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Journal of Petroleum Geology, vol.23(4), October 2000, pp 425-447 425

DEPOSITIONAL ENVIRONMENT AND DIAGENESIS OF THE EOCENE JDEIR FORMATION, GABES-TRIPOLI

BASIN, WESTERN OFFSHORE, LIBYA

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

The late Ypresian (early Eocene) Jdeir Formation was deposited in the Mesozoic-Cenozoic Gabes-Tripoli Basin, offshore Libya. The basin developed on the northern passive margin of the African Plate and was relatively unstable being affected by syn-sedimentGly tectonic movements. Deposition was coeval with a relative rise of sea-level and the subsequent highstand. A lower, thinly-developed nummulitic bank facies with restricted distribution records the transgressive event and is succeeded by more micritic sediments that record the time of maximum flooding. The succeeding sea-level highstand is represented by a thick, and widely developed, progradational-aggradational nummulitic sequence that displays lateral changes across WNE­ESE trending facies belts.

Three major lithofacies are recognized in the Jdeir Formation: Nummulites packstone­grainstone, Alveolina-Orbitolites wackes tone-packs tone, and Fragmental-Discocyclina-Assilina wackestone-packstone, deposited in bank, back-bank, and fore-bank environments, respectively. The formation passes to the NNE into the pelagic lithofacies of the Ha/lab Formation; landward, to the south, it passes into shoreline evaporiticfacies of the Taljah Formation. The lithofacies were structurally controlled by contemporaneous and/or syndepositional tectonic movements, with nummulitic facies tending to develop on uplifted areas.

Petrographic and petrophysical studies indicate that porosity in the Jdeir Formation is controlled by depositional environment, tectonic setting and diagenesis. The combined effects of salt tectonics, a major unconformity at the top of the formation and meteoric diagenesis have produced excellent-quality reservoir facies at the Bouri oilfield and in other areas.

Porosity is highest in the nummulitic bankfacies and lowest in the Alveolina-Orbitolites micrite facies . Good to excellent reservoir quality occurs in the upper part of the nummulitic packstone-grainstonefacies, especially where these sediments overlie structurally high areas. High rates of dissolution found at the crests of domes and anticlines suggest that early diagenetic processes and features are, in part, structurally controlled. Future exploration success will depend on investigation of similar structures within the Gabes-Tripoli Basin.

* Department of Earth Sciences, University of Manchester Manchester Ml 3 9PL. anketell@fs1.ge. man. ac. uk

**Petroleum Research Centre, Tripoli, Libya.

I 426 The Eocene Jdeir Formation, Gabes-Tripoli Basin, offshore Libya

Both porosity initiation and preservation are related to early depositional and diagenetic I processes. The wide time-gap between hydrocarbon generation and reservoir formation points to the role of the seal in porosifYrpreservation and rules out the assumption that early emplacement of oil had preserved the porof ity. I

INTRODUCTION

The study area is situated offshore Western Libya and encompasses the major part of the ESE-WNW trending, Mesozdic-Cenozoic Gabes-Tripoli Basin (Fig. 1 ). This basin developed over the extreme northern part's of the passive margin of the African Plate and includes a number of sub-basins and platforms, otably the Jifarah Trough and Jifarah Platform.

The late Ypresian Jdeir Folitnation forms the uppermost unit of the Farwah Group which is the main target for hydrocarbon exploration in the offshore area (Fig. 2). The formation comprises shallow-shelf carbbnate bank, fore-bank and back-bank facies. To the south (toward the palaeoshoreline), these NSS into the restricted platform deposits of the Taljah Formation, and to the north (i.e.seawardi), they pass into the deep platform-basinal facies of the Hallab Formation (Fig. 2). The bank facies occur on structurally high areas that are related to salt doming and fault uplift.

This paper investigates the lithofacies and depositional environments of the Jdeir Formation. The relationship between depositional facies and diagenesis is assessed and the impact of the different diagenetic processes on reservoir characteristics is assessed.

A total of 53 well logs were evaluated for lithofacies variation and 300 thin sections of core samples from wells B2-NC41, B3-09-NC41 , EJ-NC41 and EJ-NC35A (Fig. 3) were examined using conventional microscopy, cathodoluminescence (CL), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thin sections were impregnated with blue-coloured resin, left uncovered and polished~ Representative samples were stained with Alizarin Red-S and potassium ferricyanide. The chemistry of different cement morphologies was determined using an electron microprobe, and their precipitation temperatures were obtained from 8180 ratios using an ion probe.

THE GABES TRIPOLI BASIN

Geological Setting The Gabes-Tripoli Basin was initiated in response to widespread, Late Triassic-Middle

Jurassic extensional movem!ents that developed over a broad zone of strain within Gondwana which led to the development of the African and European Plates (Guiraud, 1998). Analysis of the basin-fill history indicates that the region developed from an intracontinental basin located within Gondwana to an epicratonic rift basin on the northern margin of the African Plate (Mriheel, 2000). Lithospher~c extension probably resulted from dextral strike-slip movements between the Coastal Fault system and the northern basin-margin faults (Fig. l ). Basin subsidence was mainly due to cooling

1 following lithospheric thinning, and a simple stretching model

predicts long term patterns o~tectonic subsidence (Mriheel, 2000). When extensional movements ceased (middle Middle Jurassic), the basin subsided thermally and developed as a passive continental margin. Reacti ation of the faults occurred in the Early Cretaceous (Anketell, 1996) and Late Cretaceous (Hammuda et al., 1992). ·

The sedimentary fill in he Gabes-Tripoli Basin is 10-km thick and ranges in age from Triassic to Recent. The fill is divided into a pre-rift, early-Middle Triassic succession ofnon­marine and marine clastics j a syn-rift, Late Triassic-Middle Jurassic succession dominated by shallow-marine carbon~tes and evaporites; and a post-rift succession of Middle Jurassic­Recent marine carbonates ard elastics. These tectono-stratigraphic units have been divided into nineteen second order sequer ces (Mriheel, 2000). For most sequences and sequence boundaries, either a eustatic or tectonioally-enhanced origin has been established

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J.M. Anketell and I. Y. Mrih eel

O/J ~ Kerkenna Island~

Mediterranean Sea

TUNISIA

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Fig. 1. Major structural elements of the Gabes-Tripoli basin, offshore Western Libya (after Mriheel, 2000).

u Tellil Gr.

M EOCENE

Taljah

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PALAEOCENE

Tellil Gr.

Jdeir Fm.

Jirani Doi.

Bilal Fm.

Al Jurf Fm.

Ghallil Fm.

Hall ab

Fm.

Fig. 2.Stratigraphy of the Farwah Group and equivalent units, offshore western Libya (after Mriheel, 2000)

427

428 Th e Eocene Jde1 Formation, Gabes-Tripoli Basin, offshore Libya

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Tripoli

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Fig. 3. Location map showing distribution of wells studied, together with major oilfields, including I Bouri and the A,C,D,E and F structures. Traps include anticlinal structures over salt domes (e.g.

Bo11r1) and fault blocks (e.g. F structure).

I The hydrocarbon system in the Gabes-Tripoli Basin

Hydrocarbon accumulati ns in the Gabes-Tripoli Basin have mainly been discovered in I carbonate reservoirs that range in age from Late Cretaceous to Early Tertiary (Mriheel, 1991) (Fig. 3). The nummulitic facies of the Lower Eocene Jdeir Formation and the non-anhydritic dolomite facies of the Jira~i Dolomite (Farwah Group) are the principal reservoir rocks (Fig. I 2) . The nummulitic facies ?osts a major hydrocarbon accumulation at the Bouri oilfield (Fig. 3). Middle-Upper Eocene carbonates of the Tellil Group also constitute a target for hydrocarbon exploration, particularly in the central and southern parts of the basin (Fig. 3).

The main source rocks ip the Gabes-Tripoli Basin are the widely distributed Maastrichtian I Al Jurf Formation and lowf r Eocene deposits of the Farwah Group, which are mature in most parts of the basin. Geochefll ical analyses and basin modelling have confirmed their potential to generate and expel hydrocarbons (Mriheel, 2000). Generation commenced in the basin

1 centre at about 30Ma (early Oligocene) from the Al Jurf Formation, and at 22.5Ma (early Palaeocene) from the Farrah Group. At the basin margins, however, generation began some 15 Ma later from both sourf. es (Mriheel, 2000). The hydrocarbons migrated laterally up-dip and vertically along active faults . The early-generated basin-centre hydrocarbons were subject to I secondary thermal cracki g, resulting in the occurrence of major gas accumulations in the central parts of the basin.

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J.M. Anketell and I. Y. Mrih ee/ 429

THE JDEIR FORMATION LITHOFACIES

The Jdeir Formation is subdivided into three main lithofacies: Orbitolites-Alveolina wackestone­packstone, Nummulitic packstone-grainstone, and Fragmental-Discocyclina-Assilin a wackestone-packstone (Figs. 4 and 5). The facies are intimately related one to the other in terms of their environments of deposition. Each facies is described below in terms of its lithology, geometry, distribution and relationship to adjacent facies. The environment of deposition of all three facies is then discussed. A fourth "dolomitic" fac ies was encountered in the extreme SW of the study area in only two wells, and appears to have a very limited distribution (Figs. 4 and 5). This fac ies is described first.

The dolomitic facies

Information on thi s facies is limited, but it generally consists of dolomitized limestone together with sandy, glauconitic and bioclastic limestones. These deposits may tentatively be assigned to a restricted shallow-platform environment, probably nearshore. Their occurrence to the WNW of the Taljah Formation broadly defines the position of the shoreline at the time of deposition of the Jdeir Formation, showing it to trend in a general ESE-WNW direction (Fig. 5).

Orbitolites-Alveolina wackestone-packstone

This facies occurs in the southern parts of the Gabes-Tripoli Basin and occupies the Jifarah Trough, with a depositional axis running roughly east-west parallel to the palaeo-shoreline (Fig. 4). Its absence in areas affected by salt movements and from the Jifarah Platform suggests that its distribution is structurally controlled.

The facies was encountered in eight wells (Fl-, F2-, Gl-, HJ -, Kl-, Pl-, N J- andRJ-NC41: Figs. 3 and4) and thus has a very wide distribution offshore Libya. Its maximum thickness (175 m) was penetrated at well F 1-NC4 J, and its minimum thickness ( 14 m) was penetrated at well F2-NC41. The facies thins toward the west, reaching 25 m at well Kl-NC41and33 .5 mat well Pl-NC41. It thickens generally toward the central part of the basin (Fig. 5) to 141 m at well Hl-NC41and101 m at well N l -NC41 . In the western and the SW portion of the study area, at wells MJ-NC41, El a-, Jl-, Kl- and Ll-137, the facies is missing, as is the rest of the Farwah Group and the entire Palaeocene sequence (Fig. 4).

The Orbitolites-Alveolina facies is laterally equivalent to, and is bounded to the north and NW by, the nummulitic packstone-grainstone facies which developed on relatively structurally high areas. Traced shorewards towards Tripoli, the facies grades into the evaporitic and restricted shallow-platform facies of the Taljah Formation (Fig.4).

In all the sections studied, the facies consists predominantly of whitish-light brown and light grey, compact wackestone-packstone sometimes grading to wackestone-mudstone, with thin intervals of grey shale. Microfoss ils which characterize the facies are, in decreasing order of abundance: Orbitolites sp. (including Orbitolites complanatus), Alveolina sp. together with Ophthalmididae, pelecypods, miliolids, gastropods, dasycladacean algae, ostracods, rotalids and echinoderms.

Nummulitic packstone-grainstone

The nummulitic facies was recorded in 17 wells and occupies the greater part of the Jifarah Platform and the salt wall area of the northern and NW parts of the Gabes-Tripoli Basin (Fig. 5). The depositional trend runs in an east-west direction from west of well Jl-NC3 5A passing via EJ-NC35A and C2-NC41 to well Bl a-137, more or less parallel to the palaeo-shoreline (Figs. 3 and 5). It also extends further to the west into the Tunisian offshore and crops out onshore in central Tunisia (Bishop, 197 5).

The nummulitic facies is distributed over a wide area in the northern and western parts of the Libyan offshore, extending east-.west for approximately 230 km (Fig. 4) . The extensive

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Jdelr Formation

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Hallab Formation (Pelagic facies)

J.M. Anketell and I. Y. Mrih eel

D ~::~;;:~ wackestone, shale and marl with planktonlc

431

Fig. 5. Palaeogeographic and facies model of the rimmed carbonate shelf sequence, early Eocene Jdeir Formation and age-equivalent Taljah and Hallab Formations. Note that the clinoform pattern

indicates an early retrograding phase followed by sets of prograding units at the shelf margin.

development of the facies in areas to the west where the main salt tectonics are located, and its gradual eastwards narrowing towards the structurally-low Jifarah Trough, strongly implies that its distribution is both topographically and structurally controlled. The nummulitic facies is bounded to the north by the Fragmental-Discocyclina-Assilina facies , and landward by the Orbitolites-Alveolina facies (Fig. 4 and 5). It generally thickens toward the north and the NW reaching its maximum thickness of 180 mat well B7-NC41, 169 mat well B4-NC41and158 mat well Dl-13 7.

The facies is composed mainly of light brown-grey, nummulitic packstone-grainstone and wackestone-packstone; occasionally intervals of wackestone-mudstone were also observed. Nummulites rollandi-Nummulites irregularies are characteristic, and associated fauna in descending order of abundance include: Nummulites dis tans, Nummulites globulus, Operculina sp., Bolivin idae, Buliminidae, ostracods , rotalids, echinoderms, Discocyclina sp., Polymorphinidae, miliolids, Orbitolites sp., Orbitolites complanatus, Alveolina sp., molluscs and algae.

Fragmental-Discocyclina-Assilina wackestone-packstone This facies was penetrated in four wells, ll-NC35A, Al-NC35A, El-NC41 and Sl-NC41 . It

occupies the extreme northern portion of the Libyan offshore, running in an arcuate WNW-ESE trend parallel to the nummulitic facies belt, and passing to the north of the Bouri oilfield close to well B7-NC41 (Fig. 4 ). To the north, the facies grades into open-marine facies of the Hallab Formation. To the south, it grades landward into less thick nummulitic facies.

I 432 The Eocene Jd ir Formation, Gabes-Tripoli Basin, offshore Libya

A maximum thickness of 18 m was encountered at well E l-NC4 l, and a minimum thickness I of 126 m was penetrated at well Il-NC35A. It reaches 180 m at well A l-NC35A.

In the east, the facies belt ocaurs some 35 km north of the palaeo-shoreline, while in the west it is 30 km or so further away (Fig. 4). This suggests that during deposition of the Jdeir I Formation, the depositional slope shelved less steeply in the west. This can be attributed to the presence of salt domes which bused uplift and led to the formation of a wide, gently sloping carbonate platform in the western region.

The facies consists predomipantly of a thick sequence oflight grey and tan to buff, medium- I to coarse-grained packstone-{vackestone, with intercalated grainstone intervals. The fossil content and its mode of pres rvation are clearly distinct from those of the adjacent facies . Abundant fragments of diffe~ent kinds of fossils such as Nummulites and echinoderms are I typical, and the fauna is characterized by the presence of Discocyclina sp., Assilina sp. and Operculina. Other fossils include Alveolina, Miliolidae, dasycladacean algae, Orbitolites and peloids. Discocyclina sp. is abundant in the upper parts of well El-NC41 and Assilina sp and Operculina are also present. I I Facies Depositional Model ror the Jdeir Formation

The Orbitolites-Alveolina f~cies is restricted to the region of the Jifarah Trough. On the basis

1 of its lithology, fauna! assemO!age and the geological setting, the facies is interpreted as having been deposited in lagoonal or bay environments on a restricted shallow platform with moderate circulation and essentially n9rmal marine salinities. Traced shorewards near Tripoli, it passes into the evaporitic and restr~cted shallow-platform shoreline facies of the Taljah Formation I (Fig. 5). To the north and NW

7 the Nummulitic packstone-grainstones appear to have constituted

a barrier separating the lagoons and/or bays in which the Orbitolites-Alveolina facies was deposited from open marin~ conditions.

Because Nummulites ar extinct, modelling the sedimentological and palaeoecological I parameters which control heir environment of deposition (i.e. water depth, salinity and temperature) is rather uncert~in, a factor which complicates their interpretation. Sedimentological studies ofnummulitic facie~ have been published by Arni (1965), Arni and Lanterno (1972), Aigner (1983 and 1985), Sernasconi et al. (1984, 1991) and Sbeta (1984). Based on these I studies and our own data (comprising well logs; thin sections from cores of wells B2-NC4 J and El-NC35A; and facies and ~tructural maps of the Jdeir Formation), we propose the following interpretation of the depositional environment of the nummulitic facies. I

The nummulitic facies is restricted to the Jifarah Platform and the region dominated by salt domes and walls, and is t~us clearly associated with palaeohighs. Nummulites-dominated deposits built up on the ou~er platform (Fig. 5) to form a barrier that separated the landward lagoonal or bay environm nts from open-marine conditions to the north. The facies is thus 1· interpreted as having bee 1 deposited in a bank setting with the Orbitolites-Alveolina facies comprising a back-bank d posit

Accumulation of the m1mmulitic deposits over topographic highs implies a relationship

1 between nummulite accumulation and water depth (Aigner, 1983) which may have ranged from wave-base to low tidal level (Fournie, 1975), possibly with a maximum depth of about 30m (Arni and Lanterno, 1972), Wave reworking is the main factor responsible for the accumulation of different nummulitic biiofabrics within the bank facies (Aigner, 1983; Bernasconi et al., I 1991). Storms may also have played a significant part in controlling fabric development. For example, the nummulitic /facies at the Bouri oilfield consists of a lower nummulitic bank of large nummulites, an inte~ediate bank with both small and large nummulites , and an upper bank oflarge nummulites. fthese, the uppermost bank is the most extensive and the intermediate I the least. Bernasconi et al. ( 1991) suggested that formation of the banks was favoured by regression, with extensive lateral migration and expansion of the deposits being forced by the increased wave and curre~t energy resulting from the shallowing conditions. In contrast, during transgressive phases, the/banks contracted and micrite of open-marine origin was deposited. I Sea-level fluctuation, ho ever, is not the only process resulting in exposure of the nummulites

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J.M. Anketell and I. Y. Mrih eel 433

to shallow-water conditions. Ami (1965) proposed that bank growth as a consequence of the high biological productivity of the nummulites would have produced a submarine relief, and thus have caused shallowing and exposure to higher-energy environments.

High-energy conditions caused fragmentation of the nummulites, carrying them towards parts of the back-bank setting as shown by the occurrence of fragments at well H J-NC41 (Figs. 3 and 5). Most of the fragmented material was, however, transported to the northern (frontal) margin of the bank, as evidenced by an enrichment of the fossil fragments at wells El-NC41, Al- and ll-NC35A. Ami (1965) and Ami and Lantemo (1972) proposed that the association of abundant fossil fragments withDiscocyclina sp.,Assilina sp. and Operculina can be regarded as characteristic of a fore-bank setting. The presence of abundant fragments of echinoderms andnummulites at well/ I andAJ-NC35A, and the presence ofnummulite fragments,Discocyclina sp. , Assilina sp. fragments and Operculina at well El-NC41 indicates that these deposits are of fore-bank type. Traced seawards, the fore-bank facies passes into the open-marine, deeper­water facies of the Hallab Formation (Figs. 2 and 5) .

The nummulitic facies is thus located at the extreme northern flanks of the outer platform that dominated the northern part of the area during deposition of the Jdeir Formation. It is interpreted as having been deposited in an open-marine environment, probably at the shelf­slope break where relatively high-energy conditions prevai led.

Discussion

Both the geometry and distribution of facies of the Jdeir Formation are compatible with a rimmed carbonate shelf model (Ami, 1965; Read, 1985; Moody, 1987; Buxton and Pedley, 1989) for the western Libyan offshore region. The well-developed, moderate to high energy nummulitic facies constituted a "rim" or bank which was separated from near-shoreline facies by a broad zone of Orbitolites-Alveolina wackestone-packstone forming a back-bank facies in a lagoonal setting. The nummulite bank passed seawards into fore-bank deposits which are represented by the Fragmental-Discocyclina-Assilina facies, before pass ing into the deep shelf-basin.

By contrast, in the Tunisian offshore, a ramp model has been proposed by Loucks et al. ( 1998) for the Jdeir Formation equivalent, the El GariaF ormation. In this region, the discocyclinids concentrate landward of the nummulitic facies and are interpreted as a shoal within a middle­ramp depositional setting (Loucks et al. , 1998). The relationship is clearly different from the Jdeir Fomrntion where the Discocyclina-Assilina facies lies seawards of the nummulitic bank facies and developed in a shelf slope environment. In addition, the broad lagoonal setting characterised in the Jdeir Formation by the Alveolina-Orbitolites back-bank facies is absent from the El Garia Formation.

It would appear therefore that lower Eocene sediments in the Tunisia and Libyan offshore regions were deposited over two different palaeo-depositional profiles. The palaeo-depositional profile over which the El Garia Formation was deposited was the gently dipping surface of a ramp platform. To the east, the Jdeir Formation was deposited on a rimmed shelf. Here, the offshore build-up of Nummulites was facilitated by the palaeohigh of the Jifarah Platfom1, and lagoonal deposits developed over the gently-subsiding Jifarah Trough which is absent in the Tunisian region. A comparable variation in platform type resulting from reg ional variation in structural setting was reported by Spring and Hansen ( 1998) in a study of the Palaeocene Harash Formation in the Eastern Sirte Basin.

In summary, the lower Eocene Jdeir Formation in the Western Libya offshore is composed of a shallow-water, rimmed carbonate shelf deposit, whose main facies can be interpreted as having accumulated in fore-bank, bank and back-bank settings . The nummulitic facies accumulated over a palaeohigh composed of the Jifarah Platform and the salt wall region, forming topographic banks in an outer platform environment (Fig. 5). These acted as a barrier separating a lagoonal Orbitolites-Alveolina back-bank facies from open-marine conditions. The nummulitic banks were deposited in shallow-marine conditions where wave and storm action winnowed and reworked the Nummulites during high-energy phases, expanding their

434 Th e Eocene Jdei Formation, Gabes-Tripoli Basin, offshore Libya

Diagenetic Events Early I Increasini burial

I Late Products

Micritisation I ' ii I I 1 Fonnation or micritic envelope

Fibrous rim cement 11, 11

11 Early ri m cement fo rmation

Mechanical compaction I ·.1 I Repacking and fracturi ng ofbioc lasts

xx x x Poros ity enhancement Dissolution

xx x x

Equant sparry cakilc ~ Reduction in porosity

Drusy cement '" Reduction in porosity

Syntaxial overgrowth . ·~ Reduction in po rosity

Coarse blocky and poikilotopic ~· l Reduc1ion in porosity

calcitcccmcm

NeomorphismO 11)1' 1, 1' 1' ~f icrosparice

Dolomitiz:uion 1111 1: 11111 Fine planar replacement dolomite

Pressure dissolution , ' Pressure solution seams and stylolites

Fractures ~A Fracture porosity ~

I 111 11111 11 Replacement o f bioc lasts by

Silici fication I authigenic sil ica

Pyrite filling cavities J I • l Reduction in porosity

Hydrocarbons 11 1111 l l11l

11l

1

l1 1l1

Cessation of diagenctic processes

18888883 Processes creating porosity Processes destroying porosity 111 ~ ~ : I 1 I : I Processes not affecting porosity

Fig. 6. Diagenetic sequence in the Jdeir Formation.

areal distribution. Fragmentaiion of the Nummulites resulted in typical fore-bank facies deposits. Shallowing to high-energy t onditions was largely forced by changes in relative sea-level, although autocyclic processeJ were also important. Variations in the degree of relative subsidence and uplift of the inner and ou er platform caused a change from a rimmed shelf to a ramp setting westwards towards the Tunisian offshore

DIAGENESIS OF THE JDEIR FORMATION

Mriheel (1991) described the diagenetic history of the Jdeir Formation in the Gabes-Tripoli Basin on the basis of petrographic studies. Diagenetic processes and products are further evaluated here using data frJ m CL, SEM, and geochemical analyses, together with 8180 isotope signatures and basin modelling techniques.

Petrographic investigatio1ns indicate that diagenetic processes which have modified the Jdeir Formation include micritiFation, cementation, dissolution, neomorphism, dolomitization, compaction, silicification and interaction with hydrocarbons (Fig. 6). These processes are discussed below as are the rplationships between porosity and diagenesis in the Jdeir Formation and burial history of the s~diments .

Diagenetic Sequence Diagenetic modificatiop of the Jdeir Formation limestones . (Fig. 6) commenced with

micritization as a result ofr oring by micro-organisms. This was accompanied and followed by fibrous and bladed isopachous magnesian calcite cementation at a very early stage (Fig. 7 A and 7B). Dissolution then parti~lly removed this rim cement, together with aragonitic bioclasts and carbonate mud matrix. Precipitation of non-ferroan sparry calcite cements (Figs. 7C-7F) and recrystallization ofmicrit 1 into microsparite (which probably continued with increasing depth of burial) was followed ijy dolomitization (Fig. 7F). The final diagenetic stages involving increasing burial and compaction led in sequence to:

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J. M. Anketell and I. Y. Mrih eel 435

Fig. 7. Photomicrographs of early diagenetic features and different calcite cement morphologies in the Jdeir Formation.

(A) Eariy rim cement, probably of marine origin, in nummulitic grainstone.Well B3-09-NC41, depth 3471.9m

(B) Micritic envelopes on molluscan bioclasts. Well El-NC35A, depth 2295m (C) Equant mosaic of non-ferroan calcite cement in bioclastic packstone. Well El-NC35A, depth

2296.6m (D) Syntaxial overgrowth cement around echinoderm fragment, in bioclastic packstone. Well El­

NC35A, depth 2297.6m (E) Coarse, blocky calcite cement adjacent to biomouldic porosity, in bioclastic packstone. Well

EJ-NC35A, depth 2296m (F) Poikilotopic calcite cement, in bioclastic packstone. Well El-NC35A, depth 2296.6m.

436 The Eocene Jde1 ·Formation, Gabes-Tripoli Basin, offshore Libya

• grain orientation. • breakage and deformation of bioclasts. • development of pressure solution seams, micro-stylolites and stylolites. • formation of non-ferroan coarse sparry calcite and poikilotopic cements which probably

developed as a result of reprecipitation following pressure dissolution. • fracture development. • silicification resulting in the replacement of some bioclasts and formation of micro and

chalcedonic quartz. • hydrocarbon emplacement (Figs. 8C and 8F) and the cessation of diagenesis.

DIAGENESIS AND RESERVOIR PROPERTIES

Porosity in the Jdeir Formatfon is in general either primary (intergranular and intragranular) or secondary, enhanced by dissolution and fracturing of the limestones. It varies from zero to 40 % (based on core analysis) and includes both fabric-selective and non-fabric-selective types. Fabric-selective porosity includes mouldic, intergranular and intragranular types . Non-fabric­selective pore spaces are generally vuggy and fracture types.

Porosity is at a maximum ~· . the nummulitic packstone-grainstone facies , decreases in the fore-bank facies and is !owes in the Orbitolites-Alveolina wackestone-packstone facies (Fig. 9). Intensive subaerial dissolu ion within the nummulitic facies during early diagenesis resulted in significant porosity develo ment. Reductions in porosity are mainly due to cementation by calcite and compaction.

Diagenetic sequence and pqrosity development The diagenetic history can be divided into three principal stages: marine, near surface, and

burial (Fig. 6). Each of thes~ is characterised by differing degrees of porosity formation and cementation.

Early marine diagenesis This stage involved micriti(lation and precipitation of early-marine rim cements. Micritization

predominates in the upper part of the formation and was caused by boring by micro-organisms. Primary porosity is largely naffected by cementation. The rim cements, occurring as fibrous and prismatic fringes ofnon"ferroan calcite which were precipitated originally as aragonite or high magnesium calcite, are volumetrically unimportant, indicating that the formation was not affected to any considerabl~ degree by early marine diagenetic processes (Fig. 7 A).

Near-surface diagenesis This stage mainly invoh ed carbonate dissolution and cementation by non-ferroan calcite

(Figs. 7 A-7E). Neomorphi m is rare and only occurs within some mud-supported limestone units (Fig. 8E). It has not een observed within the skeletal grains.

Dissolution is dominant n the upper part of the Jdeir Formation and is the main diagenetic process responsible for the c eation oflarge volumes of secondary porosity within the nummulitic facies . Vuggy and some mouldic porosity (Figs. 7C and 7E) greatly enhanced primary porosity and is mainly responsible for the formation's high reservoir potential. By contrast, where dissolution was inactive, such as in parts of the back-bank facies , porosity is poor (Fig. 9). Cementation by non-ferroan calcite dominates the lower part of the formation, increasing downwards with the deve~opment of syntaxial, drusy, equant and isopachous growths that progressively occlude priipary porosity.

Cathodoluminescence Cf L) analysis of the different cement types reveals that zoning is rare, although two different zo9e types (non-luminescent and dull orange luminescent) are present. The non-luminescent zone/includes microcrystalline calcite (micrite), early marine rim cements, syntaxial, equant, drusy mf saics, and sparry calcite cement between grains and filling bioclastic

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Fig. 8. Photomicrographs showing the effects of compaction and fracture in the Jdeir Formation carbonates.

(A) Primary porosit)' reduced by compaction and locally by syntaxial overgrowth cements (arrow). Well B3-09-NC41, depth 3448.Sm.

(B) Reduction of primary intergranular porosity by mechanical compaction. Well B3-09-NC41, depth 3447.3m.

(C) Nummulitic packstone with fractures infilled by bitumen. These fractures are possibly hydrofractures related to pore fluid overpressure. Well B3-09-NC41, depth 3454.3m.

(D) Mechanical compaction has induced enhancement in the permeability via initiation of microfractures. Well B3-09-NC41, depth 3452m.

(E) Stylolites stained with oil in nummulitic packstone. Note the presence of neomorphosed micritic substrate. Well Il-NC35A, depth 2389m.

(F) Dolomitised packstone exhibits well-developed, wave-like stylolite seams rich in insoluble bitumen residue.Well EJ-NC35A, depth 2354.Sm.

438 The Eocene Jd ir Formation, Gabes-Tripoli Basin, offs hore Libya

_/ HI-Heel

ll · NC35A

"""°'' 0 10 20 JO _,.,.) o.piti., {m) 0 10 20 JO

~ Otc>cri .. 1 ... 1

For• Bank·Oeep SheK E:ZJ For• Bank m Bank D Back Bank

Fig. 9. Porosity logs from w lls Il-NC35A, B2-NC4, and HJ-NC41 through the main lithofacies of the Jdeir Formation. Note that porosity decreases with depth and high porosity values generally

occur within the hig1h energy nummulitic bank lithofacies. Note scale differences.

cavities (e.g. nummulite c~ambers). Dull zones occur occasionally in some crystals of the syntaxial, equant and coar~e calcite cements.

Burial diagenesis . This was largely a pornsity-occlusion phase with some fracture porosity development.

Porosity generally decrea~es with depth and large amounts of both primary and secondary porosity were destroyed as) a result of cementation and compaction. Additional loading during progressive burial produced sty lo lites in places, further reducing porosity. However, stylolitization also significantly affected /the reservoir by providing pathways for petroleum migration, as is clearly indicated by the rnsidue of bitumens found along most stylolites (Figs. 8E and 8F). Stylolite dissolution may ~hus have been enhanced by decarboxylation of organic matter.

Interpretation Differences in porosity j'Vithin the nummulitic facies are most readily attributed to variations

in the nature of t~e diageDietic fluids w~th de~th. The great7r de wee of ~issolution in the upper part of the formation prob~'bly reflects circulation of aggressive flutds which were undersaturated with respect to calcite (presumably meteoric waters). An influx of meteoric waters can be linked directly to exposu] ofthe succession to subaerial conditions following the relative sea­level fall during the late presian. Alteration may have taken place in the vadose zone or in the upper part of an acti

1ely circulating phreatic zone.

Precipitation of non-ferroan syntaxial, drusy, equant and isopachous, calcite cement in the lower part of the successibn may also be ascribed to this period of exposure. As a result of calcite

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Eguant calcite cement Elements ~ Oxide % Sxntaxial overgrowth Elements % Oxide %

Cao 40.700 56.948 Cao 40.592 56.790 Mgo 0.069 0.116 Mgo 0.173 0.288 Na20 0.018 0.025 Na20 0.021 0.029 Si02 0.000 0.000 Si02 0.002 0.006 Al203 0.000 0.000 Al203 0.000 0.000 SrO 0.035 0.040 SrO 0.0 18 0.022 FeO 0.003 0.004 FeO 0.003 0.003 MnO 0.004 0.005 MnO 0.004 0.005

Coarse blocb' calcite Elements % Oxide % Poikiloto11ic calcite cement Elements % Oxide % Gao 40.994 57.360 Gao 40.163 56.196 Mgo 0.096 0.161 Mgo 0.079 0.131 Na20 0.015 0.020 Na20 0.004 0.010 Si02 0.000 0.001 Si02 0.000 -o.ooo Al203 0.000 0.000 Al203 0.000 0.000 SrO 0.048 0.057 SrO 0.037 0.045 FeO 0.002 0.002 FeO 0.007 0.010 MnO 0.003 0.003 MnO 0.004 0.005

Table 1. Mean values of electron microprobe results obtained from different calcite cement types of the Jdeir Formation.

dissolution in the upper part of the formation, circulating pore fluids became progressively less aggressive with depth due to increasing saturation with respect to calcite until eventual oversaturation led to cementation.

Lack of association of the calcite cements with compactional features such as fractures and stylolites, and absence of hydrocarbon inclusions within the cements together with the non­ferroan nature of the calcite, suggest that cementation was early, near-surface rather than burial­related.

Interpretation of the cements as early burial stage on petrographic and probable process grounds is supported by the results of geochemical analyses which are discussed in the following section.

Geochemistry of Cements

Electron Microprobe Analysis

Major and trace element compositions of the equant, syntaxial overgrowth, coarse blocky and poikilotopic cements were identified during the petrographic study and were analyzed using an EX JOO Cameca electron microprobe. Results in wt. % (0.01 wt. % in electron microprobe analysis is equivalent to 100 ppm) are summarized in Tables 1 and 2 and illustrated in Fig. 10.

Interpretation

The electron microprobe results clearly show a general similarity between the modal ranges ofresults from different cement morphologies (Fig. 11 ). All the calcite cements studied contain less than 1.00% MgO and are therefore composed of low-magnesium calcite. Various trace elements are present at very low concentrations, largely below the limit of analytical detection. Minimal trace element concentrations and a low magnesium content in a calcite cement are characteristic of a meteoric influence on the fluid source (Morse and Mackenzie, 1990; Tucker

440 The Eocene Jd ir Formation, Gabes-Tripoli Basin, offshore Libya

O.•

0 Analysis No.

Analysis No.

©

,_,

Fig. 10. Elemental compositiops of different calcite cements from microprobe analysis, expressed as weight%. (A) equant calcite. (B) syntaxial overgrowth. (C) coarse blocky calcite. (D) poikilotopic

calcite.

and Wright, 1990). This supports the petrographic interpretation that the different calcite cements were precipitated fir/, om a meteoric source during early diagenesis, prior to significant burial. ·

A low Fe concentration ·n coarse, blocky fracture-filling calcite which is interpreted as a burial cement can, however, be interpreted to have formed in an alternative way. Such cements

. are commonly ferroan. Lo~ concentrations of Fe can occur if the pore fluid contains high concentrations of H2S. In his case, most of the Fe is removed from solution as FeS2 and is therefore not available for iincorporation into the carbonate cement (Allan and Wiggins, 1993). The presence ofH2S in many of the offshore wells (i.e. Bouri oilfield wells, CJ, C2, D2, Hl­NC41 , CJNC35A and Bia 137) (Fig. 3) suggests that such a process may have been operative.

Stable Isotope Analysis / Oxygen isotope geothe~n10metry was applied to the various crystal morphologies with the

aim of estimating the tenw eratures at which the cements formed and identifying the fluid source. The analytical resr lts are presented in Table 2.

Stable isotopic ratios frf m the different cement types show similar isotopic signatures, with slightly negative oxygen ~alues. The first group which encompasses the coarse blocky and poikilotopic calcite ceme~ts has 8180 values between -1.98 to - 6.29 %0. The second group is represented by the equant and syntaxial overgrowth cements. These cements have 8180 values between -0.45 to - 6.65 f6o.

Progressive decreases bn oxygen isotope ratios primarily reflect increasing temperature or fractionation of the precir itating waters. The slightly negative ratios support the conclusion

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E3J Temperature range for equant calcite cement.

D Temperature range for syntaxlal cak:ite cement

g) Temperature range tor poikilotopic and coarse blocky calcite cements.

441

Fig. 11. Equilibrium relationship between calcite 0180, temperature and 0180 of water. X-axis represents 0180 value of water (SMOW); Y-axis represents temperature °C. The curved lines

represent constant 0180 values (PDB) for calcite (Prezbindowski, 1985; Tucker and Wright, 1990).

Well No. Depth (m) Cement Type o"O(PDB) 0180 (SMOW)

. El-NC35A 2296 Coarse blocky -5.50 25.19

El-NC35A 2296 Coarse blocky -5.74 24.94

El-NC35A 2296 Coarse blocky -6.29 24.37

El-NC35A 2296.6 Poikilotopic -1.98 28.81

El-NC35A 2296.6 Poikilotopic -2.02 28.77

El-NC35A 2296.6 Poikilotopic -2.59 28.18

El-NC35A 2296.6 Poikilotopic -2.72 28.05

El-NC35A 2296.6 Poikilotopic -3.60 27.14

El-NC35A 2297.6 Syntaxial -0.45 30.39

El-NC35A 2297.6 Syntaxial -l.3 1 29.50

El-NC35A 2297.6 Syntaxial -l.85 28.95

El-NC35A 2297.6 Syntaxial -2.18 28.61

El-NC35A 2297.6 Syntaxial -2.60 28.17

El-NC35A 2296.6 Equant -6.65 24.0

El-NC35A 2296.6 Equan~ -5.17 25.53

El-NC35A 2296.6 Equant -4.58 26.13

El-NC35A 2296.6 Equant -3.48 27.27

Table 2. Results of oxygen isotope analyses of cements from Jdeir Formation. Results are expressed in parts per mil.

442 Th e Eocene Jd ir Formation, Gabes-Tripoli Basin, offs hore Libya

that the cements are near-surfa e in origin, since burial cements are, due to higher temperatures of precipitation in the burial emrironment and the fractionation effect, more depleted in 180 than marine or earlier meteoric cements virtually without exception (Choquette and James 1987; Tucker and Wright 1990). Sinqe trace element geochemistry of the calcite indicates a common fluid source, temperature or fr, ctionation effects may also explain the slight variation in the signature patterns.

Palaeotemperatures can be estimated by using an assumed or determined value for the oxygen isotopic compositions) for the waters , and with reference to standard graphs of 8180 (SMOW) versus temperature (Fig. 11) (Prezbindowski, 1985), estimating the temperature ranges involved .

A range of -4 to -6%0 was/ recorded for meteoric-derived groundwater by Lawrence and White (1991) and Hays and C!Jrossman (1991), and this has been used to interpret the data.

The 8180 values obtained f9r the poikilotopic and coarse blocky calcite cements range from - 1.98 to -6.29%0. Using a fluid source composition of - 5.0 8180H2o (SMOW) gives a palaeotemperature range of2 to 20°C (Fig. 11). By contrast, the 8180 values obtained for the syntaxial calcite cement ranges from -0.45 to - 2.60%0, giving a range of 0° to 5°C (Fig. 10). Similarly, the 8180 values for the equant calcite cement ranges from -3 .74 to -6.65%0, giving a range of 10° to 21°C (Fig. ~ 1 ).

Both the low absolute values and also the low range in the palaeotemperatures of the fluids precipitating the different ~es of cement indicate that they formed early in the diagenetic history (i.e. during near-surface cementation), and preclude an origin during later (burial) diagenesis.

HYDROCA~ON GENERATION AND DIAGENESIS 1F THE JDEIR FORMATION

The nummulitic facies of the Jdeir Formation together with the dolomitic facies of the underlying Jirani Formation (Fig. 2) comprise the main reservoir rocks in the NW Libyan offshore. The absence of hl drocarbon fluid inclusions in the calcite cements indicates that hydrocarbon generation pos -dated cementation. In addition, the absence ofpost-hydrocarbon­migration cement, such as hat observed within the Smackover Formation by Heydari and Moore ( 1989) suggests tha diagenesis had ceased after hydrocarbon emplacement.

Petrographic evidence fot the relative ages of major diagenetic processes and oil generation is supported by burial histoP'. modelling (Fig. 12). This shows that oil generation commenced at about 30 Ma (late Oligoc

1ene) from the Late Cretaceous-Palaeocene Al Jurf source rocks in

the centre of the Gabes-Tri~oli Basin. More significantly, at the basin flanks , in the vicinity of the nummulitic banks, hydrocarbon generation did not begin until the middle Miocene about 15 Ma ago, and is still continuing at the present day (Mriheel, 2000).

The lengthy gap betwee 1 the time of hydrocarbon generation and the time of deposition of the early Eocene reservoirs (Fig. 12) indicates why hydrocarbon inclusions are absent from the cements. Furthermore, it ru~es out the involvement ofC02, produced during thermal maturation in the source rocks, in the enhancement of secondary porosity in the near-surface dissolution phase. The relatively late emplacement of hydrocarbons to the Jdeir Formation reservoir also precludes their having had/ a role in porosity preservation. Instead, this can be ascribed to the presence of an effective c~p-rock seal.

POROSITY DEVELP PMENT AND RELATIVE SEA-LEVEL CHANGES

The Jdeir Formation records a relative sea-level rise following the restricted platform-tidal flat environments typical 1 fthe underlying Jirani Dolomite and Bilal Formation (Bernasconi et al., 1991 ). At the Bouri oilfield, detailed analysis of the nummulite facies by Bernasconi et al. ( 1991) recorded a subsequent regression followed by transgression, and then a more extensive

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Immature --""""'o"'il_z_o-ne ____ O.?

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\.,..;llooor-ll:=:l3 U . Aljurf

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Hydrocarbon accumulation Paleo1 cene Eocene I Oligocen~ Miocene I Pliocene

Micritization I Calcite cements I I Dissolution

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Neomorphism D()<I

Dolomitization ~ Compaction I I Fractures V /// ////,;i Pyrite filling cavities I===! Deposition of source rocks -=- -=I Deposition of reservoir unit V,,i Deposition of seal rocks

Porosity preservation kYXxl Hydrocarbon generation I Central I Basin flanks

443

Fig. 12. Burial history, diagenetic sequence and hydrocarbon accumulation in the Jdeir Formation in relation to time. Note that hydrocarbon generation began after trap formation. Note also that

porosity preservation is largely related to sealing rather than the emplacement of oil.

444 The Eocene Jd ir Formation, Gabes-Tripoli Basin, offshore Libya

regression in the late Ypresian. This later regression led to the emergence of the entire Farwah Group into subaerial condition , where leaching of both the micritic matrix and skeletal grains in the nummulitic banks resulted in the initiation of a well-developed secondary mouldic and vuggy porosity.

It is tempting to relate th5 initial transgression to the eustatic sea-level rise at 51 Ma recognised by Vail et al. (1977D, and the ultimate regression to a major sea-level fall in the late Ypresian-early Lutetian at 49i .5 Ma; however, this correlation is speculative, since the age of the Jdeir Formation is poorly constrained. At the same time, the close association between the distribution of major structural elements in the region and facies types may imply that sea-level variation and facies developn/ient are in part controlled by local tectonism.

The main hydrocarbon ace mulations in the Jdeir Formation occur in structural traps, where the nummulitic facies overlie

1structurally high areas. These palaeotopographic highs occur in

areas of salt-dome uplift and positive fault blocks for example at the Bouri oilfield ("B" structure) and El-NC35A ant}clinal crest (Fig. 3). The log-derived porosities, and the average absolute porosities from cor[ analysis, for the formation at wells in the "B" structure and adjacent wells (Fig. 9) indicate that higher porosities developed at structurally-higher levels. The restriction of good secontlary porosity to the upper parts of the formation may indicate that tectonism caused emergence! of the formation into subaerial conditions. The development of high porosities above salt-cor~d structures is explained by inferring that doming in the underlying Triassic _salt created areas o~ topographic relief on the s~a :t:ioor, creating ~hallows in wh!~h nummuhte banks accumulate~. Fault-controlled topographic highs were also sites for nummuht1c bank development. During tpe subsequent sea-level fall , the nummulitic banks on highs and positive fault blocks would 1-ave been subaerially exposed. Lenses of meteoric waters would have been established in the

1exposed crestal parts of the banks, leading to leaching ofunstable

grains and micritic matrix w~th the development ofvuggy, mouldic and matrix porosity. Thus, positive palaeostructural settings were important for the development of higher porosity and better reservoir quality wit~in the Jdeir Formation reservoir carbonates.

The extent to which the highs were actively uplifted as opposed to passive palaeotopographic features controlling deposition is, however, not clear; nor is it clear iftectonism was involved in the major relative sea-le el fall that exposed the Jdeir Formation to subaerial conditions in the late Ypresian

St MMARY AND CONCLUSIONS I

In the Gabes-Tripoli Bas(n (offshore Libya), the late Ypresian Jdeir Formation was deposited on a relatively unstable platform affected by syn-sedimentary tectonism. Deposition commenced with a transgresive facies /tract with the development of a lower nummulitic bank. During maximum flooding, the deposits became more micritic in character, and during a succeeding sea-level highstand, an upper nummulitic facies was extensively and thickly developed as a result of aggradation andl progradation at the shelf margin. Lateral facies changes in the formation developed in broad WNW-ESE trending zones parallel to this margin. Geometry and facies relationships point tb deposition on a rimmed shelf that changed westward in Tunisia to a ramp setting. This can be related to regional variations in structural setting.

Three major lithofacies/ in the Jdeir Formation can be recognized: Numrnulites packstone­grainstone; A lveolina-OrbtNolites wackestone-packstone; and F ragmental-Discocyclina-Assi l i na wackestone-packstone. T ese lithofacies were deposited in bank, back-bank and fore-bank environments, respective . Basinwards, the Jdeir Formation passes laterally into the pelagic lithofacies of the Hallab Fbrmation, and landwards into shoreline evaporitic facies of the Taljah Formation (Fig. 13).

Diagenesis of the Jdei~ Formation mainly involved marine phreatic, meteoric phreatic and burial stages. The main ~iagenetic processes include micritization, dissolution, cementation, neomorphism, compaction and fracturing.

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Nummulites

Nummulite­Clasts

Alveolina Orbitolites Miliolides

Echinoids

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Discocyclina

Assil ina Planktonics

Dissolution

Porosity Hydrocarbon

Cementation

Stylol ites

Fractures Dolomitization

Anhydrite

J.M. Anketell and I. Y. Mrih eel

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Deep Shelf - Basin

Sh., Marl , & M/W W/P P/G

----- -

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Restricted Shelf

Back Bank Evaporite

W/P Doi. & Anhy.

c:J Dolomitic Facies CJ Orbitol ites-Alveol ina Facies

Symbols ~ Nummulitic Facies E:Z3 Fragmental-Discocyclina-Assilina Facies

[

--- High ~ Pelagic Facies Frequency Scale 1.1..ii.i..1 -Moderate to Low

Fig. 13. Depositional environments and main diagenetic features of th e J deir Formation and its lateral equivalents, the Taljah an d Hallab Formations.

446 Th e Eocene Jd ir Formation, Gabes-Tripoli Basin, offshore Libya

Porosity in the reservoir is complex and includes primary and secondary types. Porosity is highest in the nummulitic baiJ facies and lowest in the Orbitolites-Alveolina micrite facies. Good- to-excellent reservoir quality resulting from intergranular primary porosity occurs in the upper part of the nummulitic packstone-grainstone banks, especially where these sediments overlie structurally high areas. Primary porosity and reservoir quality were enhanced by extensive dissolution. Concen~ration of calcite spar in the lower part of the banks, together with compaction and pressure solution caused porosity destruction.

The analysis of diagenetic flf:brics demonstrate that dissolution was effected by the influx of meteoric waters in the upper p rt of the banks, indicating emergence of the Jdeir Formation and its exposure to subaerial pro, esses. Eventual saturation of the pore waters with respect to calcite, and circulation of the waters at depth, culminated in precipitation of cements with a variety of morphologies. Major and trace element geochemistry and stable oxygen isotopic data confirm that the majoritY of the calcite cements within the reservoir formed early in the diagenetic history from meteo[ic fluids . Trace element and oxygen isotope analyses respectively record element concentratio9s and low precipitation temperatures that indicate an early near­surface origin for the cements. They also indicate that recrystallization or neomorphism of the cements at high temperatures t as very rare. Exceptions are minor developments found associated with burial diagenetic phenomena (i .e fractures near to stylolites) that are undoubtedly oflate burial origin (Mriheel et al.,/.1993).

Both porosity initiation anr, preservation are thus related to early depositional and diagenetic processes. Burial history an~lysis, showing a wide time gap between hydrocarbon generation and reservoir formation (> 35 Ma in the northern part of the basin), rules out the assumption that early emplacement of o~l has preserved the porosity, and draws attention to the role of the seal on top of the Jdeir Fo~ation reservoir as the means of porosity preservation.

Emergence of the Jdeir F0rmation into subaerial conditions may be related to the proposed late Ypresian eustatic fall in sea-level at 49.5 Ma. However, the close association between the distribution of facies and u9derlying salt domes and fault-block highs suggests that tectonism as well as eustasy may hav~ played a part in controlling sea-level changes. Major pools of hydrocarbons are associated with salt-doming and positive fault blocks, and there is no doubt that topographic highs relatf d to such structural features were areas in which the formation of nummulitic banks was enhanced. It is not possible to confirm whether the tectonic elements were active during deposit~on and/or were eventually instrumental in raising the formation

The combined effects of elative sea-level fall , subaerial exposure, meteoric diagenesis and above sea-level. ~

amajorunconformity at the op of the Jdeir Formation have produced excellent-quality reservoir facies at the Bouri oilfiel and in other areas. Future exploration success within the Jdeir Formation will benefit fro/1 identification of similar relationships elsewhere in the Gabes­Tripoli Basin.

ACKNOWLEDGEMENTS

This study was sponsoJ d by the Petroleum Research Centre (Tripoli) and conducted at the Earth Science Department~ Manchester University. The authors wish to express their thanks to both institutions for the pr vidision of facilities . Special thanks are due to Dr Ian Lyon for ion microprobe analyses. Journal review was by Maurice Tucker (Durham University) and Peter Gutteridge (Cambridge d arbonates) whose comments are acknowledged with thanks.

REFERENCES

AIGNER, T., 1983. Facies and origin of nummulitic buildups: an example from Giza Pyramids plateau (Middle Eocene, Egypt). Neues Jahrb. Geo/. Palaont. Abh., 166, 347-368.

AIGNER, T., 1985. Biofabrio1s as dynamic indicators in numrnulitic accumulation. Journ. Sediment. Petrol.,

55, 131-134.

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ALLAN, J. R. and WIGGINS, W. D., 1993. Dolomite reservoirs: Geochemical techniques for evaluating origin and distribution. AAPG Continuing Education Course Note Series 36, 129p.

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