lithofacies of dahra “b” member in concession nc102, sirt basin,...

LITHOFACIES OF DAHRA “B” MEMBER IN CONCESSION NC102, SIRT BASIN, LIBYA 1

Lithofacies of Dahra “B” Member in Concession NC102,Sirt Basin, Libya

Ahmed m. muftAh1, AbuessAoud h. AddAloush2 And hAni J. dAniels 2

ABSTRACT

The Dahra “B” Member (Upper Palaeocene) in seven wells in Concession NC102 has been investigated. Sequences of this member are composed of light brown fine crystalline dolostone, partly calcareous dolostones, light brown/off-white foraminiferal limestone and light brown bioclastic limestones.

The faunal suite comprises benthonic foraminifera, Amphistegina sp. (in H1-92) a species commonly associated with reef build-up with bryozoan and echinoderm remains, the latter on a local basis. Dolostones are mostly very fine crystalline. Porosity in dolomites is intercrystalline, followed volumetrically, by solutional and interparticle types. It has been diagenetically enhanced by dolomitization and dissolution (in the vadose zone), but reduced by cement.

Based on lithology, existing benthonic foraminifers and correlation with nearby wells, the main interpretative palaeoenvironments are: a) intertidal, with a semi-restricted and shallow subtidal influence partly comprising mudstone-textured dolomite; b) subtidal, with intertidal and semi-restricted influences; and c) outer shelf with, possibly, a barrier island in the eastern part of the area.

The Dahra “B” Member in this concession was deposited on a shallow to very shallow shelf, mainly within a low-energy partly semi-restricted environment with an embayment. The appearance of Lockhartia cf. L. haimei denotes, stratigraphically, a Palaeocene age.

1University of Binghāzī, Earth Sciences Department, P.O. Box 9480, Binghāzī, Libya2Arabian Gulf Oil Company, Geological Laboratory, P.O. Box 263, Binghāzī, Libya

Geology of Southern Libya 2012, vol. 0, pp 00-00 - SL-194

INTRODUCTION

Dahra Formation was described by Barr and Weegar (1972) from the type section located in the Oasis F1-32 well (3040-3350’). This formation has been subdivided by petroleum geologists from different oil companies, including AGOCO, into three informal members (Figs 1 and 2). Dahra “B” Member is traced in six subsurface wells by means of E-log (Fig. 3) with emphasis on: a) petrographic and textural characteristics, b) porosity types and relative volume, and c) depositional environment. This study is mostly based on cuttings retrieved from the Dahra “B” Member in wells A1-NC102 (3350’-3500’), B1-NC102 (3652’-3760’), H1-92 (3560’-3700’), KK1-11 (3790’-3900’), M1-92 (3545’-3620’), MM1-11 (3600’-3700’) and C1-92 (3580’-3720’). This member was chosen because of its palaeogeographical importance. In addition, core #3 (3382’- 3412’) in Well A1-NC102 also

examined. Fifty-two hand specimens of overall mediocre to good quality, and 32 thin sections have been examined. The examined Dahra “B” sequences in these wells are mainly composed of dolomites (partly intermixed with limestones) and limestones. The dolomites, mostly encountered in A1-NC102, M1-92 and C1-92, are fine crystalline, reflecting a mudstone texture; some are dolomitized wackestone-packstone with dolomitized bioclasts.

PETROGRAPHICAL ANALYSIS

The Dahra “B” Member in B1-NC102 and the upper part of KK1-11 is composed of off-white to light brown chalky foraminiferal limestones, with no significant visible porosity, except locally. The Dahra “B” sequence in MM1-11, H1-92 and the lower part of KK1-11 overall is light brown, coarser grained and fossiliferous. In A1-NC102 it comprises light grey to light brown, fine crystalline dolomites (in part with biomolds) and partly dolomitized and argillaceous limestones. Similar rocks form part of the succession in M1-92, C1-92 and H1-92.

Packstones close to the base and top of the Dahra “B” Member in B1-NC102 are composed dominantly of

2 MUFTAH AHMED M., ET AL.

Fig. 1. Location map of the studied concession, NC102 with six wells in the concession.

Fig. 2. Nomenclature chart of Palaeocene rock units in Sirt Basin (modified after Barr and Weegar, 1972).

planktonic foraminifera, such as Planorotalites sp. with calcispheres (Plate II, 4). A similar lithology (although partly within dolomitic wackestone-packstone) is found at the top of the sequence. These two horizons are separated by a mixture of echinoderm-bearing packstone, pyritic wackestone with benthonic foraminifera, and mudstone/wackestone-textured carbonate with planktonics. Local shears are noted in the lower part.

Packstones, occupying the lower part of Dahra “B” Member in KK1-11 include partly micritized (on the surface) shell fragments and unidentified micritizied grains, with some ostracodes. The upper part of the sequence in this well is a partly dolomitized wackestone with planktonic foraminifera, and trace shell fragments and ostracodes. It also contains locally sheared argillaceous limestone.

The Dahra “B” carbonates in MM1-11 are variably dolomitized wackestones with benthonic foraminifera (Quinqueloculina sp. and Textularia sp. in the middle part) with some echinoids and minor shell fragments (Plate I, 6).

In H1-92, the Dahra “B” comprises a mixture of echinoderm and bryozoan-bearing packstones (with Nummulites cf. N. partschi, N. sp., Lockhartia cf. L. haimei and Rotalia sp. and pectinids), and mudstone-textured


Fig.3. Log response, Dahra “B” Member in six wells in Concession NC-102.

dolomites (partly with biomoldic porosity); the latter becomes more common with depth. This unit also comprises rocks with comminuted very fine-grained bioclast debris.

In thin sections, samples from M1-92 are mainly mudstone- and wackestone-packstone textured dolomites and dolomitized limestones, respectively. At the base the latter contains benthonic foraminifera (Quinqueloculina sp. and Rotalia sp.) and ostracodes, while in the upper part it exhibits vadose leaching and vug filling by calcite spar (Plate I, 1).

The Dahra “B” in C1-92 is composed dominantly of variably dolomitized packstones with a major benthonic foraminiferal bioclast component (some Quinqueloculina sp. and Lockhartia cf. L. haimei present in the upper part) (Plate II, 1). It is partly biomoldic, with some biomolds infilled by gypsum and calcite in the lower half. The sequence also contains mudstone and wackestone-textured dolomites.

Packstones at and close to the base in A1-NC102 are bioclastic limestones with benthonic foraminifera (including a trace of Quinqueloculina sp. and Rotalia hensoni) (Plate I, 2) with minor amounts of echinoderm spines and shell fragments also present. The rest is dolomitized mud, with argillaceous calcareous dolomite/dolomitic limestone and partly dolomitized fossiliferous limestone. Dolomitization, overall, appears best developed in this well.

In general, dolomite becomes overall more abundant towards the north and northwest, indicating increasing dolomitization towards the strandline, as the depositional environment changes to that of a tidal facies.

DIAGENESIS

An understanding of post-depositional processes is important, as they affect and modify the sediments in


Fig. 4. Paragenetic sequence of diagenetic events. 1 - length of stage (arbitrary); 2 - bar location (comparative); 3 - drusy; 4 - sparry; 5 - stylolitization.

terms of mineralogy, texture, porosity and permeability. These processes can thus be a major control on reservoir characteristics. Of these, early diagenetic events, such as dolomitization and leaching, appear to have imprinted the most favourable effects on the Dahra “B” Member in Concession NC102.

Based on mineralogical and textural interrelationships, various processes and diagenetic products have been observed (Fig. 4). Starting with the oldest, the main processes and minerals are:

a) Deposition of allochems, some of which are aragonitic, and precipitation of lime-mud and, probably, primary dolomite (a few crystals are approximately 2 microns across). Some primary porosity is still preserved in samples from H1-92. Fenestral porosity was formed locally at an early stage of diagenesis in upper intertidal sediments in A1-NC102 (Plate I, 3) and there is local precipitation of fluorite.

b) Micritization of particles (mainly at grain surface) within a marine phreatic environment, with some particles suffering extensive degradation. At a later stage of diagenesis, partly micritized aragonitic bioclasts were dissolved, yielding micritic envelopes.

c) Filling of pore space with drusy calcite (equant, coarsening towards pore centre) within a freshwater

phreatic environment. There is no indication of a younger generation marine phreatic environment. Cementation is a porosity-destroying process.

d) Leaching of aragonite within the freshwater vadose zone, resulting in biomoldic and vuggy porosities which are major components of the total porosity in some rocks (as in A1-NC102).

e) Dolomitization of mudstone-texured limestone in part resulted in fabrics containing good intercrystalline porosity ( as in A1-NC102 and M1-92) (Plate I, 5).

Physical and chemical compaction is an ongoing process, having an adverse effect on porosity. Stylolitization, forming at a later stage of diagenesis, is present on a local basis.

AGE

Dahra “B” member in the Sirt Basin is of Palaeocene age (Landenian) based on planktonic foraminifera from shale interbeds, especially those near the Dahra Formation/Khalifa Formation lateral boundary (Barr and Weegar, 1972). Dahra “B” Member of Dahra Formation in Concession NC131 is of Palaeocene age based on the presence of Ranikothalia soldadoensis, Daviesina cf. D. langhami (Senoussi and Saad, 1995) and in Concession


Fig. 5. Dahra “B” facies distribution: a) upper part, b) lower part. Note the relative shift in configuration towards the north, from older to younger units.

102 a Palaeocene age is suggested, based on the presence of Lockhartia haimei in H1-92.

PALAEOENVIRONMENT OF DEPOSITION

Various depositional environments, based on facies type and fossil content, existed during the deposition of the Dahra “B” Member in Concession NC102. Three facies types, partly intermixed, and in association with other less well-defined types, are present. The palaeoenvironment may have been dominated by the presence of an embayment (Fig. 5). The main facies are:

Flacies I (Outer Shelf)

This facies, deposited in quiet outer shelf waters, is represented by planktonic-calcisphere wackestones and packstones. It is more evident in B1-NC102

(Fig. 5) (Plate I, 4), and the upper part of the KK1-11 intersection as well. In KK1-11 this facies grades with depth to facies II.

Facies II (Subtidal)

Subtidal deposits, recognized by the diversity and abundance of organisms, are prominent features in H1-92, C1-92, and MM1-11, but are also present in part of the sequences in KK1-11 and A1-NC102 (Fig. 5). They comprise bioclastic packstones and wackestones containing grains that are micritized on their surface, echinoderms, bryozoans, benthonic foraminifera, micritic envelopes and minor glauconites; the presence of pre-existing bioclasts/allochems is suggested by biomolds. This facies exhibits some degree of influence of a semi-restricted environment as indicated by the diagnostic presence of miliolids in the middle parts of the Dahra “B” intersection in C1-92 and MM1-11, and at its base


in A1-NC102. Rocks from A1-NC102 also comprise dolomitized/dolomitic muds at top and base, suggesting an affinity to an intertidal environment.

Facies III (Intertidal)

This facies, recognized by common porous and non-porous dolomites, fenestral porosity (local), desiccation cracks? (local) and lithoclasts (local) appears most common in A1-NC102 (Plate I, 5) and less so in M1-92 (Fig. 5).

Facies IV (Barrier/Shoal)

The presence of Palaeocene Nummulites, together with relatively common echinoderms and bryozoans with a few corals (Plate II, 3) in H1-92 packstones suggests proximity to a barrier island/shoal (Fig. 5). Some of the samples in this well also contain the foraminiferal species Amphistegina sp., which is known to thrive within coral communities.

In conclusion, it appears that there is a palaeogeographic shift in depositional environment (facies) towards the north, as the Dahra “B” Member evolves from older to younger units, indicating a receding shoreline (transgression).

POROSITY TYPE AND DISTRIBUTION

This analysis is of limited scope, as porosity is mainly assessed visually from thin section (point-count data is not available). Core analysis is only available on core #3 in A1-NC102. With the exception of “barrier island” rocks, porosity is dominantly of two types: a) intercrystalline, mainly within dolomitic rocks, and b) biomoldic.

Intercrystalline porosity is difficult to detect in samples containing dolomite crystals in the order of 10 microns across, as a normal thin section of a 30 micron-thick rock slab would comprise three layers of dolomite crystals. Any two layers would obstruct the light passing through intercrystalline pores of the third, thus rendering them difficult to discern.

Examination of visual porosity in thin section (Fig. 6) shows a drastic increase in overall porosity volume from south (outer-shelf) to north (intertidal).

Figure 6 also indicates that porosity of the Dahra “B” sequences in these wells (with the exception of B1-NC102 and KK1-11) tends to increase from base to top. Porosity at depth, however, remains volumetrically significant, as in A1-NC102, M1-92 and C1-92. Lower

porosity at depth is attributed to: a) depositional characteristics of the rocks (facies), and b) weak diagenetic effects.

Depositional control is evident locally at the base of A1-NC102 (in part wackestone-textured limestone), while diagenetic control is exemplified by filling of biomoldis and intercrystalline pores by gypsum and calcite (in C1-92).

Porosity type also appears to vary according to depositional environment. Dolomitized intertidal deposits in A1-NC102, M1-92, C1-92 and MM-11 contain intercrystalline porosity (Plate I, 1) with a biomoldic component (mainly in C1-92).

Porosity in subtidal deposits in H1-92, on the other hand, is of solution-enhanced interparticle (Plate II, 2), moldic and intercrystalline types. Pore interconnectivity appears poorest in limestones, being poor where moldic porosity is dominant, and fair to good in mudstone-textured dolostones.

Outer shelf deposits in B1-NC102 and KK1-11 contain poor intercrystalline porosity within limestones. The best visible porosity is present in A1-NC102. It is mainly intercrystalline within dolomite, with a local moldic/biomoldic component; vuggy and fenestral porosities are locally present (core #3). The sequence in this well best exemplifies the upward increase in porosity in Dahra “B” Member (Fig. 6).

Core analysis of the interval 3383-411’ in A1-NC102 indicates a range of 8.3-40.9%, averaging 26.6%, and a range of 0.1-295 md permeability, with seven samples (out of 11) exceeding 100 md. Porosity in wells M1-92 and MM1-11, also mainly of intercrystalline type, is overall rated “fair-good” in the former, and “fair” in the latter (Fig. 6).

CONCLUSIONS

Three main facies are identified from Dahra “B” Member of Dahra Formation in Concession NC102: a) Intertidal, with a semi restricted and shallow subtidal environment partly comprising mudstone-textured dolomite, mainly in NW part of the area; b) subtidal, with an intertidal and semi-restricted environment, mainly in the central-eastern part of the area; and c) outer shelf, in the SE corner of the area. There may also be a barrier island in the eastern part of the area. There is a shift in depositional environment towards the north, as older rocks evolve into younger strata, suggesting transgression.

Intercrystalline porosity (in dolomite) is the most prominent type, volumetrically followed by biomoldic,


Fig. 6. Schematic representation of visual porosity with depth (not to scale).

solutional and interparticle types. Porosity has been diagenetically enhanced by dolomitization and dissolution in the vadose zone. It has been partly reduced by calcite cement and evaporates.

Porosity increases drastically from the south, (poor intercrystalline in outer shelf limestones) towards the north (fair-very good, dominantly intercrystalline in dolomites in A1-NC102, M1-92 and MM1-11); fair (both intercrystalline and biomoldic in C1-92); fair-very good (mainly interparticle, solutional and intercrystalline porosity in subtidal carbonates in H1-92).

A Palaeocene age is assigned to this member based on micropalaeontological correlation and the foraminiferal assemblage.

ACKNOWLEDGEMENTS

The authors are indebted to the geological manager of the Arabian Gulf Oil Company (Mr. F. Buargoub) for his permission to publish this study. Thanks are also due to the geological laboratory staff members for their technical assistance, especially Mr. A. Salama for thin-section preparation. Our appreciation is also extended to the drafting section, especially Mr. A. Corpuz for his meticulous computer-aided drafting of the diagrams.


PLATE I

(1-6: intertidal facies; 7-9: Subtidal facies) 1- A general view of a dolomitized wackestone, with ghost allochems, and intercrystalline porosity (blue). M1-92 (3590’), plane polarized light, x40.

2- An overview of a dolomitized wackestone, with allochems mainly composed of benthonic foramini fera (Quinqueloculina); light blue patches are biomoldic and intraskeletal porosities, and black crystals of pyrite. Other parts of this rock are mudstone-textured. A1-NC102 (3410’ 6”), plane polarized light, x40.

3- Part of a fenestral pore, probably with gas-escape conduits. A1-NC 102 (3790› 7”), plane polarized light, x40.

4- Detail of a mudstone-textured dolomite, with mosaic fabric. This sample contains good intercrystalline porosity (blue) partly enhanced by dissolution, indicated by the presence of frayed dolomite crystals bordering larger pores. H1-92 (3610-20’), plane polarized light, x32.

5- An overview of lithoclastic dolostone. Lithoclasts (dark brown), composed of foraminiferal dolomite, display a different texture than the surrounding rock fabric, moldic pores (blue, right) are lined with freshwater carbonate cement; dolomite crystals are in the order of 2-5µ across. This sample represents a tidal channel deposit. A1-NC 102 (3383’), plane polarized light, x10.

6- Detail of miliolids, indicative of a restricted environment. MM1-11 (3630-40’), crossed polarized light, x32.

7- Detail of dolomite partly replaced ? by gypsum (light), a relatively common feature restricted to mudstone-textured dolomites in H1-92 (3650’), plane polarized light, x50.

8- An overview of a poorly sorted echinoderm-bearing packstone, with shell fragments (centre), algal ‘grains’ (extreme right), and mudstone-textured lithoclasts (extreme left); echinoderm fragments are common (lower left and upper centre). This sample probably represents a storm deposit, and proximity to a barrier. It occurs within dominantly planktonic limestones. B1-NC 102 (3690-700’), crossed polarized light, x15.

9- An overview of an echinoderm-bearing packstone, showing interparticle porosity (blue, lower centre) and intraskeletal porosity (upper right). H1-92 (3560-70’), plane polarized light, x32


PLATE II

1 -2: subtidal facies; 3: reefal facies; and 4: outer shelf facies

1- Detail of a wackestone-packstone, with common miliolid, representing a semi-restricted environment. C1-92 (3630’), plane polarized light, x35.

2- Interparticle porosity (blue) enhanced by solution. H1-92 (3660-70’), plane polarized light, x60.

3- A coral ? fragment. H1-92 (3560-70’), plane polarized light, x 75.

4- A general view of a calcispherid packstone; Note Planorotalites sp. B1-NC 102 (3700-40’), plane polarized light, x50.


REFERENCES

bAnerJee, S. (1980). Stratigraphic Lexicon of Libya. Ind. Res. Cent., Tripoli, Bull. 13, 300 p.

bArr, F.T. and WeegAr, A.A. (1972). Stratigraphic Nomenclature of the Sirt Basin, Libya. Petrol. Explor. Soc. Libya, Tripoli, 179 p.

DAniels, H.J. and muftAh, A.M. (1993). Sedimentological analysis of Dahra “B” Member in seven wells in Concession NC102, north-central Libya. AGOCO Internal Rept.

dunhAm. R.J. (1962). Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (ed. W.E. Ham). Mem. Am. Assoc. Petrol. Geol., 1, 108-121.

senoussi, E. and sAAd N. (1995). Geological and geophysical evaluation of Dahra “B” Member in Conc. 131 and vicinity. AGOCO Internal Rept., 16 p.

lithofacies of dahra “b” member in concession nc102, sirt basin,...

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