Journal of Petroleum Geology, vol. 18(4), October 1995, pp. 439-452 439

ORIGIN OF THE MID-YPRESIAN JIRANI DOLOMITE - A MAJOR RESERVOIR ROCK IN

THE NW LIBYA OFFSHORE

I.Y. Mriheel" and J.M. Anketell**

The Jirani Dolomite was formed during a major relative fall in sea-level in middle Ypresian (early Eocene) times, in shallow, hypersaline lagoons on a restricted shallow platform, where supratidal and intertidal and semi-arid to arid conditionspredominated. Localisation of the shallow lagoonal facies to the region of the Jifarah Trough suggests that restriction was caused by downwarp relative to mild uplift of the Jifarah Platform to the north.

Two main facies are recognised: an anhydritic dolomite facies, composed of dolomite and dolomitic limestone with anhydrite nodules; and a non-anhydritic dolomite facies, composed of dolomite and dolomitic limestone only. Lithologic andpetrographic evidence leads to the conclusion that anhydrite was also present in the non-anhydritic dolomite facies initially, but was removed by dissolution processes. Petrographic studies show that dolomitisation proceeded in three stages. Stage I involved penecontemporaneous/ early diagenetic dolomitisation of the precursor limestones with gypsudanhydrite under hypersaline seepage rejlux conditions. Stage II dolomitisation, mainly confined to the non-anhydritic dolomite facies, wasprobably formed in the mixing zone between meteoric and sea water, probably at shallow depths of burial. Stage III dolomitisation occurred at depth in the late stage of basin evolution, causing some infilling of mouldic and vuggy porosity by coarsely crystalline, saddle dolomite. Evidence of mixing zone dolomitisation indicates that the Jirani Dolomite was exposed subaerially. Studies of the Metlaoui Group show that this event can be related to a major relative fall in sea level in early Lutetian times following deposition of the Jdeir Formation. Exposure of the deposits to flushing by meteoric waters explains the dissolution of the anhydrite from the non- anhydritic dolomite facies, and the accentuation of the well-developed porosity in this facies, which make it one of the most important reservoir rocks in the ofSshore region.

* Petroleum Research Centre, PO Box 6431, Tripoli, Libya. ** Dept. of Geology, University of Manchester, Oxford Road, Manchester M13 9PL.

440 Origin of the Jirani Dolomite. offshore Libya

INTRODUCTION

The Jirani Dolomite (Sbeta, 1981, 1984) occurs in the NW Libyan offshore region (Fig. 1). and constitutes the second-most important target for hydrocarbon exploration in the region. Oil and gas have been discovered at the Bouri oilfield associated with the Jirani Dolomite and the overlying nummulitic facies of the Jdeir Formation, which constitutes the main hydrocarbon reservoir so far located (Bernasconi et aZ,, 1991).

This paper discusses the origin of the Jirani Dolomite in relation to current ideas on mechanisms of dolomitisation. Data from fifty lithological, electrical and radioactive well logs were used to construct facies and isopach maps. Thin-sections from cores of wells El-NC35A, B2 and El-NC41 were also studied.

Structural Setting The NW Libyan offshore encompasses the eastern part of the WNW-ESE trending

Gabes-Tripoli-Misratah Basin, which occupies the southern part of the Pelagian Platform, lying between the Malta-Lampedusa block to the north and the Libyan onshore to the south (Fig. 2). The basin was probably initiated as a dextral strike-slip pull-apart structure in the Triassic by reactivation of major Hercynian basement fault systems, the South Graben Fault Zone in the north and the Coastal Fault System in the south. Subsequent movements on the faults were predominantly dextral strike-slip. These movements caused development of sub-platforms and troughs within the basin. Following major reactivation in the Late Cretaceous, the NE part of the basin was uplifted to form the Jifarah Platform separated from the main platform to the north by a narrow graben (Fig. 2). The bounding faults are oriented NW-SE and parallel to the Sirt trend. To the south of the platform. the WNW-ESE trending Jifarah (Jeffara) Trough was bounded to the south by the Coastal Fault System and to the west by a zone typified by salt-dome uplift.

Stratigraphy A stratigraphic nomenclature for the Lower Eocene in the NW Libyan offshore was

proposed by Sbeta (1981, 1984, 1990) in an attempt to resolve confusion arising from different usages by the various prospecting companies and correlation with onshore counterparts in Tunisia which “is not always possible or correct” (Sbeta, 1990). He introduced the term Farwah Group and its constituent formations (Fig. 3 ) for sediments equivalent in age to the Metlaoui Group of Tunisia (FourniC, 1978) and the various units were formally proposed and defined by Hammuda et crl. (1 985). The gross lithological characteristics do not appear to justify the change in name at group level; however, local lithological variation does support changes at formational level.

The Jirani Dolomite is comprised predominantly of dolomite and dolomitic limestone, and occupies most of the central portion of the Libyan offshore. It is of middle Ypresian (P8) age and forms the middle unit of the Metlaoui Group (Fig. 3 ) . The type section at well B2-NC41 is 124-ft thick, with upper and lower limits defined where limestone becomes dominant. Throughout the greater part of the region, it is conformably overlain by the Jdeir Formation and is conformably underlain by the Bilal Formation, into which i t also grades laterally towards the north (Fig. 3 ) . In the north-central part, it probably grades into the deep-marine Hallab Formation (Hammuda et al., 1985); however, at present there is insufficient data fully to support this interpretation. The Jirani Dolomite extends towards the Tunisian offshore, as do all of the Metlaoui Group Formations, where the upper beds of the Chouabine Formation (FourniC, 1978) are equivalent.

SPATIAL DISTRIBUTION

The distribution of the formation displays an oval geometry in plan, with the long axis trending in an approximate NW-SE direction from well CI-NC41, where it reaches its maximum thickness of 386ft, passing through Cl-NC35A to Ll-NC41 (Figs. 1 and 4).

34"

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I. Y. Mriheel and J. M . Anketell

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Oil & Gas Well LIBYA

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Fig. 1. Location map of the studied area, showing distribution of wells.

441

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Fig. 2. Major structural elements of the Pelagian Platform.

442 Origin of the Jirntii Dolomite. ofisshore Libyir

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Fig. 3. Stratigraphic nomenclature of the late Palaeocene and Eocene in NW Libyan offshore (after FourniC, 1978; and Hammuda et al., 1985).

I t shows little variation in thickness compared with the overlying and underlying units of the group, indicating that the overall subsidence rate during the time of deposition of the formation was low, a condition which meets one of the requirements for early dolomitisation (Greensmith. 1981). From the depocentre. it gradually thins to both north and south (Fig. 4); however. in the central northern region, towards the northern part of the Bout-i oilfield, thickness contour lines open NE-ward. suggesting the presence of an "inlet" between two barriers to west and east.

Sbeta (1990) records a comparable pattern, but shows a greater maximum thickness. of over 500ft, and the depocentre to lie further north, centred on well C2-NC41. The log of this well records 128ft of dolomite with anhydrite nodules, overlying 246ft of whitish to brown wackestones and packstones with local dolomitic limestones and argillaceous and arenaceous levels. Comparison with the type section (B2-NC41) (Hammuda et al., 1985) suggests that the base of the Jirani Dolomite should be placed at the level where wackestones and packstones become dominant, in which case the dolomites with anhydrite nodules are assigned to the Jirani Dolomite and the underlying beds to the Bilal Formation (Etourbi. 1989). Even if the underlying beds are included in the Jirani Dolomite, the total thickness is still less than that proposed by Sbeta (1990).

LITHOFACIES DESCRIPTION

The Jirani Dolomite is comprised predominantly of dolomite and dolomitic limestone, and has been subdivided into two main lithofacies on the basis of its association with anhydrite (Fig. 5 ) (Sbeta, 1984).

Anhydritic dolomite facies This facies, composed predominantly of light brown-grey, hard to medium-hard,

microcrystalline dolomite and dolomitic limestone with anhydrite, mainly occupies the southern part of the region (Fig. 5) . The presence of anhydrite nodules in well C2-NC41 shows the facies to extend further north than is recorded by Sbeta (1981, 1990). The anhydrite content ranges from traces to less than 10% and characteristically occurs as discrete nodules up to 4in diameter although. at well HI-NC41, there are beds approaching Sft in thickness. Thin levels of poorly fossiliferous wackestone-mudstone and shale are also present. The dolomite is generally non-fossiliferous; however, the poorly fossiliferous

I . Y. Mriheel and J. M. Anketell 443

0 10 20 30 40 50km - I /' ,..'

Fig. 4. Isopach map of the Jirani Dolomite.

wackestone-mudstone and shales in wells HI and LI-NC4Z yield a fauna of restricted shallow-platform type, including miliolids, ostracods and molluscs (Wilson, 1975). The facies has been recorded at wells C1-137, CI-NC35A, C I , C2, H I , and Ll-NC41 (Fig. l), and displays appreciable variations in thickness with a maximum of 386ft at CI -NC41 and a minimum of 128ft at C2-NC4Z (Figs. 1 and 4).

Non-anhydritic dolomite facies The non-anhydritic dolomite facies, comprising light brown-grey, medium-hard, very

fine-grained dolomite and dolomitic limestone with a few levels of mudstone-wackestone, is closely comparable to the anhydritic dolomite facies, except that anhydrite is absent. Bioclasts again occur in a few sections. Coarse-grained dolomite occurs at well EI- NC35A. The non-anhydritic dolomite facies occupies most of the northern part of the Libyan offshore, and is encountered in fourteen wells (Figs. 1 and 5 ) . It was penetrated at the Bouri oilfield in wells B1 to B7-NC41, and elsewhere at El-NC41, F2-NC41, B la - 137, 01-137, E l , ZI, and GZ-NC35A, displaying a wide variation in thickness with a maximum of 3 1 lft at B7-NC41 and a minimum of 105ft at F2-NC41.

At the Bouri oilfield, the facies has been subdivided into two units by Bernasconi et al . (1984). The lower unit is comprised of light grey-brown, medium-hard dolomitic packstone-wackestone, with Miliolidae, Peneroplidae, Ostracoda, Mollusca, echinoderm fragments and lithoclasts. Phosphatic remains and calcareous skeletal algae occur but are scarce (Etourbi, 1989). At well B5-NC41, the sequence passes into limestone and, at wells B3 and B6-NC41, into brown, medium-hard dolomite with good vuggy porosity. The upper unit consists of light grey-brown, medium-hard, very fine-grained dolomite. At wells B5 and B6-NC41, the dolomite section passes into dolomitic limestone with good vuggy porosity. Throughout the Bouri field, the dolomite is intensely burrowed, with rare

4.444 Origin of the Jirarli Dolomite. offshore Libya

Dolostone Dolomitic Limestone

Dotostone Dolomitic Limestone with Anhydrite nodules

1 Emergent land - arid to semi-arid conditions 0 10 20 30 40 50km - ./. .’

/’

Fig. S.Lithofacies map of the Jirani Dolomite.

anhydrite. kaolinite-filled vugs and detrital quartz grains. Repeated interlayering of thin intraformational breccia levels were observed by Bernasconi et crl . (1984). Porosity within the non-anhydritic facies is very good (see bekow), and ranges from mouldic to vuggy and intercrystalline types.

PETROGRAPHIC CHARACTERISTICS

Petrographic study of the dolomite and dolomitic limestone reveals a number of features which clearly indicate replacement of precursor limestones. Traces of miliolids can be observed in some sections in which the matrix has been completely dolomitised (Fig. 6). Partially-replaced matrix is indicated by remnants of former micritic substrate (Fig. 7 ) . Partially-replaced lithoclasts also occur (Fig. 8). but quartz grains are generally unaffected (Fig. 9).

Fabric-selective dolomitisation, where micrite matrix has been replaced before large allochems such as alveoliae, microforams and peloids, becomes apparent as the degree of dolomitisation increases. Observation of this relationship has largely been restricted to the non-anhydritic dolomite facies near the gradational contact between the Jirani Dolomite and the overlying Jdeir Formation. where such allochems, generally absent elsewhere, become more common.

The typical light-grey, very fine-grained dolomites are well displayed in well B2- NC41, although the lower parts of the sequence are composed of locally-dolomitised limestone. The typical dolomite is very fine-grained with rhombs ranging in size between < 1 Opm and 30pm with some ranging from 30-50pm. The rhombs are subhedral/euhedral in shape, and are generally characterized by equigranular fabric. Straight or uniform extinction is characteristic, and staining shows that the dolomite is non-ferroan. In most cases, the dolomites are associated with very rare subangular, detrital quartz which lacks

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346 Origin of the Jirani Dolomite, offshore Libw

any sign of replacement or inclusions. Secondary authigenic quartz filling precursor allochem moulds occurs, but is equally rare. Burrow mottling, seen in a few thin sections is, according to Bernasconi et al. (1984), very common throughout the dolomites at the Bouri field. Pressure-solution phenomena have not been documented.

At well El-NC35A, dolomitic limestone intervals occur mostly in the upper part of the sequence, together with rare undolomitised peloidal limestone layers. The main allochems in the dolomitised limestones are Nummulites, Echinoderms and Alveolinae, with rare microforams, peloids and lithoclasts, embedded in a dolomitised micritic matrix. The dolomites here display two very distinctive types; the characteristic light-brown, fine- grained dolomite; and zones composed largely of a mosaic of coarsely-crystalline, clear dolomite, cloudy dolomite. cloudy dolomite with clear dolomite rims, and coarse, sparry calcite cement. These are particularly well-developed in subtidal bioclastic deposits. The dolomite crystals range from 5Opm to 200ym in diameter, with very rare examples exceeding 300ym. The crystals are dominantly of subhedral/euhedral shape, light brown to brown, occasionally cloudy and interloclung to form a mosaic texture, although some are euhedral. Staining indicates that the dolomite is non-ferroan. The sparry calcite commonly occurs as cement in interstitial spaces among cloudy mosaic dolomites (Fig. 10). Large dolomite rhombohedra, as much as 200pm across, frequently float in the calcite \par.

A third type of dolomite, recorded by Bernasconi et al. (1984) from the non-anhydritic dolomite facies, is saddle dolomite. This is again coarsely crystalline, but commonly displays strongly curved crystal faces and cleavages, undulose extinction, and is generally ferroan in composition.

Porosity types and reservoir properties A most important feature of the Jirani Dolomite is its very good porosity of mouldic,

vuggy and interciystal types. These are particularly well developed in the non-anhydritic dolomite facies, within which the reservoir for one of the largest oilfields of the region, Bouri, was discovered. No hydrocarbons have yet been discovered in the anhydritic dolomite facies.

Mouldic and vuggy porosity (Fig. 1 1) are commonly of mesopore size, and are generally of secondary origin and related to the effects of meteoric waters. Intercrystal porosity is generally of micropore size.

Porosity varies from well to well, but on average ranges from about 25% for the fine- grained dolomite and 19% for dolomitic limestone.

DEPOSITIONAL ENVIRONMENT OF THE JIRANI LITHOFACIES

The absence of fauna in most of the dolomitic sections, and the presence of very limited fauna of restricted shallow-platform type in the dolomitic limestone and wackestone- mudstone intervals, together with the existence of anhydrite nodules in the anhydritic dolomite facies, implies the involvement of hypersaline marine waters in its formation. Moreover, the preservation of the anhydrite within the facies suggests that arid to semi- arid conditions prevailed in the southern parts of the Libyan offshore during deposition of the formation. The anhydritic dolomite facies of the Jirani Dolomite is therefore interpreted as having been deposited in a restricted shallow-platform environment dominated by hypersaline conditions. The nodular anhydrites indicate a supra- to intertidal setting. Development of discrete beds of gypsum at well HI-NC41, near the depocentre, suggests that deposition of sulphates in this region may have been in more permanent subaqueous conditions such as a hypersaline lagoon. While it is likely that the anhydrite was originally deposited as gypsum, no evidence supporting this has so far been observed.

The major difference between the anhydritic and non-anhydritic dolomite facies is the absence of anhydrite nodules in the latter; otherwise, both facies exhibit closely similar

I . Y. Mriheel and J. M. Anketell 447

Fig. 10. Sparry calcite occurring as cement in interstitial spaces among cloudy dolomites. Jirani Formation, Well EZ-NC35A, 7,767 ft. Cross-

polarised light, stained Alizarin Red. Scale bar: 0.25 mm.

Fig. 11. Mouldic porosity. Formerly a fossiliferous micrite which has now been replaced by dolomite. The more soluble calcite crystals have been dissolved to leave a mouldic porosity. Well B2-NC4Z. Cross-

polarised light, unstained. Scale bar: 0.25 mm.

lithologies and faunas. The absence of anhydrite may be due to non-deposition, in which case the non-anhydritic dolomite lithofacies must represent less saline conditions; alternatively, the facies originally contained anhydrite which was subsequently removed. The latter interpretation is supported by the following observations: 1. The presence of repeated intraformational breccia levels within the non-anhydritic

dolomite facies and their general absence in the anhydritic dolomite facies. These breccias are closely comparable to those produced by the collapse of strata when underlying evaporites are dissolved (Scoffin, 1987; Friedman, 1980). Therefore, gypsum was originally deposited in the northern parts of the investigated area, within the non-anhydritic dolomite facies during the time of formation of the Jirani Dolomites.

2. The presence of the kaolinite patches and residual traces of anhydrite filling vugs a n d or birds-eye structures at the Bouri oilfield (Bernasconi et al., 1984), suggests leaching of a precursor gypsum within the Jirani Dolomite, probably by freshwater flushing during late diagenesis following emergence, either due to eustatic sea-level change and/or uplift.

3. The presence of very good secondary vuggy and mouldic porosity. These confirm that dissolution processes were involved during diagenesis of the non-anhydritic dolomite facies, and may have helped leach the former anhydrite within the facies. In contrast, the continued presence of anhydrite within the anhydritic dolomite facies has occluded its porosity and reduced its reservoir potential.

In conclusion, it is proposed that deposition of the precursor limestones of the Jirani Dolomite took place on a restricted, shallow carbonate platform in supratidal to intertidal flats and very shallow, hypersaline lagoons, where arid to semi-arid conditions are inferred to have been dominant. Some levels of shallow subtidal deposits are also observed in the upper part of the sequence, e.g. at EI-NC35A. Evaporites were originally deposited throughout the basin; however, in the northern parts, these have since been obscured a n d or obliterated by post-depositional solution processes. Dissolution may have occurred

448 Origin of the Jircini Dolomite. ofSshore Libya

due to freshwater flushing during penecontemporaneous, periodic exposure; however, it is likely that it can be related to a major phase of meteoric diagenesis brought about by subaerial exposure of the Metlaoui Group in the early Lutetian (Bernasconi et al., 1991; Mriheel and Anketell. in press).

GENESIS OF THE DOLOMITES

Dolomite formation in the Jirani Dolomite developed in three stages:

Stage I: Main dolomitisation of the Jirani Dolomite In the area investigated, the Jirani Dolomite lies between two essentially unaltered

formations - the overlying Jdeir Formation and the underlying Bilal Formation; local gradations do, however, occur in association with development of Stage I1 dolomite. According to Greensmith ( 198 1). dolomites which lie between unaltered limestones indicate that dolomitisation occurred shortly after deposition and is early diagenetic (penecontemporaneous), and show that a magnesian solution cannot have been introduced subsequently. The presence of dolomitised fossils within the dolomitic limestone intervals and remnants of precursor micritic matrix within the dolomite also indicates that the Jirani Dolomite is not a primary deposit. A number of features, which are interpreted as indicating that the dolomites formed in a peritidal environment, support this conclusion. These are laminated microcrystalline dolomite associated with anhydrite nodules; the absence of cloudy cores or a cloudy zone surrounding the core of the dolomite rhombs; the presence of detrital quartz grains, fenestral fabric and collapse breccias (Scoffin, 1987); and probable stromatolitic algal structures reported by Sbeta ( 1 984). The scarcity andor absence of fossils within the dolomite sections, and the presence of a very limited fauna (e.g. miliolidae, ostracoda, mollusca, and algae) within the dolomitic limestone intervals, is also typical of very shallow lagoons, where peritidal-shallow subtidal conhtions prevail (Wilson, 1975). As noted above, the dolomite rhombs are usually subhedral or euhedral, straight-edged, and range between <10pm and 50pm along their longest face- edges. In addition, staining indicates that the dolomite is non-ferroan. This is in strong contrast to late-diagenetic dolomite, which is coarsely-crystalline, often with strongly curved cleavages and crystal faces, showing undulose extinction. In addition, it is commonly ferroan and can approach ankerite in composition (Scoffin, 1987).

It is concluded that the Jirani Dolomite was formed mainly by eogenetic replacive dolomitisation during middle Ypresian time prior to burial.

Early Diagenetic: Stage 11 The coarsely-crystalline, clear and cloudy dolomites of the non-anhydritic dolomite facies are interpreted as typical of dolomite resulting from the mixing of fresh and marine waters (Folk and Land, 1975). Where these occur, notably at well E l -NC35A, the passage from the Jirani Dolomite into the formations above and below is gradational, rather than abrupt, implying that the dolomitisation process was later than the main eogenetic phase. Stage I1 dolomitisation can thus be attributed to the seawater/meteoric water mixing model (Badiozamani, 1973; Land, 1973a, 1973b) and related to the inflow of meteoric waters following and congruent with subaerial exposure following deposition of the overlying Jdeir Formation. However, this conclusion needs to be supported by isotope studies, at present under way.

Mesogenetic: Stage 111 Saddle dolomite formation has been associated with hydrocarbon migration, chemical

compaction of dolomites, thermochemical sulphate reduction, and epigenetic sulphide replacement (Beals and Hardy, 1980; Radke and Mathis; 1980; Machel, 1987). They are thus, characteristically, late-stage diagenetic in origin and related to burial-phase processes.

Y. Mriheel mid J . M. Atiketell 449

Porosity generation Meteoric waters played a major part in producing the porosity, which is particularly

well-developed in the non-anhydritic dolomite facies. Mouldic porosity is interpreted to have formed by leaching of precursor allochems, probably in a freshwater vadose diagenetic environment. Vuggy porosity is interpreted to have been generated by enhancement of both mouldic and intercrystalline porosity and/or, as with the mouldic porosity, by leaching in the freshwater phreatic environment. Alternatively, some may have been developed during dolomitisation or in the late diagenetic stage from flushing by C0,- enriched waters that precede and accompany maturation and migration of hydrocarbons. Intercrystal porosity between dolomite rhombs probably developed during dolomitisation as a result of dissolution of precursor calcite cement.

MODEL FOR DOLOMITISATION OF THE JIRANI DOLOMITE

Dolomi tisation of the Jirani Formation was predominantly eogenetic, with subsequent modification resulting from the influence of meteoric waters. It is proposed that Stage I dolomitisation was brought about by magnesium-rich brines concentrated from sea water through precipitation of evaporites. The model is similar to the seepage reflux models proposed for the Permian reef complex in West Texas and New Mexico (Adams and Rhodes, 1960), and the south end of the island of Bonaire, Netherlands Antilles (Deffeyes et al., 1965). The presence of evaporite minerals, indicating that hypersaline waters were involved in the formation of the Jirani Dolomite, suggests that the model of Adams and Rhodes (1960) is more closely comparable. Initial dolomitisation of the Jirani Formation thus occurred during middle Ypresian time in a lagoonal setting with a peritidal environment similar to that in which many modern eogenetic replacement dolomites have been reported. Examples include Florida and the Bahama platform (Shinn and Ginsburg, 1964); Funafuti and Eniwetok Atolls (Berner, 1965); Bonaire, Netherlands Antilles (Deffeyes et al.. 1965); and the Persian Gulf (Illing et al., 1965).

A lagoonal setting, albeit shallow, requires a barrier to separate it from the open sea. The confinement of the Jirani Dolomite to the Jifarah Trough region suggests that the barrier was formed by a palaeohigh produced by relative uplift of the Jifarah Platform, and salt dome uplift to the west. Uplift on these structures also appears to have controlled facies distribution in the succeeding Jdeir Formation (Mriheel and Anketell, in pwss ) . Accordingly, the environment of deposition may be interpreted in terms of a back-barrier lagoonal setting which was generally, but not exclusively. shallow. The isopach map suggests that the barrier may have been cut by an inlet near to the Bouri oilfield (Fig. 4).

Development of Stage 11 dolomites by mixing of meteoric and marine waters indicates that the Jirani Dolomite was subjected to subaerial exposure. This event, resulting from relative sea-level fall in early Lutetian times, is recorded directly by the presence of a major unconformity at the top of the Metlaoui Group, and indirectly by the development of mouldic porosity and its enhancement to vuggy porosity as well as in the presence of collapse breccia indicative of solution of evaporite deposits from the anhydritic dolomite facies to form the non-anhydritic dolomite facies. Development of Stage I1 dolomite suggests that lowering of sea level was gradual, since a rapid fall would have moved the mixing zone through the sediment profile too quickly for dolomitisation to occur. Modern examples of mixing-zone dolomitisation have been reported from north Jamaica (Land, 1973a and b) and Barbados (Steinen et ul., 1978).

Development of Stage I11 dolomites followed subsequent burial of the deposits. The occurrence of saddle dolomites in the oil “window” at the Bouri oilfield (Bernasconi et al., 1984) suggest that the saddle dolomites are at least contemporaneous with hydrocarbon migration which began in the late Miocene (Mriheel and El Bakai, 1992). The most probable temperature of formation of saddle dolomite ranges from 100°C to 150°C (Radke and Mathis, 1980). Based on the burial history of the study area and using this

450 Origin of the Jirani Dolomite. ojfshore Libya

temperature range, the formation of saddle dolomite would have occurred at depths of approximately 6,000 to 10,000 ft. As such, it is suggested that the saddle dolomites are late-burial cements, which formed at considerable burial depths during the late Miocene.

CONCLUSIONS

Formation of the Jirani Dolomite marks the culmination of a major regressive phase in late Ypresian times. The deposits are restricted to the region of the Jifarah Trough, a sub-trough of the Tripoli-Gabes-Misratah Basin. Precursor carbonates and gypsum were deposited in sabkha and very shallow, hypersaline lagoons on a restricted, shallow platform, where supratidal-intertidal and arid to semi-arid conditions were dominant. Some levels of shallow subtidal deposits are also present.

Absence of anhydrite in the non-anhydritic dolomite facies seems to provide an exception to this relationship; however, it can be shown that evaporites were probably originally deposited with the dolomites, but were subsequently removed by solution processes. Evidence of their former presence can be inferred from sedimentary and faunal characteristics.

Dolomitisation of the Jirani Formation was mainly early diagenetic (eogenetic dolomite) formed by a seepage refluxion mechanism during deposition of the formation prior to burial. The isopach map suggests that a low rate of subsidence, a requirement for early diagenetic dolomitisation, predominated during the deposition of the Jirani Dolomite. The concordance between lithofacies and isopach trends suggests that the “Zero-line’’ of the formation probably represents the margin of the depositional environment, rather than a line of post-depositional erosion. Restriction of the lagoonal facies was probably controlled by relative movements of the Jifarah Trough and the Jifarah Platform, which bounded the trough to the north.

Development of Stage I1 dolomites by mixing of meteoric and marine water confirms that the Jirani Dolomite was subjected to meteoric processes. This is related to subaerial exposure of the Metlaoui Group following a major relative sea-level fall. which probably resulted from a combination of eustasy and local tectonism.

Subsequent burial of the deposits to depth resulted in the formation of mesogenetic saddle dolomite.

ACKNOWLEDGEMENTS

We express our thanks to the Petroleum Research Centre, Tripoli and the Department of Geology. Manchester University, for providing facilities during the course of this research. In particular. we wish to thank Mr Mustapha Idries for his support and encouragement, and our colleagues in both institutions for advice and discussion. Journal review and useful comments by Drs C . Johns (Scott Pid$ord Group) and M.R. Dobson ( Uniivrsity of Wales, Abepstwyth) are acknowledged.

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