I I I I I I I I I I I I I I I I I I I

Volcanism, Tectonism, and Hydrocarbon Potential of Parts of Al Han1j Area, SW Sirt Basin, Libya

M.T. B USREWIL 1, l.Y. MRIHEEL 2

, ANDY.A. AL FASATWI 2

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Abstract. Jabal al Haruj lava field is a Pliocene-Holocene .. geologic feature in the SW part of Sirt Basin. The major structural trends of this area are NW-SE and NE-SW. These two different trends have been recognized and are expressed by the alignment of shield volcanoes, volcanic cones and by the pattern of emplacement of the more recent lava flows. The NW-SE trending shield volcanoes and volcanic cones, located in the southern and northwestern parts of the area, and the phase 6 lava field appear to be related to the regional NW-SE tectonic trends of the Sirt Basin. Evidence of NE-SW trends are expressed by the alignment of the phase 5 lava field and the predominant direction of the lineaments. Volcanoes arranged in a circular pattern, present in the central area, probably form the central part of the Al Haruj uplift. The lava field and its associated volcanoes appear to be related to the intersection of the tensional fault system of Dur al Qussah and the prolongation of Hun graben. It is, thus, structurally controlled and could be related to reactivation of pre-existing pre- and post-Pan-African tectonic trends during Late Tertiary time. It is inferred

1 Department of Geology, Faculty of Science, University of Al-Fateh, P.O. Box 13258, Tripoli, Libya, G.S.P.L.A.J. 2 Exploration Department, Petroleum Research Centre, P.O. Box 643 1, Tripoli, Libya, G.S.P.L.A.J.

3 18 M.T. BUSREWIL, LY. MRIHEEL ANDY.A. AL FASATWI

that subsidence was continuous during the Late Cretaceous and Tertiary and reached a maximum during the Palaeocene and Eocene due to rejuvenation of older tectonic elements. The few wells drilled in the study area have not to date yielded any reserves of hydrocarbons. This could mainly be ascribed to either poor quality of source rocks or total thermal cracking due to volcanism which post-dated hydrocarbon generation in the Early-Middle Miocene. It is thought that the large reserves of C02 are associated with igneous activity and could have migrated from the upper mantle to their present positions.

INTRODUCTION

The volcanic complex of Jabal al Hariij lies in the southwestern part of the Sirt Basin and covers an area of over 40 000 km2• It extends as a broad north-south band from Zallah Oasis in the north to Waw al Kabir. Within the Al Hariij area a few isolated patches of Lower Tertiary carbonates occur in windows in the basalt cover (Goudarzi, 1970). The area of this study is the eastern part of Al Hariij lava field and includes the Al J:Iulayq high and Zallah and Abu Tumayam troughs (Fig. 1). The surface geology of parts of Jabal al Haruj al Aswad was dealt with by Klitzsch (1968), Busrewil and Wadsworth (1980), Woller (1984) and Busrewil and Swaisi (1988). Intensive exploration activities were lim­ited to the northern and NE parts of Al Hariij area where Zuetina Oil Company is now producing from Aswad; Zallah and Sabah oil fields (Fig. 1). The south-central part of the ~ea, based on geophysical studies, is con­sidered to be devoid of significant structures (Mockel et al., 1983). A few scattered wells were drilled in Conces­sion 85 and although no hydrocarbons were discovered, C02 gas was recovered from the Bahi Formation at a few wells. An aeromagnetic survey of the area was carried out by Pacific Aero Survey Company on be­half of the Industrial Research Centre, Tripoli, in 1979. The magnetic response of the underlying sediments and basement rocks was mostly obscured and unmappable due to the volcanic cover. The aim of this study is to investigate the subsidence history, the hydrocarbon po­tential and the relationship between volcanism and tec­tonism in the area. Subsidence rates and history curves were constructed using subsurface data, and structural analyses were made using remote sensing techniques and field observations. Two main phases of subsidence have occurred since the time of the breakup of the Sirt arch (Late Cretaceous and Early Tertiary phases) and they were separated by a tectonically quiescent phase during the Maastrichtian. The greatest subsidence rate occurred during Palaeocene and Eocene times.

Geological setting

The Sirt Embayment is a Mesozoic-Cenozoic in­tracratonic basin located at the northern margin of the

stable African craton (Fig. 2). The basin architecture is represented by a mosaic of northwesterly trending plat­forms and troughs. The geometry of these horsts and grabens is probably related to strike-slip fault zones di­verging into Sirt Basin from the Gulf of Guinea and dissi­pating their shear movements into the development of ex­tensional basins, oriented perpendicular to the fault zone during the Upper Cretaceous (Fairhead and Binks, 1991 ).

The stratigraphy of the study area is shown in Fig. 3 and the vertical and lateral distribution are also pre­sented (Figs 4, 5). The basement rocks have exerted a profound control over the stratigraphic and structural evolution of the study area and the Sirt Basin at large. During Precambrian and Cambrian times an extensive tract of the rigid cratonic block was intruded by granitic and gabbroic intrusions along zones of weakness. The intrusion of these magmatic sequences, during the post-Pan-African thermal episode of Kennedy (1964), caused upwelling of the overlying country rocks. Reju­venation phases in the emplacement of these plutonic rocks, as deduced from the range of Palaeozoic ages they display (Cahen et al. , 1984), reactivated deep faults and may have eroded the pre-granitic country rocks sufficiently deeply. The upward force might be sought at deeper levels prior to the emplacement of the plutonic rock at its present level. Within the study area shallow subsurface intrusives gradually dip eastwards from the surface outcrops of Precambrian granites just west of Al Hariij to a depth of about 2 km (Fig. 1). Surface outcrops of Cambra-Ordovician sedimen­tary rocks have been described from the SW periphery of Al Hariij area by Klitzsch (1966). Similar litholo­gies overlying unconformably the basement rocks in the low area of Sirt Basin were also described (Bon­nefous, 1972; Barr and Weegar, 1972) and considered to represent the Early Palaeozoic passive margin sag stage within Sirt Basin (Harding, 1984). When African Gondwana glaciers were melted during the Early Sil­urian, the sea level rose sharply and the Palaeo-Tethys Ocean transgressed North Africa depositing a thick se­quence of organic-rich shales (Berry and Boucot, 1973; Husseini, 1991). However, at the NW periphery of Al Hariij a thick sequence of Tanezzuft Formation sedi­ments (693 m) were penetrated at Al Fuqaha water well T 2D 0035076. Similarly, within Sirt Basin, Silurian

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VOLCAt"IISM, TECTONISM, AND HYDROCARBON POTENTIAL OF PARTS OF AL HAROJ AREA

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320 M.T. BUSREWIL, I.Y. MRIHEEL AND Y.A. AL FASATWI

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graptolitic shales were encountered in a few boreholes (Dl-32, Hl-87, El-114, Cl-83). Although Devonian and younger Palaeozoic sediments have been described from southern Libya (Pierobon, 1991), Al I:Iamadah al I:Iamra', (Bracaccia et al., 1991; El-Rweimi, 1991) and the Cyrenaica platform (El-Arnauti and Shelmani, 1988), they have not been reported in the subsurface of the study area or in the rest of Sirt Basin. The thin-

ning of the Silurian graptolitic shales and the absence? of subsequent Palaeozoic strata were not accompanied by any major sea-level changes and, therefore, it is attributed to local orogenic movements and regional up­lift, which in turn resulted in erosion of older sequences and depositional hiatuses. At this stage uplift and ero­sion eventually combined and brought the intrusive rocks to the surface.

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VOLCANISM, TECTONISM, AND HYDROCARBON POTENTIAL OF PARTS OF AL HAROJ AREA 32 1

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A global sea-level drop during the Triassic and Juras­sic periods (Vail et al., 1977) placed the Sirt upland in a terrestrial environment and resulted in poorly dated elastics (Rossi, et al., 1991 ; Eland, 1992). Initial rift­ing and faulting of the pre-existing Sirt arch due co tensional movements may have commenced during the Triassic and Lace Jurassic. Since the breakup of the Sirt arch two major episodes of subsidence are recognized. These are the Late Cretaceous and the Early Tertiary subsidence phases.

The sedimentary sequence of the Late Cretaceous, in the study area and in most parts of Sirt Basin, comprises Cenomanian-Maastrichtian transgressive sandstones of the Bahi Formation, the Cenomanian shallow platform carbonates of the Lidam Formation, the Rakb Group, and the Maastrichtian outer platform-open marine car­bonates of the Kalash Limestone (Barr and Weegar, 1972).

The encountered sedimentary section of the Palaeo­cene encompasses the thickest succession in most parts of the Sirt Basin. In the study area, the sequence com­prises Hagfa Shale, the Beda, Dahra and Khalifa for­mations, Zelten Limestone, and the Harash and Kheir formations (Fig. 3).

The Palaeocene section is succeeded conformably by the Eocene sequence which is comprised of three main formations. In ascending order these are the Gir For­mation, subdivided into two members, a lower dolomite and an upper evaporite (Barr and Weegar, 1972); the Gialo Limestone; and the Augila Formation. The lower dolomite member of the Gir Formation is known as the Facha Dolomite and the upper is named as Hon Evaporite. The first represents the principal reservoir rocks in the northern parts of the study area while the latter forms the main seal. The post-Eocene sections are represented by the Najah Group and the Al Haruj lava fi eld (Fig. 3).

Extensive volcanic activity in the very recent geo­logic past is evidenced by the abundance of erosionally unmodified spatter and cinder cones and the unweath­ered state of the surface features on lava flows . Older, more weathered volcanic rocks and their state of ero­sion, together with isolated radiometric dates (Ade-Hall et al., 1974; Vesely, 1985), suggest that the eruptions started 6-8 million years ago and have continued inter­mittently up to historic or pre-historic times.

Structure and subsidence history

Lineament analysis was carried out using Aerial Photograph (AP) mosaics (scale 1 : 250 000) and, for the southern part of the area Landsat TM images. Length

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VOLCANISM, TECTONISM, AND HYDROCARBOK POTENTIAL OF PARTS OF AL HAROJ AREA 323

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and frequency histograms were prepared and a class interval of 5° azimuth was selected. The major structural trends of Al Haruj lava field, based on a rose diagram of lineaments (Fig. 6) and as expressed by the alignment of spatter cones and the areal distribution of more recent lava flows, are NW-SE and NE- SW. The NW tectonic trend can actually be observed and ranges from 135° to 155° (140° dominant). It is indicated by the occurrence of NNW trending fractures and linear zones of volcanic cones and more generally by the areal distribution of points of outpouring of eruptive sites or vents. These faults were important in controlling development of the adjoining Zallah trough and may also have influenced subsequent oil migration and entrapment in Zallah and Aswad oil fields (Fig. 1). The NE-SW tectonic and eruptive trends range from 45° to 65°. It is expressed by the NE alignment of phase 5 lava field (Fig. 1) which in tum lies parallel to the NE systems of fractures in Libya (K.litzsch, 1966, 1968).

Judging from the direction of flow of the more recent lavas and from cone alignment, it is suggested

that the setting of the Al Haruj lava field is related to tensional lines along the Dur al Qu~~ah fault system and the prolongation of the Hun graben. The rising magma is believed to have erupted along planes of the lowermost resistance areas at the cross point of these tensional lines around Qur as Sab' ah area. Drainage divides, which display sharp changes in direction away from Qur as Sab' ah, are considered to be indicative of the general distensive character of the region. After the activity waned on Al Haruj, the area was cut by a conspicuous rejuvenated NW-SE set of faults . The rejuvenation phases of these faults clearly post-date the last phase of igneous activity as they cross recent lava fl ows (Al Mashaqaq).

A regional Bouguer gravity map clearly exhibits the main structures of the area. A good correlation exists between the graYity expression of the fault zone and the lineaments interpreted from remote sensing images (Fig. 6). This coincidence confirms the presence of large-scale basement faults and suggests that parts of these faults are possibly still active.

324 M.T. BUSRE\VlL, LY. MRIHEEL ANDY.A. AL FASATWI

Fig. 6. Rose diagram of lineaments showing the major structural trends in Al Han1j lava field.

Subsidence history and rate curves (Figs 7, 8) were calculated for selected wells to evaluate the subsidence history of the Al Haruj area. They were constructed us­ing subsurface stratigraphic data obtained from the bore­holes and the time-scale is based on available biostrati­graphic data. The curves over the whole area are remark­ably consistent (Fig. 8) and show that subsidence was slow during approximately the first 10 m.y. (Cenomanian through Coniacian). A sudden increase of subsidence and rapid sedimentation, concomitant with the deposi­tion of the Santonian and Campanian shales was abruptly terminated during the Maastrichtian as indicated by the flattening of the curve. During Palaeocene and Eocene times subsidence accelerated but was again terminated during the Oligocene and Miocene. Correlation of the subsidence rate curves (Fig. 7) and subsidence curves show some consistency in the general pattern of subsi­dence. It is therefore suggested that the major tectonic evolution of Al Haruj area occurred during the Early Tertiary and not during the early-Late Cretaceous as was suggested by Gumati and Kanes (1985).

In summary, since the breakup of the Sirt arch, two main phases of subsidence were identified. These are the Late Cretaceous phase and the Early Tertiary phase. These two subsidence phases are separated by a quiescent phase during the Maastrichtian. The great­est subsidence rate since the breakup of the Sirt arch occurred during Palaeocene and Eocene times.

Hydrocarbon potential of Al Hara} area

Reservoirs Widely distributed Late Cretaceous reservoirs (Bahi

Sandstone, Lidarn Formation, Kalash Limestone) and Early Tertiary reservoirs (Beda Formation, Zelten Lime­stone, and Facha Dolomite Member of the Gir Forma­tion) are present in most parts of Al Hariij area (Figs 4, 5). These reservoirs are well-known producing horizons in many oil and gas fields in Sirt Basin. Of particular interest in Al Haruj area is the Facha Dolomite Mem­ber and the Beda Formation which form the target for hydrocarbon exploration in the study area and make

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I VOLCANISM, TECTOl\ISM, Al\D HYDROCARBON POTENTIAL OF PARTS OF AL HAROJ AREA 325

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Fig. 7. Subsidence rate as function of time (m/m.y.), wi thout correction for compaction.

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326 M.T. BUSREWIL, LY. MRIHEEL ANDY.A. AL FASATWI

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up the principal reservoir in the northern parts of Al Hariij area (Zallah, Aswad and ~abah oil fields). It is important to note that the Facha Dolomite Member, as well as the above mentioned reservoirs, occur under­neath the Al Hariij lava field . Accordingly, it is not a problem of the availability of reservoir rocks, as the penetrated rocks are hospitable for oil , but rather the poor source rocks, and the fact that volcanic activity may have played a vital role in the absence of oil ac­cumulation in the region. Folded and faulted units of Neogene and Palaeogene provenance were observed in remote sensing images in the SE part of Al Hariij area and these could be considered prospective structures for hydrocarbon exploration.

Source rock maturation and migration

In the northern parts of Al Hariij area the Late Cretaceous marine shales of Sirte Shale and Rachmat Formation are the best source rocks yet identified. The shale beds of the Etel Formation are most likely the second most important source rocks of Facha oil. Most hydrocarbon production occurs above the source rocks ·

in the Early Palaeocene Early Eocene carbonates in faulted anticline traps. The underlying Etel , Rachmat and Sirte shales are oil-prone source rocks lying within the oil window with respect to kerogen maturity (Time Temperature Index (TTI) of 30, 31 , 32 respectively). The complex structural pattern of platforms and troughs of most parts of the Sirt Basin is comprised of a mosaic of closely spaced horsts and grabens so the migration path of the generated hydrocarbon would be short. In the northern part of the Al Hariij area, migration into porous Beda carbonates and Facha facies progressed out of the basin from east to west some time during the Miocene.

Measured maturities (spore and vitrinite reflectance)

In order to define maturation levels and estimate the palaeotemperature of the Al Hariij area, spore coloura­tion and vitrinite reflectance values were obtained from A58-NC74B well. The measured values range from 0.33 to 2 % of vitrinite reflectance and from l+ to 2 to 2+ of thermal alteration index. The observed or­ganic maturities data indicate that the Tertiary sequence

I

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VOLCANISM, TECTONISM, AND HYDROCARBON POTENTIAL OF PARTS OF AL HAROJ AREA 327

and the Maastrichtian beds of the Kalash Limestone are immature (0.33- 0.48% R0) and (l+ to 2- TAI), while the Campanian to Cenomanian sequences are in an early stage of maturity (0.53- 0.7% Ro and 2-, 2 to 2+ TAI). The observed organic maturities data (Ro and TAI) for well A58-NC74B indicate that the Tertiary and Maastrichtian sequences are immature, while the Campanian-Cenomanian section is in the early stage of maturity. The gradual increase in maturities between Late Cretaceous and Tertiary sedimentary organics may indicate that there is no break in thermal maturity his­tory; this would imply that neither unconformities nor higher heat-flow levels have occurred in the past in the northern parts of Al Hariij area.

Thermal maturation history using lopatin method

In addition to the available published observed mat­uration values of Vitrinite Reflectance (Ro) and Ther­mal Alteration Index (TAI), maturation of source rocks was investigated by the Lopatin method. The numeri­cal analysis method of Lopatin (1971), as modified by Waples (1980), was used to estimate the present-day level of maturation of sedimentary sequences and to calculate the timing of hydrocarbon generation in the Al Hariij area. Lopatin modelling was applied to the postulated burial history of a deep well (A58 NC74B) in the northern parts of Al Hariij area (Zallah Oil Field) using a geothermal gradient of 2.6°C/100 m and assum­ing that the surface temperature was 20°C (Fig. 9). The computed organic maturity (TTI) values which were obtained give an estimated timing of hydrocarbon gen­eration in the Early-Middle Miocene. This signals the oil-bearing potential of the pre-Miocene traps in the Al Hariij area. The principal source rock (Sirte Shale) may have entered the major oil generation stage at a depth of about 10 400' during the Middle Miocene and at present it entered the major oil generation at a depth of 9980' . With the exception of the Maastrichtian Kalash Limestone which shows a maturity value of7.0 TTI, the calculated organic maturity from Lopatin-Waples TTI plots in the northern parts of Al Hariij area indicate that the Late Cretaceous sequence is in a liquid-hydrocarbon state of maturity (30 to 60 TTI) and that the Tertiary section is immature.

Observed and predicted maturity

The organic maturities for well A58-NC74B (Fig. 9), predicted by using a geothermal gradient of 2.6°C/ 100 m, show a reasonable correlation with the observed maturities (Thermal Alteration Index and Vitrinite Re-

flectance) . These results suggest that a thermal model which uses a gradient of 2.6°C/100 m accurately pre­dicts the organic maturi ty of the encountered sedimen­tary sequence in well A58-NC74B. It is concluded that the Lopatin model yields a suitably accurate prediction of the organic maturities of the Campanian-Cenomanian section and that for this well the present-day geother­mal gradient is similar to the palaeogeothermal gradient during the Late Cretaceous. The modelling of the north­ern Al Hariij area reveals that the depth to the top of the "oil window" in the area was at about 10 400' dur­ing the Middle Miocene (Fig. 9). The predicted present position of the top of the "oil window" is at a depth of 9980' . Lopatin modelling of the thermal and burial histories in the area indicates that source rocks were mature as early as the Campanian.

In conclusion, the measured organic maturities for A58-NC74B well in the Sirt Basin indicate that the Cenomanian-Campanian sequence is in the liq­uid-hydrocarbon state of maturity (0.53-0.7% R0 ) and 2-, 2 to 2+ TAI) and that the Tertiary-Maastrichtian section is immature (0.33-9.48% Ro and 1 + to 2- TAI). The present position of the oil window is estimated at 9900' using observed organic maturity values of Ther­mal Alteration Index and Vitrinite Reflectance, while it is estimated at a depth of 9880' using the Time Tem­perature Index. It is thus concluded that the present-day geothermal gradient has remained unchanged since the Late Cretaceous and the applied model fits much of the northern part of Al Hariij area. This work will undoubtedly help in future exploration.

Seal The Lower Eocene Hon Evaporite of the Gir For­

mation is the cap rock for the Facha reservoir. The producing Facha reservoir is interbedded upwards with evaporite layers which are considered to be ineffective seals. The Hon Evaporite has a thickness of 1614 ft in the type section (Y 1-59) well and reaches a maximum thickness of about 3500 ft in western concession 11 (Barr and Weegar, 1972). The Hon Evaporite Member consists of a sequence of interbedded anhydrite and dolomite with minor shale.

Source of C02 The C02 distribution in Al Hariij area and adjacent

parts suggests a preferential distribution pattern near regional faults . C02 was recovered from the Bahi For­mation on Al l:fulayq high at Cl-85, Fl-85 and Hl-78 wells . The presence of C02 near the boundaries of this uplifted area within, or at very close proximity to, the Al Haruj main lava fie ld may suggest that C0 2 is related to

328 M.T. BUSREWIL, I.Y. MRlHEEL AND Y.A. AL FASATWI

Age (My)

100 90 80 70 60 50 40 30 20 10 0 0

30

2000

50 400 0

6000 Q) Q)

u 70 IJ..

0

ci. E

Well Q)

I-

.c a.

8000 Q)

0

90 A 58 NC 74 B

Z all a h Field 10,000

110

12,000

Fig. 9. Burial history of the southwestern Sirt Basin (A58-NC74B well), Zallah Oil Field.

local igenous acivity. C02 rising from a magma body at depth along deep-seated faults is thus believed to have cooled and condensed, or dissolved and been entrapped in the thick water-saturated zone of the Bahi Formation. A similar mechanism was proposed by Maisonneuve (1991) for the origin of C02 in C02-rich hydrothermal water in the cratonic plates of Africa (e.g. in Camer­oun, Chad, Algeria (Hoggar), Niger (Air) and Egypt). Thermal degradation of carbonate rocks is possible but unlikely. However, isotopic work is needed for a more definitive conclusion.

SUMMARY AND CONCLUSIONS

Jabal al Hariij lava field is a Pliocene-Holocene geologic feature in the SW part of Sirt Basin. The major structural trends of this area are NW-SE and NE-SW. The lava field and associated volcanoes appear related to the intersection of the tensional fault system of Diir al Qu~~ah and the prolongation of Hun graben. The rising magma is believed to have erupted along planes of the lowermost resistance areas at the cross-point of these tensional lines around Qiir as Sab' ah area.

Subsidence was continuous during the Late Creta­ceous and Tertiary with rates reaching maxima during

the Palaeocene and Eocene due to rejuvenation of older tectonic elements. Following the breakup of the Sirt arch two main phases of subsidence are identified. These are the Late Cretaceous phase and the Early Tertiary phase. These two phases are separated by a non-active phase during the Maastrichtian. The great­est subsidence rate occurred during the Palaeocene and Eocene.

The observed organic maturity data indicate that the Tertiary and Maastrichtian sequence are immature, while the Campanian-Cenomanian section is in the early stage of maturity. The gradual increase in maturity between Late Cretaceous and Tertiary sedimentary or­ganics may indicate that there is no break in the thermal maturation history which implies that neither unconfor­mities nor higher heat-flow levels have occurred in the past in the northern parts of Al Hariij area.

The most obvious hydrocarbon emplacement mech­anism is vertical migration along reactivated trough faults. Since the structure of the basin is a mosaic of closely spaced horsts and grabens a short migration path is suggested for the generated hydrocarbons.

The unsuccessful attempts to find important hydro­carbon reserves underneath the Al Hariij lava field area to date could mainly be ascribed to either the poor

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VOLCANISM, TECTONISM, AND HYDROCARBON POTENTIAL OF PARTS OF AL HARUJ AREA 329

quality of source rocks or to total thermal cracking due to volcanism which post-dated their generation in the Early-Middle Miocene. The origin of large reserves of C0 2 is thought to be associated with the igneous ac­tivity and the gas may have migrated from the upper mantle to its present habitat.

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