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Middle East Models of Jurassic/Cretaceous Carbonate Systems

SEPM Special Publication No. 69, Copyright ©2000 SEPM (Society for Sedimentary Geology), ISBN 1-56576-075-1, p.287–297.

Abstract

The Minagish Oolite occurs in the Middle Minagish Member of the Minagish Formation (Berriasian-Valanginian) in Kuwait. Ten distinct lithofacies are recognized, which suggest sedimentation on a homoclinal carbonate ramp. A relatively small proportion of the Minagish Oolite (< 15%) consists of oolitic grainstone (Lithofacies 2), and this is confined to the lower part of the oil column. The dominant lithofacies comprises peloidal packstones to grainstones (Lithofacies 3). Sedimentation was highly storm-influenced, with significant reworking of shallow-water, inner-ramp skeletal allochems into the midrramp. The high level of reworking is believed to account for the relatively high proportion of grainstone and poorly washed packstones in the inner mid-ramp setting. The reservoir is interpreted as the product of sedimentation within late highstand, lowstand, and trasgressive systems tracts, which together represent a low- (third?) order relative sea-level change. Within each systems tract, laterally correlatable flooding surfaces at the tops of parasequences are directly overlain by thin units of bioturbated wackestones to packstones (Lithofacies 7). These wackestones to packstones are interpreted as deeper-water, outer-ramp environments, and indicative of higher-frequency, fourth- or fifth-order, cyclicity. There is strong evidence of a southwestward lateral facies change into more argillaceous limestones (“marls”) in the upper part of the Minagish Oolite. The geometry of the transition suggests that it marks the extreme fringe of a shallow-water clastic system. It represents the earliest evidence of delta progradation in the early Cretaceous of the Kuwait area. Evidence of associated shallowing is absent, and it seems that tectonic uplift in the hinterland was more influential than relative sea-level change.

Intense micritization has generated high proportions of microporosity, and it is the distribution of these micropores which mostly influences permeability. The best reservoir facies are grainstones of Lithofacies 2 and 3, where the pore network is macropore-dominated and microporosity is concentrated within micritized allochems. More heterogeneous packstones of Lithofacies 3 and 5 have mixed pore systems, whilst wackestones and packstones of Lithofacies 7 and 8 have micropore-dominated pore networks. In these samples, the pore network is dominated by interparticle micropores, and macroporosi try is rare and isolated. These microporous facies typically form laterally correlatable beds above flooding surfaces and are capable of forming baffles and barriers to vertical transmissibility. Overall, the proportion of facies exhibiting mixed and microporous pore systems increases upwards through the reservoir, and hence there is a corresponding decrease in reservoir quality. During the later stages of production, as the oil-water contact rises, increasingly detailed understanding of the reservoir architecture wil be required to maintain production levels. The lateral facies change at the top of the reservoir allied to increased compartmentalization indicates that a more comprehensive secondary recovery scheme will be required in this part of the reservoir.

Introduction

The Umm Gudair Field, Kuwait (Fig. 1) comprises two separate culminations, East and West Umm Gudair. Where East Umm Gudair straddles the boundary with the Partitioned Neutral Zone, a region jointly administered with Saudi Arabia, it is designated South Umm Gudair Field (Fig. 1). The discovery well was drilled in 1962, and there has been a major expansion of activity since 1993. Hydrocarbons are found at several levels in the Jurassic and Cretaceous sequence, but development has been focused largely on the Lower Cretaceous Middle Minagish Oolite Member reservoir (Fig. 2), which is the object of this study. This work builds upon previous reservoir description of the field (Sungur, 1996), and this paper aims to provide a detailed description of the Minagish Oolite reservoir, outlining lithofacies associations, diagenetic modification, and pore types in a sequence stratigraphic framework. These permit an evaluation of reservoir architecture and the controls on reservoir potential. The reservoir description will be a fundamental tool for long-term management of the Minagish Oolite reservoir.

Fig. 1.

Location of the Umm Gudair Field West Kuwait showing topographic contours.

Fig. 1.

Location of the Umm Gudair Field West Kuwait showing topographic contours.

Fig. 2.

—Stratigraphy of the lower part of the Thamama Group, Kuwait.

Fig. 2.

—Stratigraphy of the lower part of the Thamama Group, Kuwait.

Stratigraphy and paleogeography

The Umm Gudair Field, Kuwait, is located on the southwestern margin of the Kuwait arch and is structurally simple, comprising the two separate anticlinal culminations of East and West Umm Gudair (Carman, 1996). The Minagish Formation was deposited on an eastward-prograding carbonate ramp system on the north-south trending Arabian Shelf (Aisharhan and Nairn, 1986; Longacre and Ginger, 1988). Sedimentation was initiated in the Cretaceous, following transgression of the late Jurassic intrashelf basin, within which interbedded salt and anhydrite-limestone facies of the Gotnia Formation and the overlying Hith Formation were deposited (Ali, 1994). The Minagish Oolite is the Middle Minagish Member of the Lower Cretaceous (Berriasian-Valanginian) Minagish Formation (Sungur, 1996) (Fig. 2). The Lower Minagish Member is a low-permeability, dolomitized section, and dense limestone and marls of the Upper Minagish Member form the seal to the productive Minagish Oolite (Sungur, 1996). The Minagish Oolite is equivalent to the Ratawi Oolite reservoir of the Partitioned Neutral Zone (Longacre and Ginger, 1988), the Ratawi reservoirs of northern Saudi Arabia (Ayres et al., 1982), and the Yamama Formation reservoirs of southern Iraq (Sadooni, 1993).

Database and Analytical Techniques

Detailed core description was undertaken from the reservoir intervals in seven wells. One hundred and twelve thin sections, prepared from core plug trims, were examined from wells A, C, D, E, and F and a further thirty-eight thin sections from four wells. Thin sections were impregnated with blue-dyed resin and stained for carbonate with combined alizarin red-S and potassium ferri-cyanide (Dickson, 1965). Modal analysis was conducted on an all samples, by counting 200 points for mineral identification. Macroporosity was recorded over the same interval using isolator channels. A stage interval of 0.33 mm and track spacing of 2 mm was used to ensure a minimum of three traverses of the thin section. Core analysis data was available from all wells except well B.

Pore systems were examined in detail by scanning electron microscopy of thirty-five samples. Pieces of core that had been cleaned and gold coated prior to analysis were examined using a JEOL JSM-35CF microscope with a PGT System 4 EDS attachment. Operating conditions were set at 15 kV. Examination of ten samples under cathodoluminescence employed a Technosyn cold-cathode luminoscope 8200 MkII, fitted to a Nikon Optiphot-pol microscope. The vacuum was held at 0.2 atm and the beam current at approximately 650 μm. An accelerating voltage of 15-20 kV was used.

Lithofacies Associations and Sequence Stratigraphic Framework

Ten lithofacies can be identified in the Minagish Oolite Member, based upon core description and supported by microfacies analysis (Table 1). These lithofacies effectively form a continuous spectrum, with gradational boundaries between different types, broadly relating to deposition in increasing water depths. Individual lithofacies are described below and their depositional environment assessed with respect to the nomenclature of Burchette and Wright (1992).

Table 1.

—Summary of lithofacies characteristics for the Minagish Oolite in the Umm Gudair Field.

LithofaciesComponent lithotypesBiogenic/ sedimentary structuresVolume of core datasetDepositional environment
Thinly interlaminated peloidal packstones, grainstones, and thin mudstone beds thinly laminated 0.1% Perltidal/lagoonal 
Oolitic grainstones some trough cross-bedded 13.9% Inner-ramp sand shoal/ storm reworked into mid-ramp 
Peloidal packstones-grainstones bioturbated bed tops 45.6% Storm-dominated, mid-ramp sand sheet 
Peloidal packstones-grainstones cross-laminated-cross-bedded 2.7% Wave- and storm-dominated sedimentation in the inner to mid-ramp 
Shell rich packstones and grainstones — 2.9% Reworked rudist bloherms developed within a relatively low-energy, deep mid-ramp setting 
Bioclastlc peloidal packstones-grainstones — 6.6% Mid- to outer-ramp 
Bioclastic peloidal wackestones-packstones bioturbated 17.2% Low energy, outer-ramp. Representative of deepening events — immediately overlie flooding surfaces 
Bioclastic wackestones, peloidal packstones and grainstones dissected by coarsegrained sediment-filled burrows 9.4% Usually associated with deepening and outer-ramp sedimentation. Firmgrounds 
Nodular wackestones burrowed 1.2% Deep-water-outer-ramp 
10 Green-gray argillaceous mudstones — 0.5% Distal fringes of fluviodeltaic system 
LithofaciesComponent lithotypesBiogenic/ sedimentary structuresVolume of core datasetDepositional environment
Thinly interlaminated peloidal packstones, grainstones, and thin mudstone beds thinly laminated 0.1% Perltidal/lagoonal 
Oolitic grainstones some trough cross-bedded 13.9% Inner-ramp sand shoal/ storm reworked into mid-ramp 
Peloidal packstones-grainstones bioturbated bed tops 45.6% Storm-dominated, mid-ramp sand sheet 
Peloidal packstones-grainstones cross-laminated-cross-bedded 2.7% Wave- and storm-dominated sedimentation in the inner to mid-ramp 
Shell rich packstones and grainstones — 2.9% Reworked rudist bloherms developed within a relatively low-energy, deep mid-ramp setting 
Bioclastlc peloidal packstones-grainstones — 6.6% Mid- to outer-ramp 
Bioclastic peloidal wackestones-packstones bioturbated 17.2% Low energy, outer-ramp. Representative of deepening events — immediately overlie flooding surfaces 
Bioclastic wackestones, peloidal packstones and grainstones dissected by coarsegrained sediment-filled burrows 9.4% Usually associated with deepening and outer-ramp sedimentation. Firmgrounds 
Nodular wackestones burrowed 1.2% Deep-water-outer-ramp 
10 Green-gray argillaceous mudstones — 0.5% Distal fringes of fluviodeltaic system 

Lithofacies 1

This lithofacies forms a minor component of the cored dataset (Table 1). It is markedly heterolithic and comprises a hybrid association of finely interlaminated peloidal packstones and grainstones and sparsely bioclastic mudstone beds (Fig. 3A). A relatively diverse, though not abundant, range of skeletal allochems within the packstone and grainstone components includes foraminifera (miliolids, uniserial and biserial), algal debris (Lithocodium/Bacinella, dasycladaceans, and Permocalculus), and echinoid fragments. Beds are mostly < 1 m thick, and this lithofacies is intimately associated with Lithofacies 2. Core analysis plugs are biased to coarser cleaner laminae, and are not representative of the mudstone facies. The packstones and grainstones typically exhibit very high helium porosities (23.4-24.7%; Fig. 6) and permeabilities (97-250 md; Fig. 4A). It is anticipated that the mudstone beds have lower porosities and insignificant permeabilities.

Fig. 3.

—Selected core photos of lithofacies within the Minagish Oolite. A) Lithofacies 1. Mudstone with thin, coarse-grained peloidal and oolitic laminae (arrows). Dissected by burrow (a). B) Lithofacies 2. Laminated oolitic grainstone dissected by subtle burrow (a). C) Lithofacies 3. Laminated peloidal grainstone. D) Lithofacies 5. Skeletal packstone with large fragment of “brain” coral (a) and rudist shell (b). Matrix contains well defined burrows (c). E) Lithofacies 7. Skeletal wackestone with well developed stylolites (arrows). Note the intense cementation that surrounds these stylolitized zones. F. Lithofacies 8. Skeletal wackestone with well preserved Thallasinoides-type burrow network (a), which has focused differential cementation.

Fig. 3.

—Selected core photos of lithofacies within the Minagish Oolite. A) Lithofacies 1. Mudstone with thin, coarse-grained peloidal and oolitic laminae (arrows). Dissected by burrow (a). B) Lithofacies 2. Laminated oolitic grainstone dissected by subtle burrow (a). C) Lithofacies 3. Laminated peloidal grainstone. D) Lithofacies 5. Skeletal packstone with large fragment of “brain” coral (a) and rudist shell (b). Matrix contains well defined burrows (c). E) Lithofacies 7. Skeletal wackestone with well developed stylolites (arrows). Note the intense cementation that surrounds these stylolitized zones. F. Lithofacies 8. Skeletal wackestone with well preserved Thallasinoides-type burrow network (a), which has focused differential cementation.

Fig. 4.

—Reservoir properties of selected samples from the Minagish Oolite according to lithofacies. Symbols and numbers represent lithofacies.

Fig. 4.

—Reservoir properties of selected samples from the Minagish Oolite according to lithofacies. Symbols and numbers represent lithofacies.

Lithofacies 1 caps upward-shoaling parasequences, directly overlying oolitic grainstones of Lithofacies 2 (Fig. 5A), implying a shallow-water origin. The sparse skeletal component to the mudstone beds suggests that deposition was within a low-energy, probably lagoonal, environment with cleaner, more skeletal packstone and grainstone laminae deposited by washover into the back-shoal, behind the oolitic sand sheet, during storms.

Lithofacies 2

Lithofacies 2 accounts for a significant component of the cored dataset (Table 1). It comprises oolitic grainstones and peloidal oolitic grainstones (Fig. 3B), which are dominated by ooids, and rarely pisoids, with subordinate peloids. Although ooids usually maintain remnant concentric and radial micro-structure, they are heavily micritized (Fig. 6A, B). Intraclasts and composite grains are also present, along with a low-abundance skeletal assemblage including foraminifera (dominantly miliolids) and echinoid and bivalve debris, all commonly with an oolitic coating. The dominant form of this lithofacies is as upward-fining beds, approximately 1 m thick (Fig. 5A). A second type, comprising trough cross-bedded oolites (locally bidirectional), is also recognized. Cross-bed sets are typically less than 0.5 m thick, and some set boundaries host simple burrows (Fig. 3B). Interlaminated medium- to coarse-grained ooids and very fine- to fine-grained peloids commonly define lamination (Fig. 6B).

Fig. 5.

—Common lithofacies associations in the Minagish Oolite.

Fig. 5.

—Common lithofacies associations in the Minagish Oolite.

Fig. 6.

—Selected photomicrographs of microfacies and diagenetic features. A) Oolitic grainstone (Lithofacies 2), dominated by heavily micritized ooids (a), which display some remnant texture. Bored bivalve shell is encrusted by Lithocodium/Bacinella (b, c). Interparticle porosity is partially occluded by nonferroan dolomite often following compaction (d), which has destroyed pore throats (e). Field of view, 4 mm. B) Peloidal oolitic grainstone (Lithofacies 2) comprising very coarse-grained ooids (a) and composite grains (b), and fine-grained peloids (c), which are principally distributed along laminae. Field of view, 4 mm. C) Skeletal peloidal packstone (Lithofacies 3), dominated by peloids(a) and benthic foraminifera (b, miliolids and Trocholirm) with intraclast encrusted by Lithocodium/Bacinella (arrows). Allochems are coated by thin fringing nonferroan dolomite cement and primary and secondary interparticle porosity occluded by subsequent sparry nonferroan dolomite (c). Field of view, 4 mm. D) Skeletal peloidal packstone (Lithofacies 3) with diverse skeletal assemblage including foraminifera (a, dominantly miliolids), echinoid fragments, micritized bivalve fragments, and Lithocodium/Bacinella. Micrite matrix has been intensely neomorphosed (b). Field of view, 4 mm. E) Skeletal packstone (Lithofacies 5), with a less diverse skeletal assemblage than in lithofacies 3 and dominated by fragmented rudist debris (a). Note the complete occlusion of biomolds by nonferroan dolomite (b). Field of view, 4 mm. F) Skeletal peloidal packstone-wackestone, with a low-abundance, low-diversity skeletal assemblage comprising bivalve debris (a) and, in this sample, a reworked fragment of Permocalculus (b). Peloids are potentially intensely micritized forams. Field of view, 4 mm.

Fig. 6.

—Selected photomicrographs of microfacies and diagenetic features. A) Oolitic grainstone (Lithofacies 2), dominated by heavily micritized ooids (a), which display some remnant texture. Bored bivalve shell is encrusted by Lithocodium/Bacinella (b, c). Interparticle porosity is partially occluded by nonferroan dolomite often following compaction (d), which has destroyed pore throats (e). Field of view, 4 mm. B) Peloidal oolitic grainstone (Lithofacies 2) comprising very coarse-grained ooids (a) and composite grains (b), and fine-grained peloids (c), which are principally distributed along laminae. Field of view, 4 mm. C) Skeletal peloidal packstone (Lithofacies 3), dominated by peloids(a) and benthic foraminifera (b, miliolids and Trocholirm) with intraclast encrusted by Lithocodium/Bacinella (arrows). Allochems are coated by thin fringing nonferroan dolomite cement and primary and secondary interparticle porosity occluded by subsequent sparry nonferroan dolomite (c). Field of view, 4 mm. D) Skeletal peloidal packstone (Lithofacies 3) with diverse skeletal assemblage including foraminifera (a, dominantly miliolids), echinoid fragments, micritized bivalve fragments, and Lithocodium/Bacinella. Micrite matrix has been intensely neomorphosed (b). Field of view, 4 mm. E) Skeletal packstone (Lithofacies 5), with a less diverse skeletal assemblage than in lithofacies 3 and dominated by fragmented rudist debris (a). Note the complete occlusion of biomolds by nonferroan dolomite (b). Field of view, 4 mm. F) Skeletal peloidal packstone-wackestone, with a low-abundance, low-diversity skeletal assemblage comprising bivalve debris (a) and, in this sample, a reworked fragment of Permocalculus (b). Peloids are potentially intensely micritized forams. Field of view, 4 mm.

Helium porosities (17.4-28.0%; Fig. 4A) and horizontal permeabilities (51.0-553 md; Fig. 4A) are mostly very good in Lithofacies 2. On open-hole logs, this facies expresses a distinctive bow-shaped sonic response and a very low gamma-ray signature.

The cross-bedded oolitic grainstones are interpreted as representative of in situ deposition in very shallow water, with sedimentation influenced by wave and tidal currents. Some units may represent tidal shoals and/or channels in flood/ebb deltas, suggestive of a nearshore inner-ramp setting, and possibly characteristic of a shallow barrier to a broad lagoon (cf. Wright, 1986). The more massive, upward-fining units are thought to be storm-reworked ooids, deposited offshore of the main area of ooid production, probably within the shallow mid-ramp.

Lithofacies 3

Lithofacies 3 is the dominant lithofacies in the Minagish Oolite (Table 1). It comprises peloidal grainstone and skeletal peloidal packstone/grainstone (Figs. 3C, 6C, D), intraclastic peloidal packstone/grainstone, and peloidal packstone. A high-abundance, high-diversity, intensely micritized skeletal assemblage is recognized, which is dominated by benthic foraminifera (miliolids, textularids, rotalids, Nautiloculina, and Trocholina) and algal debris (principally Lithocodium/Bacinella, less commonly, Permocalculus, and dasycladaceans), with slightly less abundant bivalve, echinoid, gastropod, coral, and stromatoporoid debris (Fig. 6C, D). Partial micritization and allochem morphology suggests many peloids are micritized skeletal allochems (especially forams). Shallow water skeletal allochems, in particular miliolids and Lithocodium/Bacinella, are significantly more abundant in Lithofacies 3 in the tops of upward-coarsening parasequences than in the bases, within most of the reservoir. Towards the top of the reservoir, however, composition is not significantly different between the tops and the bases of individual parasequences. Locally, bimodal and trimodal beds < 1 m thick are recognized which are characterized by very coarse-sand to pebble-grade, Lithocodium-encrusted, skeletal allochems and/ or intraclasts, mostly of peloidal packstone and grainstone.

Beds are typically 1-2 m thick and upward fining, building up into larger-scale upward-coarsening sequences (Fig. 5B, C). Individual beds may be cross-bedded and are occasionally dissected on their upper surfaces by cylindrical and branching Thalassinoides-type burrows, implying firmground development. Very similar facies dominate the Ratawi Oolite reservoir of Wafra Field (Longacre and Ginger, 1988) and the Yamama Formation reservoirs of southern Iraq (Sadooni, 1993). Lithofacies 3 has heterogeneous reservoir properties, with poor to excellent helium porosities (2.5-35.3%; Fig. 4B) and horizontal permeabilities (0.01-1652 md; Fig. 4B). It is recognized by a consistently low gamma-ray curve, and somewhat variable sonic and density log porosity profiles.

The high degree of sediment reworking in these upward-fining sequences, with localized cross-bedding, suggests that these beds are amalgamated storm beds, forming a broad belt of carbonate sand in the mid-ramp (cf. Burchette and Wright, 1992). Very coarse-grained, trimodal beds probably represent individual storm events, and can be correlated to adjacent wells. Bathymetric ranges for both miliolids and Lithocodium/Bacinella suggest production of these allochems largely in water depths of up to 10 m, possibly up to 30 m, with production decreasing at increasing water depths as the depth of light penetration was exceeded (Banner and Simmons, 1994). Deposition of Lithofacies 3 between fair-weather and storm-weather wave base at water depths of potentially up to 50 m (e.g., Tucker and Wright, 1990) implies significant offshore transport of shallower-water allochems, from above fair-weather wave base, into the mid ramp. The more diverse skeletal assemblage described from facies towards the tops of parasequences suggests a progressive loss of these allochems with depth, potentially a function not only of decreasing light penetration, but also of the waning effects of storm currents at depth. The decrease in skeletal diversity towards the top of the reservoir might appear to imply a subtle overall upward deepening. A lateral transition into argillaceous limestones of Lithofacies 10, however, indicates that the lower skeletal diversity is associated with an increased clay component, and that siliciclastic poisoning is also involved.

Lithofacies 4

This lithofacies is volumetrically minor (Table 1) and comprises thinly laminated, very well sorted, and very fine- to finegrained peloidal grainstones and packstones. The lamination is typically low angle to horizontal, but hummocky and swaly cross- stratification is also commonly recognized. These intervals have a very low gamma signature but very high porosities and high resistivities in the oil zone. Hummocky and swaly cross-stratification is consistent with wave- and storm-dominated deposition of this facies in the mid-ramp (cf., Burchette and Wright, 1992).

Lithofacies 5

This is a distinctive lithofacies, which forms a minor component of the total cored interval (Table 1). Lithofacies 5 comprises almost entirely skeletal peloidal packstone (Fig. 3D). Skeletal allochems are dominated by rudist and “brain” coral fragments (Fig. 3D) and also include fragmented scleractinian corals, bivalves (including large fragments of Inoceramus), echinoid and crinoid fragments, and minor foraminifera (rotalids, rare miliolids) (Fig. 6E). No unequivocal examples of rudists and corals in life position were observed. Beds are typically < 1 m thick, commonly resting directly above flooding surfaces (Fig. 6C), and are structureless except for a weak background bioturbation mottling. Reservoir properties are variable for this lithofacies, but helium porosities are typically very good (12.8-24.2%; Fig. 4C) and horizontal permeabilities poor to moderate (1.6-395 md; Fig. 4C). Lithofacies 5 does not carry a distinctive open-hole log signature.

Longacre and Ginger (1988) interpreted a lithology similar to Lithofacies 5 as providing evidence of a reef a short distance beyond the eastern margin of the Wafra Field. A somewhat different interpretation is proposed here. The observed skeletal assemblage is suggestive of localized buildups, possibly bio-herms, of rudists and corals. Fragmentation implies breakup of these bioherms potentially during storms, although the very coarse size of much of the skeletal debris suggests transportation was minor. Given that rudists largely require relatively low rates of sediment accumulation in order to remain stable on or within the sediment (Skelton, 1997), they often favor quite low-energy environments, and are easily fragmented. Elsewhere in the Middle East, rudist bioherms are found towards the top of upward-shallowing cycles (e.g., Burchette and Britton, 1985). The relative absence of shallow-water biota and the characteristic position of this facies towards the bases of upward-shallowing parasequences are more consistent, however, with quieter-water accumulation of these buildups in the deep mid-ramp. A similar association is seen in the Cretaceous of southwest Texas, where rudist bioherms are found closely overlying maximum flooding surfaces (Trevor Burchette, personal communication). This would have permitted growth under relatively quiet conditions, with reworking only during major storms.

Lithofacies 6

Lithofacies 6 is a minor component of the total cored interval of the Minagish Oolite (Table 1). It is dominated by skeletal peloidal packstones and typically contains abundant foramin-ifera (miliolids, textularids, and rotalids) as well as echinoid debris and Lithocodium/Bacinella. Helium porosities are mostly very good (16.5-26.9%; Fig. 4C), whilst horizontal permeabilities are more variable (1.70-507 md; Fig. 4C). Lithofacies 6 does not have a distinctive open-hole log signature for identification in uncored wells. The muddy texture and relatively diverse skeletal assemblage within this facies is suggestive of mid to outer ramp sedimentation with a continuous influence on deposition by storms.

Lithofacies 7

Lithofacies 7 is a volumetrically significant lithofacies (Table 1). It comprises biorurbated peloidal packstone and wackestone (Fig. 3E), with a low-abundance skeletal assemblage including foraminifera (rotalids, rare miliolids), often fragmented and abraded bivalve and echinoid debris, and calcispheres (Fig. 6F). Towards the top of the reservoir, it becomes increasingly argillaceous, with a corresponding increase in stylolitization. Lithofacies 7 typically forms thin (< 1 m) beds that have a characteristic, pervasive bioturbation mottling. It abruptly overlies oolitic grainstones of Lithofacies 2 and skeletal peloidal packstones and grainstones of Lithofacies 3 (Fig. 5A-D). Lithofacies 7 has good measured helium porosities (2.5-27.8%; Fig. 4C) but poor horizontal permeabilities (0.01-284 md; Fig. 4C). Within the oil leg, this facies has lower resistivity than shallower, cleaner, grain-supported lithofacies, reflecting higher water saturations. Where present, the increase in argillaceous debris results in a gamma-ray response relative greater than other, cleaner facies.

The low abundance of skeletal allochems and the muddy, biorurbated fabric suggests low-energy sedimentation under slow rates of sediment accumulation. Such conditions may occur within a lagoonal environment in the inner ramp, or farther offshore in the outer ramp. Given the absence of a diagnostic shallow-water skeletal assemblage, the presence of calcispheres, and the often highly fragmented and abraded nature of the bivalve and echinoid debris, this facies is interpreted as a deep-water outer ramp packstone—wackestone. Because many of the basal contacts of Lithofacies 7 abruptly overlie Lithofacies 2 and 3 (Fig. 5), they mark the bases of parasequences and are interpreted as deepening events above flooding surfaces.

Lithofacies 8

Lithofacies 8 makes up a significant component of the cored dataset (Table 1). It comprises peloidal and skeletal packstones and wackestones, with a low-abundance and low-diversity skeletal assemblage that includes foraminifera (principally rotalids) and echinoderm and bivalve debris. This lithofacies is distinguished by prominent coarser-grained burrow fills (including Thalassinoides), which dissect the fine-grained background litholo-gies (Fig. 3F). Towards the top of the reservoir section, many examples are thin (decimeter scale) and preferentially calcite cemented (Fig. 3F), with the degree of cementation increasing upwards. In several examples, preserved burrow diameters likewise increases upward, which is consistent with early cementation retarding compaction. Furthermore, some burrows contain distinctive dark green glauconitic grains and phosphatized skeletal allochems. Beds are generally < 1 m thick. Helium porosities are mostly very good (9.2-27.2%; Fig. 4C) and horizontal permeabilities poor-moderate (0.70-771 md; Fig. 4C). Except where Lithofacies 8 is dissected by numerous stylolites, resulting in an increase in the gamma-ray response and a reduction in log porosity, it is difficult to differentiate this facies on open-hole logs.

The low-diversity skeletal assemblage and absence of shallow-water indicators in this lithofacies is consistent with sedimentation in the outer ramp. The burrows record decreases in sed ¡mentation rate, either under decreasing flow following storms or, more significantly, during deepening. In these cases, the burrowed surface is a flooding surface, and the association of cementation, burrowing, upward increases in burrow diameter, and glauconite implies firmground development.

Lithofacies 9

Lithofacies 9 makes up a minor proportion of the total cored interval (Table 1). It comprises characteristically dense, nodular wackestones and mudstones, which have a low-abundance, low-diversity skeletal assemblage comprising calcispheres and rare red algae and benthic forams. Common, incipient clay-rich stylolitic seams are common and result in low porosity zones and erratic gamma-ray profiles on open-hole logs. The presence of calcispheres and the muddy texture of this lithofacies implies relatively deep-water sedimentation, possibly at > 60 m water depth (Banner and Simmons, 1994). The association of this fades with better constrained deeper-water Lithofacies 7 and 8 is consistent with this and suggests sedimentation in the outer-ramp or basinal environment.

Lithofacies 10

Lithofacies 10 forms a negligible component of the total cored interval (Table 1). It comprises green-gray argillaceous mud-stones to wackestones and is largely featureless. A monospecific skeletal assemblage consists of localized concentrations of in situ Exogyra. A high but erratic gamma-ray signature and higher neutron than density porosities distinguish this lithofacies.

The muddy texture of this facies indicates a quiet-water environment, and the argillaceous content suggests close proximity to a contemporaneous clastic environment. Exogyra has been described from more basinal settings (e.g., Kimmeridge Clay of western Europe; N. Cross, personal communication), which seems at odds with an increase in clastic sediment supply. Conversely Hughes (1997), in a comparison between the Shuaiba Formation and the modern Great Pearl Bank Barrier of the Arabian Gulf, depicts Exogyra as a component of lagoonal faunas behind an Albian rudist-dominated barrier. It is difficult, therefore, to make a definitive interpretation on the basis of deposi-tional character alone. A more helpful observation is that Lithofacies 10 forms the basal beds of the overlying Upper Middle Minagish Member, and that it grades laterally into mid- to outer-ramp sediments of Lithofacies 3, 7, and 8. The nature of the transition is such that similar water depths must be invoked for Lithofacies 10. The depositional geometries in particular render it unlikely that Lithofacies 10 represents a deeper-water (necessarily intrashelf-basin) setting. Instead it is best interpreted as representative of the extreme fringes of a shallow-water clastic system.

A similar lateral facies change has been recognized in the uppermost beds of the Ratawi Oolite reservoir in Wafra Field (Longacre and Ginger 1988), suggesting that the change is of regional significance. For these reasons, the argillaceous limestones can be considered to represent the earliest evidence of delta progradation in the Early Cretaceous in Kuwait.

Lithofacies Organization and Trends

Lithofacies similar to those described in this study have been described from the Ratawi Oolite reservoir of Wafra Field (Longacre and Ginger, 1988) and the Yamama Formation reservoirs of southern Iraq (Sadooni, 1993). The Minagish Oolite can be broken down into four principal units, each comprising characteristic lithofacies associations.

The uppermost unit is separated from the underlying unit on the basis of the lateral facies change described for Lithofacies 10. Relatively thin interbeds of Lithofacies 3,7, and 8 characterize the central to northeastern part of the field, whereas the southwestern part is composed exclusively of nonreservoir argillaceous limestones. The base of this unit is picked at an easily correlated, strongly cemented bed that equates to the base of the argillaceous limestones in the southwestern part of the field.

The second unit, which is also the main reservoir section, is marked by a basal flooding surface in which outer-ramp pack-stones of Lithofacies 7 abruptly overlie oolitic grainstones (Lithofacies 2). The most common lithofacies association in this second unit comprises meter-scale, stacked upward-fining beds of Lithofacies 3 overlying Lithofacies 7 (Fig. 6B). Less commonly Lithofacies 4 separates beds of Lithofacies 3 and 7, or Lithofacies 5 abruptly overlies Lithofacies 3 (Fig. 5C). Over-all, this results in a common organization of broadly upward-coarsening parasequences.

A third unit (the so-called “main oolite”), beneath the flooding surface at the top of the upper unit, is separated from the basal unit by an abrupt facies change from outer-ramp packstones and wackestones (Lithofacies 7) to inner-ramp oolitic grainstones (Lithofacies 2). Within this main oolite, the principal lithofacies association is between Lithofacies 2 and Lithofacies 7, which forms the basal part (up to 1.5 m thick) of thick (up to 12 m) homogeneous sections of oolitic grainstone (Fig. 5A). Thin beds of Lithofacies 1 are recognized, particularly towards the top of this oolitic section (Fig. 5A). Finally, the lower unit, beneath the main oolite, comprises large-scale coarsening-upward units with Lithofacies 7 at the base, grading up to coarser-grained Lithofacies 3 and 6 (Fig. 5D) with no ooids recognized beneath the sharp base of the oolitic sandsheet.

Diagenetic Modification

The Minagish Oolite Member has undergone significant diagenetic modification, and a number of events have exerted an important control on the reservoir potential of individual facies.

Marine Diagenesis

The moststriking feature of the Minagish Oolite is the intense micritization that both skeletal and nonskeletal allochems have undergone as a result of boring in the submarine environment (Fig. 5A-C). In many cases, this has rendered skeletal allochems unrecognizable, such that true peloids, for example, cannot be readily differentiated from completely micritized foraminifera (Fig. 6C). Isopachous, acicular, or prismatic nonferroan calcite and dolomite locally coat micritized skeletal and nonskeletal allochems (Fig. 6C), indicating that boring preceded cementation (Fig. 7). Isopachous cements are most common within grainstones of Lithofacies 2, 3, and 4, which were deposited in the relatively high-energy inner and shallow mid ramp. They are therefore interpreted as marine cements precipitated under agitated, wave- and storm-dominated conditions.

Fig. 7.

Paragenetic sequence for the Minagish Oolite.

Fig. 7.

Paragenetic sequence for the Minagish Oolite.

Burial Diagenesis

Syntaxial overgrowths and equant sparry nonferroan dolomite and calcite (Fig. 6A, C, E) occlude primary and secondary macropores. Syntaxial overgrowths preserve interparticle volumes and inhibit compaction, whilst equant sparry cements occlude compactionally modified pores (Fig. 6A). Syntaxial overgrowths envelop isopachous marine cements and are themselves enveloped by equant sparry calcite. Under cathodoluminescence, syntaxial overgrowths are typically nonluminescent, suggesting precipitation from oxidizing fluids; potentially meteoric porewaters driven downdip from the west or southwest, which is perceived to be more landward (Sungur, 1996). In contrast, equant sparry cements exhibit a finely zoned, dull-bright luminescence more indicative of precipitation from evolved, reducing burial fluids (Machel and Burton, 1991).

Within finer-grained, muddier samples (e.g., Lithofacies 7 and 8), small, poorly connected primary interparticle macropores did not permit a ready throughput of diagenetic fluids. Significant pore-occluding cementation was, therefore, inhibited and compactional modification was more intense. Dense concentrations of stylolites are commonly observed in Lithofacies 7 and 8, particularly towards the top of the reservoir, and are surrounded by calcite cemented zones up to 10 cm thick (Fig. 3E). Micrite adjacent to stylolites has undergone intense neomorphism from the carbonate-saturated fluids generated by pressure dissolution. SEM analysis revea Is that the size and crystallinity of micrite is increased from a few microns to up to 10 urn, such that interparticle microporosity has been destroyed.

Hydrocarbon is commonly concentrated along stylolites, where it is often diffusely replaced by pyrite (Fig. 7), suggesting that hydrocarbon emplacement at least partly preceded significant pressure dissolution. Few diagenetic events postdate compaction, but rare ferroan dolomite, with a saddle morphology, is precipitated along stylolites. Fracturing in the Minagish Oolite is not significant, but impersistent fractures are seen locally terminating against stylolites, implying a syncompactional or postcompactional origin (Fig. 7). These fractures are often partially cemented by nonferroan calcite, which also envelops saddle dolomite. A postcompactional hydrocarbon charge cannot be ruled out, and the corresponding circulation of organic acids may have generated some late-phase secondary macroporosity.

Pore Systems

The Minagish Oolite Member is highly microporous, and therefore similar to many Lower Cretaceous carbonates in the Middle East (e.g., Longacre and Ginger, 1988; Moshier, 1989). Helium porosities differ little among the different lithofacies (Fig. 5) except where porosity has been occluded by carbonate cements. This has two important implications: the permeability of individual lithofacies cannot be implied by its porosity, and it is very difficult to differentiate individual lithofacies from porosity logs. It is therefore vital to understand the nature and distribution of pore types in order to understand the controls on reservoir potential.

The best reservoir rocks are the grainstones and clean pack-stones of Lithofacies 2 and 3, and less commonly Lithofacies 1,4, and 6 (Fig. 4). These rocks have a well preserved primary interparticle macropore network, which may be enhanced by dissolution of skeletal allochems that were formerly composed of arago- nite and high-magnesium calcite (Fig. 8A). Microporosity in these samples is concentrated largely in micritized allochems. Where there is a greater volume of micrite, mostly within Lithofacies 3,5, and 6, the pore system is more heterogeneous. Primary interparticle macropores are smaller than in cleaner samples, and less well connected. Secondary macropores, in contrast, are typically well developed where large skeletal allochems have been dissolved. These biomolds have commonly been enhanced by matrix dissolution, creating channelized and vuggy pores, resulting in a strong contrast in pore sizes (Fig. 8B). Microporosity in these samples is both intraparticle, within micritized allochems, and interparticle, within the micrite, and permeabilities are therefore slightly lower than the more macroporous grainstones (Fig. 4). The poorest-quality rocks are the muddiest packstones and wackestones of Lithofacies 7 and 8 (Fig. 4C), which are dominated by interparticle micropores (Fig. 8C). These rocks contain only rare, isolated macropores, which are generally intraparticle, and therefore support only minimal flow. Sadooni (1993) described comparable pore systems in the equivalent Yamama Formation of Iraq, although he reported only microporosity (his “matrix porosity”) in mud-grade sediments.

Fig. 8.

—Generalized pore types in Minagish Oolite. A) Oolitic grainstone with well preserved primary interparticle macroporosity. Microporos-ity is dominantly intraparticle, within micritized allochems. B) Skeletal peloidal packstone dominated by large (millimeter-scale) macropores created by dissolution of skeletal allochems and enlarged by subsequent dissolution of the surrounding matrix. Isolated biomolds are also recognized. Microporosity is intraparticle, within micritized allochems, and interparticle, within micrite matrix. C) Peloidal packstone-wackestone, with a pore system dominated by interparticle micropores within the micrite matrix.

Fig. 8.

—Generalized pore types in Minagish Oolite. A) Oolitic grainstone with well preserved primary interparticle macroporosity. Microporos-ity is dominantly intraparticle, within micritized allochems. B) Skeletal peloidal packstone dominated by large (millimeter-scale) macropores created by dissolution of skeletal allochems and enlarged by subsequent dissolution of the surrounding matrix. Isolated biomolds are also recognized. Microporosity is intraparticle, within micritized allochems, and interparticle, within micrite matrix. C) Peloidal packstone-wackestone, with a pore system dominated by interparticle micropores within the micrite matrix.

Discussion

Reservoir Architecture

Previous studies have suggested that deposition of the Minagish Oolite Member was on a broad, easrward-prograding carbonate ramp (Alsharhan and Nairn, 1986; Longacre and Ginger, 1988; Sungur, 1996), and the results of this study are consistent with this. Understanding the depositional evolution of the Minagish Oolite is based upon interpretation of vertical successions and fieldwide correlations, themselves based upon recognition of key surfaces. These are principally flooding surfaces (Van Wagoner et al., 1990), which are most commonly recognized where outer-ramp packstones and wackestones of Lithofacies 7 abruptly overlie shallow mid-ramp to inner-ramp packstones and grainstones of Lithofacies 2 and 3. Less commonly, forced-regression surfaces (Posamentier et al., 1992) are recognized, typically where inner-ramp coarse-grained grainstones of Lithofacies 2 or 3 directly overlie outer-ramp bioturbated packstones and wackestones of Lithofacies 7.

An approximate Berriasian-Valanginian age is assigned to the Minagish Oolite (Sungur, 1996). These two stages have a combined duration of approximately 10-12 million years (Haq et al., 1987; Harland et al, 1990). This duration suggests that up to three third-order cycles could be represented in the Minagish Formation as a whole. Haq et al. (1987) interpreted two major falls in global sea level at c. 126 and c. 128.5 Ma, respectively, sandwiching the Berriasian-Valanginian boundary. These two falls can be identified in the Cretaceous of Oman and U.A.E. (Aziz and El-Sattar, 1997), although absolute ages of c. 139 and c. 141.5 Ma are proposed there (Vahrenkamp, 1996, using time scale of Harland et al., 1990). Irrespective of their absolute age, these dates suggest that major falls in sea level could have influenced deposition and stratigraphic architecture of the Minagish Oolite. As described, detailed correlation suggests that the Umm Gudair Field can be divided into four distinct units: a basal unit, the main oolite, the main reservoir unit above the oolite, and a fourth unit characterized by a lateral facies change (Fig. 9). These units are interpreted as systems tracts within a single, low- (?third) order cycle. The component lithofacies associations form stacked, broadly upward-coarsening parase-quences, interpreted to be representative of fourth- or fifth-order sea-level fluctuations.

Fig. 9.

—Schematic diagram illustrating the reservoir architecture for Minagish Oolite in the Umm Gudair Field.

Fig. 9.

—Schematic diagram illustrating the reservoir architecture for Minagish Oolite in the Umm Gudair Field.

Insufficient core data makes correlation of the unit below the oolite difficult. By comparison with the Minagish Field (Davies and Hollis, 1996), however, it appears to be progradational and may correspond to the upper part of a highstand systems tract (HST). The main oolite comprises three upward-shallowing and -thinning units, separated by deeper-water facies. Thickness variations between the two wells with the best core coverage over this interval are suggestive of shingled geometries between wells A and C (Fig. 9), though 3D seismic data to confirm this observation were not available at the time of the study. Shingled geometries, if proven, would imply progradation of the oolitic sandsheet, either within the late HST or within a lowstand systems tract (LST). The former model would suggest increased progradation of the oolitic sandsheet in response to a progressive decrease in accommodation space as the rate of sea-level rise decreases. The absence of a sequence boundary, either as an exposure surface or manifested by an increase in the abundance of meteoric cements, which could be representative of the subsequent lowstand, is inconsistent with such a model.

The marked basinward shift in facies at the base of the main oolite can be interpreted as a surface of forced regression at the top of the late HST, placing the main oolite within a lowstand systems tract. Progradation of the oolitic sandsheet would again have taken place in response to the limited accommodation space, and evidence of exposure might be anticipated to the southwest, in a more proximal position on the ramp. This model of deposition within the LST is consistent with the basinward migration of facies belts within the LST discussed by Tucker et al. (1993) and Burchette and Wright (1992). It is comparable to the shelf-margin systems tract described in the Upper Jurassic Hadriya Formation of northern Saudi Arabia by Koepnick et al. (1994). The latter terminology is not followed herein because there is no evidence of a shelf margin either in the Umm Gudair itself or on a more regional scale, in the time-equivalent Yamama Formation (Sadooni, 1993).

The unit above the main oolite can easily be correlated across the Umm Gudair Field and is dominated by stacked, broadly upward-coarsening parasequences (Fig. 9). These are composed largely of outer-ramp peloidal packstones and wackestones (Litho-facies 7), which shallow up into shallow mid-ramp skeletal peloidal packstones and grainstones (Lithofacies 3). At the base of this unit, however, parasequences are often capped by upward-fining beds of structureless oolitic grainstone (Lithofacies 2), which were reworked into the shallow mid-ramp from the inner-ramp oolitic sand sheet. This main reservoir unit, therefore, has a subtle retrogradational signature, with a trend towards thinner parasequences in its upper parts (Fig. 9). The entire unit is therefore interpreted as a transgressive systems tract (TST), and the gradual landward shift in facies upwards through the section suggests that carbonate production did not keep pace with sea-level rise (cf. Tucker et al., 1993).

In the central to northeastern parts of Umm Gudair, the uppermost unit continues the trend towards even thinner parasequences (Fig. 9). The parasequences largely comprise mid-ramp skeletal packstones (Lithofacies 3) interbedded with bioturbated wackestones that suggest outer-ramp settings. A regressive signature in the overlying Upper Minagish Member implies that the maximum flooding surface marking the top of the TST lies within the thinly interbedded, heterogeneous outer-ramp packstones and wackestones of Lithofacies 7 and 8, which dominate the uppermost unit of the Minagish Oolite. In this respect, it is significant that the continued upward-thinning trend is associated with the marked lateral facies change to more argillaceous limestone of Lithofacies 10 described previously (Fig. 9). The increased argillaceous content of these facies suggests that it is the distal expression of a deltaic system that prograded eastwards across the ramp from the southwest. This implies that silicidastic poisoning rather than simple deepening alone gradually curbed carbonate sedimentation on the ramp. Increased suspended material in the water column would have suppressed the growth of algae, which were the dominant grain-forming organisms in the Minagish Oolite. A marked increase in the frequency of firmground development at parasequence tops suggests that significant periods of nonsedimentation or slowed sedimentation were associated with this poisoning. Given that the change from carbonate to deltaic sedimentation is located close to the maximum flooding surface, the progradation of this delta is likely to have been tectonically rather than eustatically controlled.

Controls on Fluid Movement and Implications for Reservoir Development

The responses to relative sea-level change and the strati-graphic organization described imparts strong controls on permeability. Third-order and higher-frequency cyclicity imposes distinct reservoir properties. The highstand systems tract, lowermost in the reservoir, is a distinctive high porosity/low permeability zone, reflecting the dominance of microporous, muddy, mid- to outer-ramp facies. In contrast, the lowstand systems tract, characterized by oolitic grainstones, has lower porosities but higher permeabilities. The regressive signature to the transgressive systems tract, influenced most strongly by fourth-order sea-level fluctuations, imparts significant heterogeneity. In particular, there is an upward decrease in permeability towards the top of the reservoir, in response to deepening and a more intense diagenetic overprint. This upward degradation in permeability is exaggerated in the uppermost layers of the reservoir, but this seems to be related to siliciclastic poisoning and its associated diagenetic signature rather than to continued deepening.

Fourth-order and fifth-order cyclicity offers an important control on permeability distribution. Most importantly, finer-grained, muddy outer-ramp packstones and wackestones (Lithofacies 7-8) above flooding surfaces may have permeabilities of up to three orders of magnitude less than the clean mid-inner ramp packstones and grainstones (Lithofacies 2 and 3) they overlie (Fig. 4). Because flooding surfaces and the outer-ramp facies above them are correlatable field-wide, they are capable of forming important baffles to vertical transmissibility. Field data suggest that many are capable of limiting pressure communication, with common pressure shifts of tens (and some of hundreds) of psi occurring across correlatable flooding surfaces. This clearly limits vertical fluid movement. In the upper parts of the reservoir, an intense diagenetic overprint leads to consistent pressure differentials of several hundred psi and hence results in increasing reservoir compartmentalization. Increased production is likely to limit further communication across low-permeability beds, particularly in the uppermost part of the reservoir, where the lateral facies change to impermeable argillaceous mudstones limits the potential for natural aquifer support. It is anticipated, therefore, that a more comprehensive secondary recovery scheme will be required in this part of the reservoir.

The greatest postdepositional modifier of reservoir properties is compaction. Where diagenetic fluids have circulated easily within clean packstones and grainstones, marine-burial cements have provided a rigidity, which has inhibited compaction. Larger original pore sizes within these samples also means that the effects of compaction are less extreme, such that pore throats are narrowed or occluded rather than completely destroyed. The increasingly muddy textures of deeper-water lithofacies means that cementation is less likely to have imparted rigidity, and therefore the effects of compaction are more extreme. Small primary macropores and micropores are destroyed and originally poor reservoir properties significantly degraded further, both by compaction and by corresponding neomorphism of the micrite matrix. Where secondary interparticle macropores are recorded in packstones they may locally channelize flow, but they are often not large enough to focus fluids over more than a few centimeters.

The microporous nature of the Minagish Oolite requires a detailed understanding of the pore network in order to maximize production. Clearly the most efficient production will be from clean, macroporous skeletal peloidal and oolitic grainstones. These facies are focused towards the base of the reservoir, however, and the oil-water contact is now above the oolitic grainstones of the LST. The upward degradation in permeability within the reservoir corresponds to an overall decrease in pore sizes as facies become increasingly muddy. This means that sweep efficiencies will be reduced and enhanced recovery mechanisms may be necessary for effective hydrocarbon recovery later in field life. Much of the hydrocarbon may be reservoired within micropores, and therefore recovery mechanisms must aim to achieve an efficient sweep from these pores, even where they contrast markedly in size to surrounding millimeter- to centimeter-scale vuggy porosity.

Conclusions

  1. The Minagish Oolite reservoir of the Umm Gudair Field, Kuwait, represents storm-dominated sedimentation on a ho-moclinal carbonate ramp.

  2. Ten lithofacies are identified, which typically form upward-shallowing parasequences separated by flooding surfaces.

  3. Late highstand, lowstand, and transgressive systems tracts are recognized, which together represent a low-order, potentially third-order, stratigraphical cycle. Each systems tract comprises stacked, upward-shallowing parasequences, interpreted to be the products of fourth- or fifth-order sea-level fluctuations.

  4. Intense micritization has generated highly microporous limestones, which significantly impact understanding of reservoir properties.

  5. An upward degradation in permeability and increased heterogeneity through the reservoir means that a good understanding of pore-scale to reservoir-scale heterogeneities must be achieved later in field life to maintain recovery.

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Acknowledgements

The authors gratefully acknowledge the Kuwait Oil Company for permission to publish this work. We would also like to thank Nigel Cross (Badley Ashton and Associates Ltd.), Christian Strohmenger (EXXON Exploration Company), and Harry Mueller (EXXON Production Research Company) whose comments greatly improved previous versions of this manuscript. We are also grateful to Arnout Everts and John Hopkins for thorough and detailed reviews of this paper. Gerry Hyde, Heather Nickson, and Dave Kemp (Badley Ashton and Associates) are thanked for preparing the diagrams.

Figures & Tables

Contents

GeoRef

References

References

Ali
,
M.A
,
1994
, Gotnia Salt and its structural implications in Kuwait, in
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