges, basinal sediments

have been ponded. In the north, the dip slope of the Davie Ridge forms the margin of the Mozambique Channel basin; the Beira Ridge ponds the Zambezi Delta basin sediments, while the Mozambique Ridge retains the sediments of the proximal Natal Valley fed by the Limpopo River.

Cross-structural elements These elements are the most heterogeneous

of the three groupings, but are considered together solely on the basis of a coherent ENE trend. The most northerly element simply represents the line of Precambrian outcrop which, northeast of Qualimane, ap- proaches very close to the coast. In the east, the most southerly element marks the transition from the Mozambique Channel to the basin, at about 24°S. North of 24°S the M anomalies disappear and the magnetic re- sponse of the basement changes. Onshore, and continuing into offshore along the same trend, is the axis of a gravity anomaly. The change in the Mozambique Ridge has led to the suggestion that this axis may also coincide with a major fault downthrown to the south. It is along this axis that, according to Dingle and Scrutton (1974), the ridge may be offset by about 60 km. Onshore, the trend of the Limpopo and the Zoutspanberg re-entrant, as well as the change in trend of the Karoo outcrops suggest tha this parallel feature may be controlled by basement structures. In a

similar manner, the course of the Upper Zambezi is also parallel to this trend.

The simplest interpretation of the ridges is to suppose they are merely tilted horst blocks and, given the trend of the M anomalies, mark fracture lines normal to the spreading axis on which the anomalies were generated. This suggestion is not new for the Davie fracture zone, for Norton and Sclater (1979) illustrated such a pattern, although their timing of the event is suspect. It is, as far as we know, new to suggest the other fractures per- form the same functions, providing an easy interpretation of the Mozambique Ridge, which is similar structurally to the Davie fracture zone. These lines, therefore, accommod- ated the relative southerly movement of East Gondwana from early Late Jurassic until mid-Early Cretaceous (Hauterivian-Barre- mian).

The obvious question, then, is whether the Mozambique Channel has an oceanic crust. The difference in the magnetic anomaly pattern compared to those found in the Mozambique basin would argue against oceanic crust. Darracott's (1974) interpretation of the gravity data, as showing generally isostatic equilibrium, is consistent with the existence of thinned or transitional crust.

This thinning most likely resulted from tensional forces associated with the sundering of Gondwana. The degree of extension which occurred is reflected in the pattern of block faulting, with the offset of the Mozambique Ridge (60 km) providing a minimum value. The total extension was probably not much more than double that amount. Heat generated by the movements was disseminated by volcanic activity, and the Movume basalts date from about the time movement was com- ing to an end.

Subsequently, due perhaps to the initiation of movements in the Atlantic and the isolation of Madagascar, there was a swing in the regional stress field and a new set of fractures developed which had the present Indian Oc- ean trend. Leaky transforms may have developed. The north-south fractures became inert


and, as the crust gradually cooled, further thick sedimentary wedges developed fed by the detritus brought down from the uplifted interior by the Limpopo and Zambezi Rivers. At the onset of these movements, the Agulhas fracture zone developed, with the Falkland spur separating from the African margin.

The final phase of movements was the development of the East African rift system and its extension south of Lake Nyassa through the Chire-Urema graben system. This activity is discussed later because of its importance in the development of the paleo-Zambezi delta system. In the continental slope in northern Mozambique young grabens, associated with

N

Zambezi Graben

A

A

Urema-Chire Graben

Fig. 12. The interpretation of the tectonic history of the Zambezi-Urema-Chire graben systems (after Flores, 1973).

the East African rift system have been described by Mougenot et al. (1986 a, b).

The sequence of events described essentially dates from the Late Jurassic. The nor th-south structures are imposed upon older structures, but do not entirely obliterate them. Predicting what form the older structures may take is difficult since there is little onshore seismic data and virtually no bore- hole data. Some inferences may be drawn from the exposed Karoo sequence, which terminated with the extrusion of lavas in the Early Jurassic. From the exposures in the Zambezi valley graben, as well as the pattern of outcrops from the surrounding regions, tensional fracturing, with the development of horsts and grabens, also occurred during Late Permian and Triassic times.

It is not clear from the literature (cf. Borges, 1952) whether the oldest (Ecca) sediments preceded or were contemporaneous with graben formation, or if they are merely pre- served within a graben. It seems likely that there was Late Permian-Early Triassic fracturing as the Karoo Basin widened and sediment spread over the Upper Zambezi region, as may be inferred from the geological record in Zimbabwe. A similar event is known in Madagascar, where, in the south an Upper Permian marine horizon occurs. Unfor- tunately, little is known of the transport di- rections of the clastic Karoo sediments in Mozambique.

Turning to the smaller-scale features onshore, Flores (1973) showed a pattern of faults and intersecting graben systems. The Chire-Urema-Chissenga graben system has been related to the continuation of the Nyassa Rift, thereby providing a link to the East African rift system. The faults of the system cut across the Zambezi Rift, which to the east is scarcely identifiable (Fig. 12). The Zambezi Rift could continue to the east at depth under the depression of the Zambezi Delta, or it could die out in a fault splay, producing a number of small semi-grabens. Although the latter is the preferred interpretation, it must be stressed that there are no data which would

98 A . E . M . N A I R N E T A L

help discriminate between the two interpretations.

The earliest age of activity in the Chire- Urema-Chissenga system, during the Early Cretaceous, is in part defined by the role played in the development of the paleo- Zambezi Delta as illustrated very clearly by Flores (1973) (Fig. 12). The grabens are shown as if a single system, but are more probably a series of straight segments with offsets. The southern Chissenga graben is traced south- eastward by a combination of indirect techniques, geomorphological and geophysical. In the offshore continuation of the trend, dark Domo shales imply that the continuation is reasonable. South of the Save, in the Sul da Save area, which is the region where the

Mozambique basin is widest, the structure is difficult to infer. Outcrops are restricted to young rocks, and seismic data are absent. Flores (1973) interpreted the gravity and magnetic data in terms of "magnetic arches." As the magnetic highs tended to coincide with gravity lows, he suggested they marked troughs of a sedimentary fill with "magnetic" sediments, i.e., conceivably significant thicknesses of tuff. The pattern of magnetic arches is consistent with the general structural trend and the existence of grabens north of the Save River. More arches may exist in the western part of the basin and, potentially, some may simply be tilted blocks.

The tectonic activity of the fault systems, except for some subsequent rejuvenation, pre-

A LEBOMBO MTNS, 0 ~ COAST

km

B 0

l "~ " MOZAMBIQUE -~:.:..~ R,OGE

" . ' . ' . 249 WE B , E . .

L ~ t - ' . ' - ' . ' - ' . ' . ~ - ; " ~ MOZAMBIQUE \ .,oDE

4 ~" " " ' * ' " "* °* "" °" "" """ ? \.8 j + ~ - - : . : - : . : - : - ]

6-~ + + + ' + + + ~:-:.:.:.:.:. km + + + + + + . . . . vvvv vv~vvvvv

C LEBOMBO 02 ~ M T N $ , COAST

:::::" • . . . . . - . : . : : : : : :

21 , , \ ' -'- o 0

• PROXIMAL ,'o NATAL MOZAMBIQUE ". ' , VALLEY RIDGE

• ." ' ' ' ' . ' . ' " • WESTERN • " . . . . . . MOZAMBIQUE

4 " ' : ' : ' : ' " " 2 . BA~'N

...:.:.:.:." 6 "-:-:.'.'"

S e d i m e n t

C o n t i n e n t a l C r u s t

O c e a n i c C r u s t

• S i t e • " 7 /]. / !/::

l O O m l J

• i I O O k m

Fig. 13. Sections across the Zambez i Cone and the proximal Nata l valley (A ~nd B) modif ied to show asymmetry (C and D).


sumably ended within the Late Cretaceous when the current spreading pattern became established in the Indian Ocean. With the cooling and gradual subsidence of the thinned crust, a wedge of sediments built cut into the Mozambique Channel. The sections of Dingle and Scrutton (1974) which show the gradual submergence of early structures under this sedimentary prism have been modified to suggest an asymmetry (Fig. 13). There is some support for this model, for sediments derived from the east have been described in offshore wells.

Relevance of the model to petroleum exploration

Because the sedimentary pattern, in both thickness and distribution of the different lithologies, was controlled by the underlying structure, the tentative conclusions drawn from the model are relevant to exploration.

The Karoo sequence has proven disap- pointing whenever examined in South and East Africa; reservoirs are numerous, but it is difficult to isolate a good source rock. The most suitable source would lie within the Ecca where the presence of coal indicates the preservation of organic matter. Marine horizons occur within Ecca rocks (in both Natal and Mozambique), but little evidence exists that such marine horizons have ever been either extensive or of long duration. Coal horizons have been recorded and coal has been worked in the Zambezi Valley, but its subsurface extent is unknown. The southeast continuation of Ecca beds in the Inhaminga area depends upon which model for the Karoo is used.

The most promising prospects rest in the beds of the Sena Formation, a diachronous sequence which overlies the Belo Formation in the Zambezi graben. Facies equivalents elsewhere rest upon volcanics. The Sena For- mation consists of continental clastic beds which pass southward into the marine horizon of the Maputo Formation in the general

area of the Sul da Save region. Above the Maputo beds lie the rocks of the Domo For- mation, which are facies equivalents of higher levels of the Sena Formation. The Domo For- mation has a middle sandstone division en- closed between shales which have been described as euxinic. A considerable thickness of the Domo Formation has been drilled in wells which appear to penetrate Domo shales laying within the continuation of the Chis- senga graben. The distribution and thickness of these shales and enclosing beds has been indicated in the structure contour and isopach maps of the region (Figs. 4-6). This pattern may be repeated on the eastern side of the Beira Ridge in the Mozambique Chan- nel basin.

Because Cretaceous beds lap onto volcanics of probable Lower Cretaceous age, no control exists on whether these beds may be pre- served in an offshore basin, the marine equivalent of the Belo Formation. Such a suggestion is incorporated in an interpretation in the Western Geophysical report. The presence of marine Jurassic in this region is unknown, but its occurrence in both South Africa (Knysna) and southern Tanzania suggests that it may well be present.

The mixed clastics of the Grudja Forma- tion, in which there are several stratigraphically distinctive sandstone horizons (gas- bearing), contain organic matter. The formation tends to develop a more open marine character eastward where it may have source rock potential. These "tilted beds", recognized in seismic lines, are unconformably overlain by Eocene, so that migration and trapping potential exists. The whole paleo- Zambezi delta complex deserves more attention.

The preservation of evaporites (Temane Formation, Fig. 6) near the mouth of the Save River during the regressive conditions of the Oligo-Miocene probably has little significance from the standpoint of hydrocarbon potential, for the evaporites are not associated with a "black shale" facies. The regressive Oligo- Miocene clastics seem to hold little potential,

1 0 0 A.E.M. NAIRN ET AL.

nor do their equivalents seem to have much of a potential.

H Y D R O C A R B O N PROSPECTS

History of exploration

This history of exploration in Mozambique can be divided into three main phases. The early phase covers the period from the first exploratory wells up to the granting of a concession to Gulf Oil in 1948. The middle phase lasted until 1976 when, due to local political activities, the last company withdrew. The present phase began with promulgation of new hydrocarbon legislation and the formation of a national agency to promote active exploration as part of a development program in the early 1980's. This history has been reviewed (Hn77ard et al., 1971; Kamen- Kaye, 1983; Kihle, 1983; Armstrong, 1985)

and in various reports (Salman, 1982; SECH, 1982; Western Geophysical, 1983). The most succinct exploration history of the area is that of Kihle (1983), the most detailed is that of Salman (1982). A summary of exploration activity and of gas production and reserves is provided in Tables 2-5.

Basically, until Gulf was granted a concession in 1948, exploration had been sporadic and seemingly uncoordinated. Exploration was concentrated around the known gas seep, with shallow wells of less than 500 meter depth being drilled during the periods 1904- 1907 and 1927-1934. One deep well, Inham- inga-5, did result in a gas flow of short duration. Salman (1982) provides most of the de- tails and refers to gas and oil seeps, but elsewhere reference is to a single gas seep.

The second phase of activity began with Gulf's initial exploration efforts which combined field and photogeology, geomorphic

T A B L E 2

Summary of gas field product ion

Pande gas field (5 wells) Producing horizon (principal)

Initial bo t tom hole pressure Final bo t tom hole pressure Flow Reserves est imated (1981) Reserves est imate (1973)

Temane gas fieM (2 wells) Producing horizon Bot tom hole pressure Flow

Buzi gas fieM (2 wells) Producing horizon Bot tom hole pressure Flow

Estimated gas reserves: Pande field 1972 Gul f -Amoco 1973 F ran lub 1979 Rompelrol 1981 Braspetrol

G6

118.1 a tm 100.8 a tm 260.0 x 103 m 3

9.9 × 109 m 3

30.0 x 109 m 3

G9 141 a tm 189.0 x 103 m 3

G9 158.0 a tm 246.2 x 103 m 3

39.0 × 109 m 3 30.0 x 109 m 3 15.4 X 109 m 3

9.9 X 109 m 3

Sands 8 - 1 6 m thick ( three other sands produce)

some condensa te

recoverable

G E O L O G Y , BASIN A N A L Y S I S A N D H Y D R O C A R B O N P O T E N T I A L O F M O Z A M B I Q U E 101

studies, and gravity, seismic, and magnetic studies of the 46,000 klTl 2 concession. The seismic work, although of generally poor quantity, did result in the production of isochron maps, on the basis of which the locations of the first wells were determined. More detailed regional survey work followed the partnership of Gulf with Amoco in 1958, and by 1967 the Pande gas field had been found, with five of the six wells drilled yielding gas from sandstone horizons within the Grudja Formation (Table 2). Gulf-Amoco faced a reduction in their concession area, and contracts were then let to Aquitaine, Sunray, and Hunt. In 1969 Gulf began offshore seismic work and spudded their first offshore well.

Sunray, operating in the south, had a program in which most of the activity was offshore, with six wells compared to two onshore. The offshore wells reached a basalt basement at 1.3-3.0 km, although in some narrow grabens the basalt was attained at 3.5-4.5 km. A much thinner sequence is recorded here than further north where, in the paleo- delta of the Zambezi, both Hunt and Aqui- taine found that the sediments range from 6-9 km in thickness.

In the north, Hunt spudded Zambezi-1 and 3 which helped define a Cenozoic deltaic and reef complex that overlies the Upper Creta- ceous marls and argillites. Although these wells located Lower Miocene viscous oil, asphalt-impregnated sands, and some gas shows, no commercially viable prospects were discovered.

Gulf withdrew in 1970, a year during which Aquitaine reduced its acreage. Aquitaine, Sunray, and Amoco all withdrew in 1972, and with Hunt withdrawing in 1974, all exploration activity came to an end.

The Mozambique government subsequently attempted to stimulate and renew exploration activity in the 1980's. Newer and more favorable petroleum legislation was passed and a government department was set up to coordinate activity with foreign oper- tors. The international consulting firm, A.D. Little, was retained to advise, and Geco and

Western Geophysical were engaged to shoot offshore seismic during 1981-82. Flores was retained in 1983 to make formal government- sponsored presentations in an attempt to generate company interest. An optimist ic summary of the data was presented by Kihle (1983). Towards the end of 1983, 17 offshore blocks were opened to bidding, an opening delayed until production sharing contracts on the Rovuma onshore concession had been signed with Shell and Esso.

The offshore concessions were awarded until 1984, the first going to Standard Oil of Indiana in October. In December, BP won the second contract for the offshore, an area east of Maputo, and Amoco took the area north of Beira. The A.D. Little report showed that these offshore blocks did not generate much interest; closing dates for bids were delayed several times at the request of compa- nies. The principal scientific constraint was the general lack of a good source-rock horizon.

An adequate structural model is missing from the data package prepared at the request of the Mozambique government, and the structural figure proposed can be traced back to Flores (1973), which is reproduced by Kihle (1983) and also presented, with only slight modifications, in the Western Geophysical report (1983).

Tables 2 and 3 chronologically summarize the exploration history. Where there is a con- flict in reported dates, the dates given by Salman (1982) have been used. This is the only report which gives information on 23 of the wells (locations given in Fig. 1).

Source rocks

According to Kihle (1983), qualitative studies suggest that the shale horizons within the Paleogene Cherigoma Formation and turbi- dites and shales in the Senonian to Paleocene Grudja Formation are the most probable source rocks. He lists shales with the Mazamba / Inhar r ime Formation as possible sources. Onshore samples show an organic

TA

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104

TABLE 4

Summary of geophysical exploration activities in Mozambique

A.E.M. N A I R N ET AL.

Date Company Activities

1948-1956 Gulf

1959-1965 Gulf-Amoco

1966-1972 Gulf-Amoco

1969 Gulf rex

1968-1972 Hunt

Sunray

Aquitaine

1991/1982 Geco

Western Geophysical

Gravity, magnetic, low-quality seismic surveys; produced isochrons for 1-2 reflecing horizons

Detailed regional survey

Seismic study to define Grudja-Domo potential; multiple profiles of area 19°30 'S to 2 l°30 'S . Defined 2 highs (Nemo and Sofala) on single structural zone. Pande, Buzi, Temane gas fields.

Surveyed Maputo-Tulear (line MC-7); E - W through Islas Europa (MC-9); Mozam- bique shelf project (MC-5, 6). Spudded first offshore well (Nemo-9)

Northern part of Mozambique Basin, 12,000 km seismic profiling on 2 × 2 km grid; isochron maps of 2 reflecting horizons (in Cretaceous and Cenozoic), est. thickness 9 km.

Low-density profiles, 5 x 15 km grid, area south of 25 ° S; some anticlines and complex faulting discovered.

Regional and reconnaissance lines, areas A, B, and C (part of A only in offshore); onshore seismic grid 10 × 20 km, number of anticlinal highs located, and Zambezi paleo-delta.

Seismic profiling of 13,018 km from Tanzania border to 17°S: 15 krn grid, coast perpendicular lines; 15-20 km spacing, coast parallel lines. From 17 °S to Beira: 10 km separation coast parallel, 5 km separation coast normal lines. All out to 200 m; more open grid out to 5,000 m. Five well tie lines. Using a 3103-in 3 conventional airgun, 48-fold recording to 6 sec. Shot point interval 25 m. Processed standard sequence to 6 sec. Wave migrated. Data quality good.

Seismic profiling of 12,275 km area from Maputo to Beira treated in three sections: (1) Off and SE of Beira, lines tied to Sofala-1 and Nemo wells. Nemo well tied to Senyo-1 (Aquitaine), Zambezi-3 (Hunt). 4,500 km of lines. (2) Off and ENE of Maputo, lines tied to all 5 Sunray wells. Lines cover concession and unexplored area of Maputo. 5,000 km of lines. (3) Along narrow continental shelf, poorly explored, only one well in south near Inhambane, lines between areas 1 and 2 . . . . . . 500 km of lines. Using Maxipulse energy source, 50-m shot point interval, 30-fold recording to record length of 7 sec. Data quality good.

content dominated by gas prone, humic material. However, marine influences become increasingly important eastward, hence oil prone sources become possible. Kamen-Kaye (1983) described the shales of the Grudja Formation as sublittoral to neritic with a planktonic foraminiferal assemblage. He stated, "They may have re ta ined . . . an important part of their original [organic] content," using as corroboration an unsigned report on the Micaune-1 well.

Source rocks of the Grudja Formation and

younger Cenozoic beds are generally associated with the paleo-Zambezi delta. The sedimentary sequence is thick, the inclined beds favor migration up the delta and the occurrence of stratigraphic pinchout traps. With burial depths of 5,000 m, the Mesozoic part of the succession should lie within the zone of mature rocks, the Cenozoic in the gas prone rocks.

The shales of the Domo Formation (Albian to Turonian) are also regarded as potential source rocks; they are described as euxinic,


dark grey to black, marly shales ranging in thickness from 300 to in excess of 1,500 m and extending northward nearly 1,200 km from the South African border. Their organic content is rated as humic or mixed. The geothermal gradient is 1.9-2.1°C/100 m and sediments buried in the deeper parts of grabens should be within the oil window.

These remarks concern only the Cretaceous and Cenozoic, but there is a possibility that marine Upper Jurassic may be encountered in the northern Rovumu basin. Not only are marine rocks as old as Upper Liassic encountered in Madagascar, but they are also known in the subsurface near Dar-es-Salaam. Kent (1965) also reported marine Lower and Mid- dle Jurassic rocks from the Mandawa graben close to the Mozambique border-- the southernmost known marine Jurassic sediments in East Africa (Flores, 1973). Western Geo- physical (1983) indicated one such possibility. No consideration has been given to the Karoo (Ecca) as a possible source rock, probably because exploration has not extended into the Zambezi graben. However, the occurrence in Zimbabwe of the dark Madumbisa shales and coal within Mozambique means that the possibility of hydrocarbon generation should not be ruled out, although in southern Africa the Ecca has proved to be generally disappoint- ing. The Western Geophysical report (1983) mentioned restricted marine Karoo, but gives no supporting data.

Reservoir rocks

The principal reservoir rocks of the Meso- zoic and Cenozoic are the sands; in the Sena Formation (Fig. 3), they are largely continental, but to the south and east the lower part is replaced by a marine, shaly, somewhat glauconitic sandstone referred to the Maputo Formation (Ka 1). The latter is found in the central part of the Zambezi River delta depression where it overlaps onto Lower Creta- ceous. A second sandstone forms the middle member of the succeeding Domo Formation (Ka 2), and is a good target since it lies within

a shale sequence which is the most highly regarded source rock. Ka 2 occurs at subsurface depths of 2,500 to 3,000 m.

Sandstones within the Upper Cretaceous to Paleocene Grudja Formation (Kb 1, 2) are the only proven reservoirs. Of nine documented, stratigraphically identifiable horizons, gas has been produced out of five, the most productive being the uppermost where it occurs in the central part of the basin between Pande and Temane at depths of 1,100- 2,000 m. The estimates of gas reserves within these sandstones varies according to the area of the field considered, revisions generally being downward (see Table 2).

Within the Oligo-Miocene the clastics of the Zambezi delta both onshore and offshore are possible reservoirs, although no lithological or structural traps are reported so far. The depths to these clastics are of order, 2,500- 4,000 m.

The only carbonate reservoirs are associated with the Eocene rocks. There are reefal buildups at depths of 2,500-3,500 m. Kamen-Kaye (1983) reports their potential as low.

Drilling history

The number of deep wells drilled in small; the total number quoted varies a little according to the period of time considered, but even in the most optimistic reports, the number is less than 70. According to Kamen-Kaye (1983), that is a surface density of less than one per 4,000 km 2, and the density is even lower in the offshore, with only 12 wells drilled. Figure 1 shows the location of 57 wells (after Kihle, 1983), and the location of 23 wells for which there is partial information. This well information is taken from Sal- man (1982).

The failure of the offshore wells, according to the Western Geophysical report (1983), was due to poor site location based upon the geophysical data then available.

It should also be mentioned that, in general, when basalts were reached in drilling


these were usually regarded as Karoo and were seldom penetrated. Present information (from Flores, 1973) suggests that the basalts were Cretaceous in many cases, and that there may be a considerable thickness, 1,000 m or more, of subvolcanic sediments.

Trapping mechanism

The Western Geophysical survey (1983) showed the existence of a rift offshore from Maputo which provides greater possibilities for both sourcing and trapping mechanisms. This offshore rift is in a region where faults are common, with tilted fault blocks, anticlines, and pinchouts, which all represent potential traps. Fault block edges provide the possibility for reefal buildup controlling dif- ferential compaction and sediment drape. In the southern area, the Western Geophysical report suggests the possibility of marine Jurassic. Some structural highs correspond to gravity minima, and hence may be halokinetic structures within the Karoo, although the existence of evaporites has still to be proven. There are other grabens known in the onshore area of the Mozambique basin, which figure in all structural maps, but for which other data are absent.

The timing of the faulting activity associated with the grabens affects their potential as traps. The offshore graben in southern Mozambique has an Early Cretaceous period of activity and Karoo activity of uncertain age possibly Late Permian and Triassic in age by analogy with Karoo elsewhere. Such grabens could contain restricted marine sediments of Late Permian-Early Triassic age, because marine Ecca was reported in the Zambezi, Tunduru Depression and in the Kidodi basin, Tanzania as well as near Natal in South Africa.

The greatest prospect for the offshore, and part of the onshore, appears to lie in stratigraphic trapping mechanisms associated with the Mesozoic and Cenozoic paleo-Zambezi delta complex where there is a proven sedimentary pile in excess of 6 km. All three

major situations exist: traps against basement topography, stratigraphic pinchouts, and sediments under unconformities. The sedimentary sequence contains many possible clastic reservoirs, of which the Grudja and Domo sandstone horizons are proven gas reservoirs. The Domo shales and horizons within the Grudja Formation are regarded as viable source rocks. The tilting of these deltaic sands and their sealing by the unconformably overlying Eocene beds provides a classic model highly productive elsewhere. Depth of burial is such that the lower part of the pile should lie within the oil window, although the Oligo-Miocene clastics may only be gas prone. The problem is lack of information.

BASIN ANALYSIS AND HYDROCARBON GEN- ERATION OF THE SOUTH MOZAMBIQUE GRABEN USING EXTENSIONAL MODELS OF HEAT FLOW

In the evaluation of the hydrocarbon potential of a frontier basin the limiting factor is the quantity and quality of available data.

In this section we demonstrate an integrated basin analysis method using mathematical models which may greatly en- hance the assessment of such a data-poor basin. The technique combines the depositional, structural and thermal histories of the region and ultimately results in cross-sections showing maturation potential and information on the timing of hydrocarbon generation.

Specifically, a seismic line selected perpendicular to tectonic strike is backstripped at pseudowells (cf. Swift et al., 1987), using one-dimensional burial history computer models (e.g. Guidish et al., 1984), and structurally reconstructed in sequential chronostratigraphic steps back in time.

Before the technique was applied to real field data from the South Mozambique Graben, the problems of backstripping wells which penetrate faults were investigated by means of a series of tests on synthetic exam- ples (see Iliffe et al., 1989). The main outcome


of those tests is the need to "redrill" the pseudowells after each chronostratigraphic unit has been removed.

In the absence of local thermal information, as in our example, it is possible to use the extension models McKenzie (1978), Royden et al. (1980) to estimate the thermal history from "observed" subsidence curves. Subjectivity in interpretation, and error in the data and analyses are partially alleviated by considering three different possible extension histories, and by using the upper and lower limit of each of the derived thermal histories when applying the hydrocarbon generation model described by Tissot (1969), Tissot and Espitali6 (1975), and Tissot and Welte (1978). In this way we dynamically bracket the hydrocarbon potential of the basin.

T H E S O U T H M O Z A M B I Q U E G R A B E N : A CASE S T U D Y

This study of the South Mozambique basin (see Fig. 14a) exemplifies the situation of a speculative basin with limited data, often of poor quality and shows how basin modelling may improve evaluation based on the limited resources available.

The available data for offshore South Mozambique consisted of 800 km of seismic, two shallow wells (Fig. 14b) and the general literature (see part A). In the analysis of this data we applied a one dimensional Burial History Program (Guidish et al., 1984), a generation model (Tissot, 1969; Tissot and Espitalie, 1975; Tissot and Welte, 1978), models of extension (McKenzie, 1978;

Lu a

).- <c .J ¢,o uJ uJ "r" LLI ,,-,,

DEGREES LONGITUDE

, o \ \ / \

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a

A'r

k A'

N

DEGREES LONGITUDE

3~3 3~4 3~5 ~ :3 6

MOZAMBIQUE /

A 24 A SM°Uzt;m I~ ....

2 3 ~ j ~ O ~itl;iOk m s ~

b

0 20krn

C Fig. 14. a. Location of South Mozambique Basin. b. Approximate location of seismic survey, and line A-A' used in this study. Location and structural trends which cut Lower Cretaceous strata in the South Mozambique Graben.

1 0 8 A.E.M. NAIRN ETAL.

Royden et al, 1980), and some basic geological knowledge.

Geologic setting of the graben

Initial interpretation of the data revealed a "V"-shaped rift basin trending N - S to NNW- SSE (Fig. 14c). The eastern border fault trends NNW-SSE, whereas the western border fault is oriented more N-S. The seismic data exhibit tilted fault blocks dipping to the east on westward verging faults.

The tectonic setting of offshore Mozam- bique is the subject of much debate (cf. Segoufin, 1978; Rabinowitz et al., 1983; Mougenot et al., 1986a, b; Martin and Hartnady, 1986; Coffin and Rabinowitz, 1987). Darracott (1974) recognized that the area lies between the continental crust of Mozambique and the oceanic crust of the Mozambique Channe l which separates Madagascar from the African continent. The general consensus is that Madagascar has been transported southwards along a N-S-trending transform zone (Coffin and Rabinowitz, 1987) (Fig. 15). This movement has been dated on the basis of magnetic anomalies as ranging from 150 to 110 Ma (Segoufin, 1978) or 160 to 125 Ma (Rabinowtiz et al. 1983). Initial rifting began as early as Permo-Triassic and preceded the major break-up of Gondwana in Late Jurassic to Early Cretaceous times.

When describing the general problem of sedimentation for southern Mozambique it is useful to refer to the seismic character of the chronostratigraphic units used in this study. This information is largely taken from Lafourcade (1984). The stratigraphic sequence used in this study is shown in Fig. 16.

The Karoo unit may be present in the seismic profile (Fig. 16), making up part of seismic package 6. The reflectors in this package are structurally disturbed and stratigraphically discontinuous with wedges of sediments stacked against fault blocks, char- acterizing them as synrift deposits. The age of the Karoo unit is speculative since it has never been penetrated offshore, Kamen-Kaye

kU

I--

tu ul eT-

tl,l r~

DEGREES L O N G I T U D E

Fig. 15. M a i n s t r uc tu r a l t r e n d s in the M o z a m b i q u e

Reg ion , s h o w i n g t r a n s c u r r e n t f au l t s in the a rea re la ted

to the s o u t h w a r d e m p l a c e m e n t o f M a d a g a s c a r .

(1978) described a Triassic fossil from onshore Mozambique and Forster (1975) reports a Late Triassic to Late Jurassic radiometric age in this region. The occurrence of marine Jurassic overlying evaporites in the Mandawa graben in Tanzania to the north (Kent et al., 1971) suggests that rifts in offshore Mozam- bique may also contain marine Jurassic deposits.

Recent geophysical surveys have suggested that a second phase of rifting prior to the break-up of Gondwana in this part of Africa occurred from Late Triassic to Early Jurassic times (Mougenot et al., 1986a, b). Accord- ingly, seismic package 6 is taken to be 195 million years old at its base and 140 million years old at its top.

G E O L O G Y , B A S I N A N A L Y S I S A N D H Y D R O C A R B O N P O T E N T I A L O F M O Z A M B I Q U E 109

Following deposition of seismic package 6, a marine transgression beginning in Early Cretaceous times (Albian?) is recorded by the

sands and shales of the Maputo Formation (Flores, 1973). This implies to the absence of most of the Early Cretaceous. Reflectors in

12 I1 10 9 8 7 8 5 4 3 2 1

~ ~ ~ . I : _ - _ : t . - - - I - - J - - - ! : - - - - : 'U: . ' - = - r : - : ~ : - - , "

. . . .

(a )

WELL 4

TWO-WAY L ITHO. TRAVEL D E P T H AGE

TIME ( rn ) (Mya) (sec)

FORMATION

- - 0

~22.5

- - 5 5

~ 7 6

m 9 6

~ 1 4 0

~ 1 9 5

MIOCENE DELTAICS

CHERINYOMA

UPPER OCMO SHALES/ GRUDJA

DOMO

MAPUTO

JURASSIC KAROO?

(hi

Fig. 16. a. Interpreted seismic section A-A', trending east-west showing "flower" structures at depth with more "domino" style faulting higher in the section, b. Stratigraphy and time-depth conversion of well 4.


seismic package 5 are discontinuous and structurally disturbed.

The Maputo Formation is overlain by the Lower Domo Shales Formation from Ceno- manian to Turonian age. There is a bright doublet visible at the top and bottom of the corresponding seismic package 4, the middle being largely seismically transparent. Where visible, reflectors are discontinuous. The doublet at the top may be the mid-Turonian Domo sands.

The Domo sands doublet is truncated by an Upper Cretaceous unconformity which is overlain by the clayey silts, sands, sandy limestones and marls of the Upper Domo Formation or Grudja Formation, the latter terminology depends on locality. This unit (seismic package 3) has parallel, horizontal, continuous reflectors which may indicate a basinal type depositional setting (Flores, 1973). It is apparent from the seismic section in Figure 16 that this unit was either very thin, eroded or not deposited on the flanks of the rift.

Above the Grudja Formation lies the oolitic, algal limestones, marls and sands of the Eocene-age platform and ramp sequence of the Cheringoma Formation (seismic package 2). The ramp, visible on the northeast side of the seismic section in figure 15 contains slump structures. The reflectors are slightly inclined, discontinuous, and non- parallel on the ramp, and faint or absent in the basinal areas.

Overlying these sediments are the deltaic sediments of seismic package 1 of Miocene to Recent age. Reflectors are gently dipping, thin and parallel.

Seismic interpretation and time / depth conversion

The first step in the analysis of this basin was the initial seismic interpretation. This was based on the recognition of six seismic packages rather than following one or two reflectors regionally, which proved impossible in the rift itself.

Basic isopach maps were produced and faults correlated to determine structural trends (Fig. 14c). A seismic section perpendicular to tectonic strike (A-A', Fig. 14c) was then selected. This seismic profile was converted to a true depth section using pseudowells 1 to 12 (Fig. 16). The pseudowells were located at shot points where interval velocities had been derived from seismic times and stacking velocities. The interval velocities were then used to calculate the thickness of each layer. The accuracy of these thickness estimates depends on the accuracy of the velocities, which were unavailable.

After obtaining a "true" section (Fig. 17a) the lengths of each pick were compared and the actual amount of extension as a ratio of the total extension was calculated for each time line, which may be converted into an extension rate diagram (Fig. 18a). Extension rate is calculated by dividing the length difference between the top and bottom of the unit by their age differences.

The structural interpretation was con- strained by bed length balancing an imagin- ary reflector below and parallel to the deepest reflector package a method described in Dahlstrom (1969). The cross-section is now ready for backstripping and reconstruction.

Backstripping and reconstruction

The inputs to the Guidish et al. (1984) 1-D isostatic burial history program in this study are: depths to formation tops, absolute ages at these tops, layer lithologies, paleobathymetry of each layer, density of rock matrix and porosi ty/depth relationships for various lithologies.

Several assumptions are needed to develop the analysis. The lithology of the lowest layer (package 6) is presumed to be a synrift sequence of Jurassic sands.

Owing to the lack of downhole sonic and density information, the lithological poros- i ty /depth functions were defaulted to those available in Sclater and Christie (1980). Con- sidering layers of thickness 1000 m or more to

• e ~ ~61 le uo!13as-sso.~D "~ " e ~ 0~1 le uo!13as-sso.to "j " e ~ 96 le uo!loos-sso.~ D "~ " e ~ 9L le uo!laas-ssoaD 'p " e ~ ~ 'uo~l!sodop auz:::,o] ol .~oud lsn.f UOI1OgS-SSOa~ "0 "S~u.l[ OLHI.I JO q l~ua I gep- luzs~a d "q "uot.loos pol.ZaAUOO-qlda o 'e %I "~!d

I 5

V A ~ c ~ 6 t

| I - - I

E ~ ~ g £ ~ 6 0t t t Zt

c t, ~ 9 L G 6 O~ t l ~t

~ A ~ 9 6

P

V A I ~ 9 Z

V A I N ~

q

Og~V'~

0 0 0 , ' 3N?]O~?

I t ~ g ~ g

I 1 9 6 O L LL ~ t

[ [ [ ~ ) I ~ P ~ V Z O ~ , :]0 l ¥ 1 £ N 3 ± O d N O ~ V D O ~ ( ] 2 ~ H O N V S I S A I V N V b~ISV~ ' A E ) O q O ~ O

112 A . E . M . N A I R N E T A L .

be monolithologic is obviously in error; in general however, such a working hypothesis provides an evaluation in keeping with the resolution of the bulk of the data. The model is more sensitive to error in the estimation of absolute ages and present day heat flow. These effects will be discussed below.

Paleobathymetry estimation is again mainly based on the geologic interpretation and literature.

The backstripping program was applied to twelve pseudowells (wells 1 to 12 in Fig. 17a). After removal of the first seismically defined unit the cross-section was redrawn (Fig. 17b). Since no faulting was evident after 55 Ma no rebalancing was necessary after Package 1 was removed.

When the second seismic package was backstripped, (76 Ma to 55 Ma) the throw on some faults had to be adjusted (Fig. 17c). The removal of throw on these faults slightly con- tracted the section such that those wells that crossed a fault had to be "redrilled". This process of backstripping, restoring, rebalancing, redrawing, and redrilling was repeated until the top of the "Ka roo" reflector ap- peared as a horizontal line at the earth's surface.

The depth to detachment and length of uppermost bed was then analyzed for each cross-section (Figs. 17a-f) using the balancing formulae in Gibbs (1983). The final lengths used in the depth to detachment calculations were between the border faults in the present-day true section, (see Fig. 17 for an example) i.e. within the zone of deforma- tion. The areas were measured using a dig- itizer and an area calculating computer program.

Results and discussion of structural and depositional reconstructions

When considering the variation of extension rate with time it was found that there was a slow acceleration of extension from 195 Ma to 96 Ma, the peak rifting being between 96 and 76 Ma (Fig. 18a). It was also observed

CU Z; I 10- ~J <

a ~ 0 2 0 -

5 a.

~ 3 o l

AGE ( M y a )

150 100 50

, t / / ~-,t .

4 / /

C H A N G E O F D E P T H T O D E T A C H M E N T

;-10

-20

~30

150

A G E (M'¢ a)

10{3 53

1 0 -

~., 2 0 -

~J 3 0 ~ ,J

4 0 4

504 E X T E N S I O N R A T E

- 1 0

. 2 0

r-SO

~-40

- 5 0

~3-

o 2 - I,i.

s J ~

i t . . . " ' . . . •

, • - - . . . . . . . T- 5 0 0 " C e . ""-... s . ,

• " " .................. T- 400°C . . . . . . . . . -:.'.:.- ' . . . . . . . . . . . . . . . " "

1 5 0 t I I 1 O0 50 0

~ime (blya) c

Fig. 18. a. D iag ram to show the change in extension rate fo r each backst r ipp ing t ime per iod (der ived f rom length

o f ho r i zons - - see text), b. Va r ia t i on o f the depth to

de tachment for each backs t r ipping t ime period, c. Vari- at ion of heat flow with t ime calculated from detachment depths.

that the depth to detachment seems to have shallowed from 195 Ma up to the period between 96 and 76 Ma and dropped gently ever since (Fig. 18b). It is significant that, the accumulation rate, subsidence rate and rate

GEOLOGY. BASIN ANALYSIS AND HYDROCARBON POTENTIAL OF MOZAMBIQUE 1 ] 3

of extension, show events of high accumulation, rapid subsidence, and quickest extension nearly coincident at 96 to 76 Ma, though this observation is dependent on the interpretational assumptions. The question now arises as to the tectonic implications of such an event.

The slow extension rates from 195 to 96 Ma are surprizing when studying the cross- sections, because faults were definitely active at that time. The occurrence of flower structures along the rift, shown here in Figure 15 and by DeBuyl and Flores (1986) in their figure 15; the resemblance the "V"-shaped graben has to shear zone (Tchalenko, 1970) and wrench fault basin settings (Harding, 1974); and the geologic data presented by Martin and Hartnady (1986) are suggestive of early strike-slip faulting manifestation of the N - S separation of the African and Antarctic plates (East Gondwana) (Simpson et al., 1979; Martin and Hartnady, 1986). Strain was therefore out of the plane of the E - W cross- sections drawn here.

During Late Cretaceous times the rift was extending more actively. The blocks were tilted in a systematic "domino" fashion on faults which often appear to sole out in the Domo shales just above basement rooted fault blocks. This event broadly coincides with the onset of rifting between Madagascar and Antarctic (Segoufin and Patriat, 1980) following the Barremian docking of Madagascar; the initial separation of Antarctica and Australia (Veevers, 1986); and the postulated change in poles of rotation of South America relative to Africa (Rabinowitz and LaBreque, 1979; Martin et al., 1982). It is thus apparent that a major plate reorganization occurred in the region at this time. Mougenot et al. (1986a, b) working independently further north, also concluded that the region underwent an initial period of strike slip faulting followed by a Late Cretaceous phase of extension and volcanism. Such a two-phase tectonic history was first proposed by Flores (1970).

The fastest extension rate measured in the basin, some 50 m per million years (5 cm per

1000 years) is slow when compared to spreading rates of between 1 and 17 cm per year for present-day opening oceans. This basin therefore does not appear to have been very active, and an extensional model to describe the heat flow and subsidence history must be applied with some caution.

Thermal history

Having assessed the structural and burial histories of the graben from the seismic line, some approximation to the thermal history of the basin must be made in order to apply hydrocarbon generation models in the area to complete the basin analysis. In regions with data that include downhole measurements of thermal indicators, it is possible to invert the information to obtain estimates of paleo-heat flux (Lerche et al., 1984). In lieu of any thermal information, [the nearest heat flow values of 1.2 and 1.29 HFU come from the Mozambique Channel (Anderson et al., 1977)] the heat flow history of the South Mozam- bique graben is estimated from the extensional models of McKenzie (1978) and Royden et al. (1980).

The geologic observations of several dikes or sills in the region, suggesting igneous activity, favors the use of the Royden dike intru- sion model. The McKenzie model does not involve any intrusive effects and is therefore probably not appropriate. However, in order to bracket the dynamic range of possible thermal histories both the Royden and McKenzie models were used.

Another constraint to consider in refining an extension model, is the heat flow history derived from depth to detachment calculations. If the extensional detachment is assumed to represent the bri t t le /duct i le mid- crustal phase boundary of greenschist to amphibolite metamorphic facies, then measured fluctuations in the depth to detachment can be used to calculate heat flow history using the equation:

Q/K = Gtz

114 A.E .M. N A I R N ET A L

where Q = heat flux at surface, K = thermal conductivity, Gtz = geothermal gradient. The thermal conductivity was assumed to be 5m cal cm -1 C -] s -1, and the temperature at which the phase change occurs to lie between 400 and 500 o C. Knowing the depth to this temperature (i.e. to the detachment) heat flow is easily calculated. The variation of paleoheat flux essentially matches the shape of the depth to detachment e.g. the shallowest depth to detachment of 7 km yields the highest heat flow value of between 2.8 and 3.6 heat flux units (HFU) (Fig. 18). These calculations also imply a present day heat flow value of around 1.6 to 2.1 HFU for this region, which is higher than observed in the Mozambique Channel (Anderson et al., 1977).

The extensional models (McKenzie, 1978; Royden et al., 1980) essentially predict the heatflow at a particular time after rifting of a basin, given certain parameters of that basin. Both models relate basement subsidence (tectonic subsidence) to the amount of extension and the heat flow. It is therefore possible to take the basement subsidence curves, which are output from the one-dimensional burial history computer model (Fig. 19a), for each of the pseudowells, and find the best-fit predicted basement subsidence curve of the model which corresponds to a specific extension factor (Fig. 19b). Inspection of the basement subsidence curves revealed two phases of subsidence which correspond to the previ- ously discussed extensional phases.

In practice, the fitting of the observed to predicted basement subsidence curves has a degree of subjectivity. In order to provide some idea of minimum and maximum error (Fig. 19b) the fit was bracketed. For each well, and for each of the two models, there was an upper and lower limit of the possible extension factors. Each of these upper and lower extension factors was then used in the paleoheat flux equation of the model under consideration.

Of the two subsidence/extension events (Fig. 19a) shown by the basement subsidence histories, only the second has any direct sup-

C3

3

(,9

P H A S E If

195 140 109 96 75 55 55

T I M E ( M y a )

a

22.5 0

- .6

H A S E I

,~ ~LI I I I ~ I I I ,~"~'~.,. i P H A S E

195 140 109 96 76 65 55 22.5 0

T IME ( M y a )

b

Fig. 19. a. An observed basement subsidence curve from well 6 shows two subsidence events, b. Method of determining the range of extension factors from inspection of best fit of the observed curves to those predicted by the extensional models.

porting field evidence. Yet, in order to accommodate all possible variations, three general heat flow histories were considered (Fig. 20): (1) a single thermal heat pulse event at the original rifting (195 Ma); (2) a constant heat flux of 1 HFU until 96 Ma at which time there was instantaneous rifting and associated thermal event; (3) an initial thermal event at 195 Ma followed by a later 96 Ma thermal event (instantaneous rifting). Such episodic rifting and subsidence has also been reported from the South China Sea by Ru and Pigott (1986).

Armed with these three possible cases and a range of extension factors for each of the events, a generation model (Tissot, 1969; Tis- sot and Espitali& 1975; Tisssot and Welte, 1978) was applied to each well, for each case. The heat flow history curve, in the form of the equation;

O( t ) = Qo exp(flt)


3-

Q

2-

(HFU)

I

3

Q

2

(HFU)

i

3 -

Q

2-

( H F U )

1-

C A S E 1

95

Old Q

96 0 T I M E ( M y a )

a

95 96 0

b

195 9~6 0

C

Fig. 20. Three case histories: (a) an initial rifting and thermal cooling at 195 Ma; (b) a single pulse of rifting and heating at 96 Ma; (c) two rifting and thermal events at 195 Ma and at 96 Ma.

where fl is a variable which determines the slope and curvature of the fine, and Q0 is the present day heat flux, was input into a 1-D fluid flow computer model (Cao, 1985). The extension factors taken from the extensional models were used to formulate a fl for the heat flow behavior with time in the model.

For the first two cases this technique posed no problems, as long as the present-day heat flow was known or assumed. A conservative estimate of 1.0 H F U was used for the present-day heat flux, somewhat lower than the nearest measurements in the Mozambique Channel (Anderson et al., 1977).

In the third case of two thermal events, it was necessary to calculate what the maximum heat flux would be at the inception of the second rifting, given the fl-value taken from the extension factors predicted from the ob-

served basement subsidence curves for the first event. The maximum heat flow of the second event can be pinned by backtracking the equation from the present day to 96 Ma with a known fl as shown below:

QR2=Qoexp[fl(Tpd-- TR2)] where QR2 = maximum heat flow at the second rift-

ing event, Q0 = present-day heat flow (1.0 HFU), Tpd = time since initial rifting till present

day, TR2 = time since initial rifting till second

rifting event. By applying the generation model to all the

strata we can ascertain which layers lie within the oil window. Those layers may then be studied more closely for their actual source potential. The generation model predicts how many milligrams of oil are produced per gram of kerogen for each layer at each well site. This value was then plotted in the center of the formation for each well and contoured to give maturat ion potential cross-sections (see Figs. 21 to 22).

Such maturat ion potential sections were generated for each of the three cases of heat flow, and for the upper and lower limits of each extension factor for both the McKenzie and Royden models of extension. In this way all the possible variations are taken into account and some degree of confidence may be placed on the results.

If there was only one rifting event at 195 Ma Royden's model predicts that the basin is extremely mature (Fig. 21a and b) (and hence oil prone if a source rock exists!). On the other hand if there was only one thermal event of minimal magnitude at 96 Ma Mc- Kenzie's model suggests that most of the basin is undermature, although still with some potential in the east near wells 4 and 5 (Fig. 22c and d).

The results of the two rifting event case suggests that the later smaller heating event at 96 Ma has more effect on maturation of the

sediment than the greater magnitude, but earlier, rifting event (see Fig. 23a-d). During the initial thermal (volcanic) event the sedi-

ments had either not yet been deposited or were still at a relatively cool temperature near the surface. During the later event the same

3 0 0 DEPTH

OOO ( m e t e r s ) OOO

1 2 1 1 1 0 9 8 7 6 5 4 3 2

116 A.E.M. NAIRN ET AL.

II@t I f f l | 01|/

m Wet O g e ~

NO V E R T I C A L E X A G G E R A T I O N

b / l"

1 2 1 1 1 0 9 8 7 6 5 4 3 2 |

M,OClENE j PLIOC~NE-RE~ENT ' I - - - - " 1 - EOC.ENE I 000

| I ~ - - ' - - - - - - ~ " " ~ C A M P A N I A N ~ DEPTH

I ~ - . . . . . UPPER ~ " / " " 7 " - - ~ " , , I . ~ " - ~ ~ 2 0 0 0

- I ~11~.U1LOIlIIL'-- w " ' ; . . . . - - - ~'~.UIIIliIlt)JIJJlT~H444JJJJ.U C R E T A C E O U S , l l l lPl l l l~l l ' l l l l~JHIl~f! l~~-~':. ' .".~ • . ~ ~ ' ~ ' U I U i I I T T ~ , ~ . - ~ u ~ H r , . P ' " ' ~ , J , ~ ' " , . , ~ ~ ~ . ~ . ,~\~.~cj~LU~?~I ,. ' 3 o o o

\ " " " ~ ' ~ " ' : . . . "~v.,~ ,,~, . :c: /,J ~ • , 7 - rf-_-'.'-,i-~.~ ~ . ; f x\'~ ~,,,io . . . . ~,/ \ I - / / 7 - " - - i ~ :'~ I X . ~ t s s i ¢ / _ -/=-~! "4 , ~ , . , . . . , o l . .

-- " "" -,. = D 2km

NO V E R T I C A L E X A G G E R A T I O N

JJltlIIIII rlIINII1111 IFfr]]]

//" d

Fig. 21. a. Royden model lower limit case l. b. Royden model upper limit case~ 1. c. McKenzie model lower limit case 1. d. Mckenzie model upper limit case 1.

G E O L O G Y , BASIN A N A L Y S I S A N D H Y D R O C A R B O N P O T E N T I A L OF M O Z A M B I Q U E ] 17

sediments were buried to a greater depth and so experienced higher temperatures.

The timing of generation shown for each

case using well 5 (Fig. 24 also suggests that the second thermal event is more important for maturation and hydrocarbon generation

12 11 10 9 8 7 6 5 4 3 2 1 ~ t I t i ,,,OCiENE i / I,'~,OC~NE-RE~EN' [

i I I I . I ~ I ~ EOCENE ]- 1000 [ I CAMPAINIAN I I - - ~ " - " ~ - . ~ DEPTH

7 " ~ ~ ~' I i 2000 ~ R , 7111 17i,'--71 i " ~ ~ (meters)

~ ~ , ; . o ~ . u ~ l l Ii ~ ~ ~ ~ ~ ~ 3000 - - - . , . ~ " . . ~ , i " ~ l l l i i ~ i l i ' ~ i 0 1 1 1

/ " / / / / a /KA"°~"!l'r:>°° NO VERTICAL EXAGGERATION

12 11 10 9 8 7 6 5 4 3 2 1

~ ~ 1000 - - - - - - - - - L ~ I N ' A N ~-- I ~ _ . . _ . ~ OE~T.

- ' I I - - - - - - - ~ ~ i _ ~ _ / ~ ' , , , - o ~ ' , , . ~ ~ 2ooo [I ooo

• ~ ~ l l l l l , ~ , , • " - i . . . . . - so ~ 25

" ~ 7 ~ , , ~ f - ~ / ~ ° ....... o'" c

NO VERTICAL EXAGGERATION

1 ' i , ! :

l • |

=-- __~ ~'~5 , , , ~ L . - - z _ " 5 0 ...... ~ ,,~

\ , / /'; . ":t d

Fig. 22. a. Royden model lower limit case 2. b. Royden model upper limit case 2. c. McKenzie model lower limit case

2. d. McKenzie model upper limit case 2.

] 1 8 A .E .M. N A 1 R N ET AL.

(Fig. 24b and c) of this basin because the later generation improves facilitation of sealing and trapping conditions. It is therefore significant that this event coincides with a

1 2 11 1 0 9 8 7 6

raising of the brittle/ductile boundary and documented lava flows in the region (DeBuyl and Flores, 1984).

The key is not how high the heat flow was

5 4 3 2 1

V E R T I C A L E X A G G E R A T I O N NO

1 0 0 0 DEPTH

2 0 0 0 ( m e t e r s )

3 0 0 0

25

ram Wero~n

12 11 10 9 8 7 6 5 4 3 2 1

~ ~ E O C E N E 1 0 0 0 DEPTH

2 0 0 0 L O W E R ( m e t e r s )

CRETACEOUS ~ I ~ '" ""~-""'-'"" 3000

/ / / / c /KAROO'/ I

NO VERTICAL EXAGGERATION

|

'i ~' 25

% .

"' | • P 5

/ d

Fig. 23. a. Royden model lower limit case 3. b. Royden model upper limit case 3. c. McKenz ie model lower limit case 3. d. McKenz ie model upper limit case 3.

at any time, but rather how hot the sediments became and how long they were hot. Thus the burial history is an important controlling factor in this basin.

Basin prognisis

Q

To evaluate a basin for hydrocarbon potential we must answer five questions.

(1) Does the basin possess potential trapping structures? In the case of Mozambique the answer is strongly affirmative. This block-faulted type of structural regime has

been exploited successfully for hydrocarbons in many areas such as the North Sea, Sirte basin, and Gulf of Suez (Harding, 1984).

(2) Does the basin possess reservoirs? Most likely the synrift seismic package 6 may provide reservoir quality sands and gravels in the form of fans at the sides of blocks. Kihle (1983) suggests reservoirs may be also present in the Maputo, Domo sands, Grudja sands and Cheringoma limestones.

(3) Are sealing conditions present? The Lower Cretaceous and certainly the Upper Cretaceous shales are potential seals. In most

4 -

H F U

A

3 -

2-


3

Q

2

HFU

B

C A S E II

195 150 100 5'0

TIME ( M y a )

a

r 195 150 100

TIME (Mya)

,3

i i 50 0

15-

10-

5 -

195 150

OIL GENERATION

(rn=lligrams oil/gram kerogen)

100 50

TIME ( M y a )

b

15

10

r a g / g i n

OIL GENERATION

(milligrams oil/gram kerogen)

o 195 150 0 r i

100 50

TIME (Mya)

b

Fig. 24. a. Oil generation rate of the Royden upper limit case 1 well 5 for unit 6. b. Oil generation rate of the Royden upper limit case 2 well 5 for unit 6. c. Oil generation rate of the Royden upper limit case 3 well 5 for unit 6.

1 2 0 A . E . M . N A I R N E T A L .

C

3

Q CASE III

HFU 2

~95 ~50 10O 5O 0 TIME (Mya)

a

15

!0 mg/gm

OIL GENERATION

(mdligram$ od/grsm kerogen}

i 195 150 0

Fig. 24 (continued).

- / A v / F / A I / / / / A r / ~

100 TIME (Mya)

b

50

cases the timing of generation from the lower- most potential sources is during and after the deposition of these upper shales units (Fig. 24b, c).

(4) Is there a source rock? This is a question which may only be answered definitively with a well. However, the fact that copious shales are present (of both terrigenous and marine provenance) enhances the possibility of a source quality organic shale being present. Flores (1973) refers to the lower Domo shales as being "euxinic" in nature, which would increase source potential.

(5) Has the source rock adequately ma- tured? Based on the limits of these analyses, which should more than adequately cover the

real situation, we predict that hydrocarbons have been produced from Lower Cretaceous or Jurassic source rocks.

The advantage of the technique described above is in the assimilation of all aspects of the basin to tie down the relative timing of structural, depositional and thermal events. It is important to determine if maturation of source rocks occurred before or after deposition of a sealing layer, or if traps were formed and sealed before maturation. The South Mozambique graben possesses potential source rocks most likely maturing after sealing rock (U. Cretaceous shales) and traps are in place (Fig. 24b, c).

The biggest problem encountered by the exploration geologist faced with a potentially oil-prone region such as the South Mozam- bique graben is potential recovery. The poros- ity at depths of between 3000 and 4000 m is not too promising unless overpressured conditions are experienced. Up-dip stratigraphic traps may be more prolific.

The questions which cannot be answered are those of hydrocarbon migration and accumulation, although we can show that generation occurred mostly around the time of deposition of the Upper Cretaceous shale and generally after faults had become inactive. Perhaps the migration potential could be assessed using a 2D fluid f low/compact ion model such as that of Nakayama (1987), but the lack of more refined data makes such an endeavor of less value at the present stage of understanding of the South Mozambique graben. If further work is undertaken, it would be better to determine source rock potential and procure a reliable present-day heat flow reading in the region.

C O N C L U S I O N S

The conclusions of this study are separated into three categories. First, the structural and thermal conclusions are presented, followed by maturation potential and, finally, predict- ions of hydrocarbon potential.


Tectonic and thermal implications

(1) The structural history of the South Mozambique graben shows two rifting events recognizable on the seismic reflection profiles. The first event is of strike/slip nature, possibly sympathetic to the emplacement of Madagascar. The second event is a straight extension manifested on the seismic section as a sequence of "domino" type fault blocks in the Cretaceous section.

This type of structural history has recently been reported from the area just to the north by Mougenot et al. (1986a, b).

(2) Subsidence history also indicates two periods of extension.

(3) Extension rates estimated from variation in strata lengths show an acceleration of extension at about 96 Ma.

(4) Area balancing and depth to detachment calculations for each sequential palins- pastic reconstruction also indicate the depth to detachment was shallowest at 96 Ma. This may well be a tectonic/thermal event, possibly related to a shift in stress regime caused by the opening of the South Atlantic at this time.

being present is enhanced by the high marine/terrigenous shale depositional setting envisaged for these strata.

Hydrocarbon potential

(1) Structural and stratigraphic traps are present due to the extensive faulting.

(2) Reservoirs may be present in the synrift sands or Cretaceous rocks.

(3) The Upper Cretaceous shales suffice as an effective seal.

(4) A source is unproven but likely, and most probably lies in or below the Lower Cretaceous unit. Stratigraphic (well) data would help considerably here.

(5) Regardless of the model used or heat flow history configuration, the basin is ex- pected to have mature hydrocarbons in the eastern part at depths of 3000 to 4000 m.

(6) The ideal situation of: relative timing of source deposition followed by faulting, then seal deposition followed by maturation of the source, seems to be the case in the South Mozambique graben.

ACKNOWLEDGEMENTS

Maturation potential

(1) Basement subsidence curves were cou- pled to extensional models of Royden and McKenzie to calculate the heat flow history of the region.

(2) Two subsidence events required the use of three different heat flow histories in order to accommodate all possible variations with upper and lower limits.

(3) Royden's model turns out to be more favorable from the maturation standpoint, and geologically more appropriate in view of the abundance of sills and dikes of Creta- ceous age reported in the region.

(4) A source rock which can reach matur- ity for hydrocarbons likely lies somewhere within the Lower Cretaceous or Jurassic strata. The probability of such a source rock

The geological portion of this paper was based upon a review of the geology of Mozambique which had been commissioned by the World Bank for its internal purposes. The views expressed, however, are those of the authors alone. Financial support was provided by members of the USC Basin Model- ing Group's Industrial Association. We would like to thank Western Geophysical Company of America for their generous help, and for providing the data for this study. We would also like to thank all the students of the modeling group for their positive ideas and logistical help with computing, and Ric Wil- liams for help with the computer area calculations. Many thanks go to Donna Black for typing the text and to Elaine Hadaddin, Ann Watkins, Yanqing Mo, and Mike Garbee for drafting.


REFERENCES

Anderson, R.N., Langseth, M. and Sclater, J.G., 1977. The mechanisms of heat transfer through the floor of the Indian Ocean. J. Geophys. Res., 82: 3391-3409.

Armstrong, A.C., 1985. Oil and gas developments in central and southern Africa. Bull. Am. Assoc. Pet. Geol., 69: 1680-1726.

Borges, A., 1952, Le systrme du Karroo au Mozam- bique, In: C. Teichert (Editor), Symposium sur les S~ries de Gondwana. Proc. 29th Int. Geol. Congr., Alger, pp. 232-250.

Cao, S., 1985. A quantitative dynamic model for Basin Analysis and its Application to the Northern North Sea Basin. Masters Thesis, University of South Carolina, U.S.A.

Cao, S., 1988. Sensitivity analysis of 1-D dynamical model for basin analysis. Ph.D. Dissertation, Univer- sity of South Carolina, U.S.A.

Coffin, M.F. and Rabinowitz, P.D., 1987. Reconstruc- tion of Madagascar and Africa: Evidence from the Davie Fracture Zone and Western Somali Basin. J. Geophys. Res., 92 (B9): 9385-9406.

Dahlstrom, C.D.A., 1969. Balanced cross sections. Can. J. Earth Sci., 6: 743-757.

Darracott, B., 1974. On the crustal structure and evolution of southeastern Africa and the adjacent Indian Ocean. Earth Planet. Sci. Lett., 24: 282-290.

DeBuyl, M. and Flores, G., 1986. The Southern Mozambique Basin: the most promising hydrocarbon province offshore East Africa: In: Michael T. Halbouty (Editor), Future Petroleum Provinces of the World. Am. Assoc. Pet. Geol. Mem. 40: 399-425.

Dingle, R.V. and Scrutton, R.A., 1974. Continental breakup and the development of post-Paleozoic sedimentary basins around southern Africa. Bull. Geol. Soc. Am., 85: 1467-1474.

Dixey, F., 1956. The East African Rift System. Bull. Overseas Geol. Min. Res., Suppl. 1, 71 pp.

Du Toit, A.L., 1937. Our Wandering Continents. Oliver and Boyd, Edinburgh, 366 pp.

Flores, G., 1964. On the age of the Lupata Rocks, lower Zambezi River, Mozambique. Trans. Geol. Soc. S. Afr., 67: 111-118.

Flores, G., 1970. Suggested origin of the Mozambique Channel. Trans. Geol. Soc. S. Afr., 73 (1): 1-16.

Flores, G., 1973. The Cretaceous and Tertiary sedimentary basins of Mozambique and Zululand: In: G. Glant (Editor), Sedimentary Basins of the African Coasts. pt. 2, South and East Coasts. Assoc. African Geological Survey, Paris, pp. 81-111.

F~)rster, R., 1975. The geological history of the sedimentary basin of southern Mozambique, and some aspects of the origin of the Mozambique Channel. Palaeogeogr. Palaeoclimatol. Palaeoecol., 17: 267- 287.

Frankel, J.J., 1972. Distribution of Tertiary sediments in Zululand and southern Mozambique, southeast Africa. Bull. Am. Assoc. Pet. Geol., 56: 2415-2425.

Gibbs, A.D., 1983. Balanced cross-sections construction from the seismic sections in areas of extensional tectonics. J. Struct. Geol. 5 (2): 153-160.

Guidish, T.M., Kendall, C.G.St.C., Lerche, I., Toth, D.J. and Yarzab, R.F., 1984. Basin evaluation using burial history calculations: an overview. Bull. Am. Assoc. Pet. Geol., 69 (1): 92-105.

Harding, T.P., 1974. Petroleum traps associated with wrench faults., Bull. Am. Assoc. Pet. Geol., 58: 1290-1304.

Harding, T.P., 1984. Graben hydrocarbon occurrences and structural style. Bull. Am. Assoc. Pet. Geol., 68 (3): 333-362.

Hazzard, J.C., Morris, A.E.L. and Wissler, S.G., 1971. Petroleum developments in central and southern Africa in 1970. Bull. Am. Assoc. Pet. Geol., 55: 1559-1602.

Heirtzler, J.R. and Burroughs, R.H., 1971. Madagascar's paleoposition: new data from the Mozambique Channel. Science, 174: 488-490.

Iliffe, J.E., Lerche, I. and DeBuyl, M., 1989. Basin analysis of the South Mozambique Graben. Am. Assoc. Pet. Geol. Bull.

Kamen-Kaye, M., 1978. Permian to Tertiary faunas and palaeogeography: Somalia, Kenya, Tanzania, Mozambique, Madagascar, South Africa. J. Pet. Geol., 1 (1): 79-101.

Kamen-Kaye, M., 1982. Mozambique-Madagascar geosyncline, I: Deposition and architecture. J. Pet. Geol., 5 (1): 3-30.

Kamen-Kaye, M., 1983. Mozambique-Madagascar geosyncline, II: Petroleum geology. J. Pet. Geol., 5; 287-308.

Kent, P.E., 1965. An evaporite basin in southern Tanzania. In: Salt Basins Around Africa. Inst. Pet- rol., London, pp. 41-54.

Kent, P.E., Hunt, J.A. and Johnstone, D.W., 1971. The geology and geophysics of coastal Tanzania. Inst. Geol. Sci. London, Geophys. Pap. 6, 101 pp.

Kihle, R., 1983. Recent surveys outline new potential for offshore Mozambique. Oil and Gas J., Feb. 28: 126-134.

King, N.W., 1957. Basic palaeogeography of Gondwa- naland during the late Palaeozoic and Mesozoic Eras. Quart. J. Geol. Soc. London, 114: 44-70.

Lafourcade, Ph., 1984. Etude grologique et grophysique de la marge continentale du sud Mozambique (17 °S at 28°S) Ph.D. dissertation, Laboratoire de Grodynamique Sous-Marine, Villefranche-sur-Mer, Universit6 Pierre et Marie Curie, Paris.

Lerche, I., Yarzab, R.F. and Kendall, C.G.St.C., 1984. Determination of paleoheat flux from vitrinite reflec- tance data. Bull. Am. Assoc. Pet. Geol., 68 (11): 1704-1717.


Little, A.D., 1984. Assistance in Petroleum Exploration Management Contract 175/82, 41 pp.

Lort, J.M., Limond, W.Q., Segoufin, J., Patriat, P., Delteil, J.R. and Damotte, B., 1979. New seismic data in the Mozambique Channel. Mar. Geophys. Res., 4: 71-89.

Martin, A.K. and Hartnady, C.J.H., 1986. Plate tectonic development of the south west Indian Ocean: A revised reconstruction of East Antarctica and Africa. J. Geophys. Res., 91 (B5): 4767-4786.

Martin, A.K., Goodland, S.W., Hartnady, C.J.H. and Du Plessis, A., 1982. Cretaceous palaeo positions of the Falkland Plateau relative to southern Africa using Mesozoic seafloor spreading anomalies. Geophys. J. R. Astron. Soc., 71: 567-579.

McKenzie, D., 1978. Some remarks on the development of sedimentary basins. Earth Planet. Sci. Lett., 40: 25-32.

Mougenot, D., Recq, M., Virlogeux, P. and Lepvrier, C., 1986. Seaward extension of the East African Rift. Nature, 321: 599-603.

Mougenot, D., Virlogeux, P., Vanney, J.-R. and Malod, J., 1986. La Marge continentale an Nord du Mozambique r~sultats preliminaires de la campagne md40/macamo. Bull. Soc. Geol. Fr., 8, II (3): 419- 422.

Nakayama, K., 1987. A dynamic, two dimensional, fluid flow model for basin analysis. Ph.D. dissertation, University of South Carolina.

Norton, I.O. and Sclater, J.G., 1979. A model for the evolution of the Indian Ocean and the break-up of Gondwanaland. J. Geophys. Res., 84: 6803-6830.

Rabinowitz, P.D. and LaBreque, J.L., 1979. The Meso- zoic South Atlantic Ocean and evolution of its continental margins. J. Geophys. Res., 84: 5973-6002.

Rabinowitz, P.D., Coffin, M.F. and Falvey, D., 1983. The separation of Madagascar and Africa. Science, 220: 67-69.

Royden, L., Sclater, J.G. and Von Herzen, R.P., 1980. Continental margin subsidence and heat flow: important parameters in formation of petroleum hydrocarbons. Bull. Am. Assoc. Pet. Geol., 64: 173-187.

Ru, K. and Pigott, J.D., 1986. Episodic rifting and subsidence in the South China Sea. Bull. Am. Assoc. Pet. Geol., 70: 1132-1151.

Salman, N.N., 1982. United report [part of "data package" with Flores 1970, 1973 papers]. SECH,

1982. Future Hydrocarbon Exploration and Produc- tion. Status Report, Secretario de Estado do Carvao e Hidrocarbonetos, 8 pp.

Sclater, J.G. and Christie, P.A.F., 1980. Continental stretching: an explanation of the post-mid-Creta- ceous subsidence of the central North Sea basin. J. Geophys. Res., 85: 3711-3739.

Segoufin, J., 1978. Anomalies m~sozoiques dans le bassin de Mozambique. C.R. Acad. Sci. Paris, S~r. D., 287: 109-112.

Segoufin, J. and Patriat, P., 1980. Existance d'anomalies M~sozoiques dans le bassin de Sanalie: Implications pour les relations Afrique-Antarctique-Madagascar. C. R. Hebdom. Seances Acad. Sci. Ser. B, 291: 85-88.

Segoufin, J. and Recq, M., 1980. La transition entre le canal de Mozambique et le bassin de Mozambique. Bull. Soc. Geol. Fr., 22: 469-479.

Simpson, E.S.W., Sclater, J.G., Parsons, B., Norton, I. and Meinke, L., 1979. Mesozoic magnetic lineations in the Mozambique Basin. Earth Planet. Sci. Lett., 43: 260-264.

Swift, A., Sawyer, D.S., Grow, J.A. and Klitgord, K.D., 1987. Subsidence, crustal structure and thermal evolution of Georges Bank Basin. Bull. Am. Assoc. Pet. Geol., 71 (6): 702 718.

Tchalenko, S, 1970. Similarities between shear zones of different magnitudes Geol. Soc. Am. Bull., 81: 1625- 1640.

Tissot, B., 1969. Premieres donn~es sur le m~canisme et la cin&ique de la formation du p&role dans les s6diments. Simulation d'un schema r6actionnel sur ordinateur. Revue Institut Francais Petrole, v. 24, 4, p. 470-501.

Tissot, B. and Espitali6, J., 1975. L'evolution thermique de la mati~re organique des s~diments: Applications d'une simulation math6matique. Rev. Inst. Fr. Pet- rol, 30:743 777.

Tissot, B. and Welte, D.M., 1978. Petroleum Formation and Occurrence. Springer Verlag, New York, N.Y., pp. 500-521.

Veevers, J.J., 1986. Breakup of Australia and Antarctica estimated as mid-Cretaceous (95_+ 5 Ma) from magnetic and seismic data at the continental margin. Earth Planet. Sci. Lett., 77: 91-99.

Western Geophysical, 1983. Non-exclusive geophysical survey, offshore Madagascar, 27 pp.

Geology, basin analysis, and hydrocarbon potential of Mozambique and the Mozambique Channel

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