ABSTRACT

The Lower Cretaceous (Berriasian to Valanginian) Habshan Formation (Lower Thamama Group) of Abu Dhabi was deposited on a broad carbonate shelf. In east onshore Abu Dhabi, the Habshan Formation consists mainly of limestone and dolomite reaching a thickness of more than 1,100 feet. The depositional environment ranged from shallow-water peritidal to deeper shelf basin. The integration of seismic-stratigraphic, biostratigraphic, lithostratigraphic and electric log data reveals three sequences (I to III) and three shelf edges within the Habshan Formation in east onshore Abu Dhabi. These high energy shelfal sediments prograde toward the basin to the east and northeast with their shelf edges trending north-northwest to south-southeast. The seismic data indicates that the basin was filled in the east during the Hauterivian, after the deposition of Sequence IV (equivalent to the Zakum formation). Good reservoir development is found in the carbonates deposited in the high energy environment along the shelf edge of the Habshan sequence, particularly within the oblique and sigmoidal clinoforms, whereas potential source rocks are expected to be developed basinward. This combination renders the Habshan and Zakum sequences an attractive exploration target, both as structural and stratigraphic traps. Recent exploration activity in the area established the presence of hydrocarbons within the Habshan Sequence III in east onshore Abu Dhabi.

INTRODUCTION

The Lower Cretaceous Thamama Group in Abu Dhabi (Figure 1) consists of five formations: Shu’aiba, Kharaib, Nasr, Zakum and Habshan. The Habshan and Zakum formations, and their equivalents in Abu Dhabi and Oman (Salil and Rayda formations), were studied by several authors in the past twenty years (Hassan et al., 1975; Murris, 1980; Alsharhan and Nairn, 1986; Simmons and Hart, 1987; Hughes Clarke, 1988; Scott et al., 1988; Haan et al., 1990; Abou-Choucha, 1992; de Matos, 1994; and Simmons, 1994). However the stratigraphic relationship between the shallow-water facies of the Habshan Formation, in western and central Abu Dhabi, and the basinal facies of Salil and Rayda formations, in eastern Abu Dhabi and Oman, remained unclear (Figure 2).

Aziz and El Sattar (ADNOC unpublished report, 1995) developed a depositional model which relates the shallow platform to the deep basins during the deposition of the Habshan, Zakum, Salil and Rayda, formations. The results of this study were presented at the Middle East Geoscience Conference (Aziz and El Sattar, 1996) and are described in this paper.

The study incorporates nearly 2,500 kilometers (km) of regional seismic data (60 and 120 fold) which were calibrated with synthetic seismograms from 13 wells, biostratigraphic and sedimentologic data from 10 wells, and wireline log data from approximately 30 wells (Figure 3).

The model identifies three major carbonate sequences in the Habshan Formation as Sequences I to III. These prograde east and northeast towards the basin, with well-defined shelf edges trending in a north-northwest to south-southeast direction. The overlying Zakum formation (Sequence IV; Zakum formation is an informal name and is equivalent to Zakum Member of the Lekhwair Formation of Hassan et al., 1975) shows a similar depositional trend culminating in the development of a thick prograding section representing the final fill of the basin.

In a related study, Landmesser and Saydam (1996) adopted Aziz and El-Sattar’s seismic stratigraphic model and they extended its application to approximately 3,500 km of seismic data which form a grid of approximately 2 × 2 km.

REGIONAL GEOLOGICAL SETTING AND PALEOGEOGRAPHY

The Habshan and Zakum formations (Berriasian to Hauterivian) of the Lower Thamama Group are present in most parts of Abu Dhabi. These are a succession of shallow marine sediments that were deposited on a broad, subtropical carbonate shelf (Figure 4). The shelf formed the northeastern margin of the Arabian plate, facing east to the Tethys Ocean with a continental margin located near the present-day shoreline of northeast Oman.

The depositional history of the Habshan and Zakum formations may have been controlled by a hinge line running in a north-northwest to south-southeast direction through eastern Abu Dhabi. Subsurface data indicates the influence of this hinge on the depositional regimes and lithofacies distribution of the Upper Jurassic (Oxfordian) to Lower Cretaceous (Hauterivian) successions. The hinge line (though shifting laterally) limits the deposition of the Upper Jurassic Hanifa and Hadriya reservoirs, the Arab-D grainstones, the Asab oolite and the Hith anhydrite.

Regional geological data and published studies (e.g. Hughes Clarke, 1988; Haan et al., 1990; Simmons, 1994) indicate that by Late Jurassic time (?Tithonian) most of northern Oman and probably easternmost onshore Abu Dhabi were uplifted, resulting in various degrees of erosion of the Upper Jurassic section. By Early Cretaceous time (early Berriasian) subsidence occurred to the east of the hinge line and much of northern Oman and easternmost onshore Abu Dhabi were submerged (Figure 4, Alsharhan and Nairn, 1986). This was accompanied by a rapid sea level rise resulting in the development of a differentiated carbonate shelf. A relatively deep marine shelf basin (Rayda Basin) was thus generated in east Abu Dhabi, separated from the platform region in the west by well-defined margin(s). In Oman, the southern limit of this basin was identified near the latitude of 21° North (Haan et al., 1990).

The Lower Thamama section in central and northern Oman (equivalent to Lower Kahmah) consists of the Rayda and Salil formations. These two formations correspond to basinal-to-slope facies respectively, and are overlain by the shallow, shoal-dominated facies of the Habshan Formation. This section is dated as Berriasian to Hauterivian (Hughes Clarke, 1988, Figure 2).

Haan et al. (1990) indicated that along the southern rim of the Rayda Basin, in Oman, the high energy facies of the Habshan Formation prograde northwards and exhibits a complex interdigitation with the Rayda and Salil formations toward the basin margin. Seismic data revealed the presence of extensive clinoforms in the Lower Cretaceous sequence, culminating in a belt of high angle clinoforms. In general this belt coincides with the depositional edge of the Infracambrian Ara salt.

It is widely believed that the Habshan Formation in Abu Dhabi is the lateral age equivalent of the Rayda, Salil and Habshan formations of Oman (Hassan et al., 1975; Alsharhan and Nairn, 1986). The following considerations, however, render this conclusion less than obvious:

  1. The Habshan lithofacies of Abu Dhabi differ from those of central Oman (Hughes Clark, 1988).

  2. The typical Habshan grainstone facies only occurs in east Abu Dhabi.

  3. The age of the Habshan Formation in Abu Dhabi differs from that in the northern Oman Mountains outcrops. The Habshan in Oman may be as young as Hauterivian (Simmons and Hart, 1987; Simmons, 1994) which corresponds to the age of the Zakum formation, which overlies the Habshan in Abu Dhabi.

SEQUENCE STRATIGRAPHIC INTERPRETATION PROCEDURE

The Habshan and Zakum seismic stratigraphic sequences were interpreted using approximately 2,500 kilometers of seismic data (Figure 3). The seismic sequence boundaries were recognized as generally continuous reflectors against which other reflectors terminate by onlap, downlap, toplap and truncation (Mitchum et al., 1977). The internal reflection in each sequence were then characterized in terms of seismic facies. The seismic interpretations are tied by synthetic seismograms to 13 wells in which the sequences are dated biostratigraphically. The interpretation was complimented by several well log cross-sections which tie all the wells and incorporate sedimentological data (Figures 5 and 6).

The seismic stratigraphic interpretation revealed three well-defined Habshan depositional sequences which prograde to the east-northeast. The overlying Zakum formation (Zakum Member of Hassan et al., 1975) is a separate sequence (Sequence IV) that also progrades to the east-northeast. It corresponds to the final infill of the basin during early Hauterivian.

LOWER THAMAMA SEQUENCE BOUNDARIES

Five main sequence boundaries form the framework of the Habshan and Zakum formations: (1) Top Jurassic; (2) Top Habshan Sequence I; (3) Top Habshan Sequence II; (4) Top Habshan Sequence III; and (5) Top Sequence IV. The fifth boundary corresponds to the top of the Zakum formation over most of the study area. Exception is in the northeastern part where it may coincide with the top of a prograding section penetrated only in wells Q and R. This section is a possible late highstand system which is referred to here as the younger prograding clinoforms (Figures 6 and 7).

The seismic expression of the top of the Jurassic boundary is generally parallel-concordant with the overlying and underlying reflections (Figures 7, 8 and 9). An exception occurs near the shelf edge of Sequence I where the prograding Sequence I downlaps on the boundary. This boundary may represent a flooding surface across which deepening occurred during Early Cretaceous.

Sedimentological data and well logs (mainly high gamma ray readings, see for example wells F, G and R in Figures 5 and 6) suggest low sedimentation rates and condensation at the Jurassic-Cretaceous boundary. The presence of a glauconite-rich bed (e.g. wells A, B, Q, R and V) also suggests that this boundary is a major flooding surface.

Habshan Sequence I

The base of the Habshan Sequence I corresponds to the top of the Jurassic. This is the lowest and oldest sequence of the Habshan Formation in the study area. The top of the sequence is generally a strong reflector and is parallel/concordant to the overlying and the underlying reflectors in the western area, near wells N and O, and in the easternmost and northeastern areas (Figures 7, 8 and 9). Sequence II onlaps this reflection near the shelf edge. In the southeast region of the study area, the top of Sequence I is downlapped by the progradation of Sequence II (Figures 10 and 11).

The internal seismic reflection geometries of Sequence I in the western region is mainly parallel to sub-parallel indicating uniform and stable depositional conditions. Cores and cuttings from this interval (e.g. well N) indicate variable lithology, but consists mainly of peloidal packstones to wackestones with a slight tendency of coarsening-up and bioturbated toward the top of the sequence. The upper surface of the unit is slightly vuggy and brecciated with evidence of exposure indicating an unconformity surface.

The main faunal content includes echinoderm debris, miliolid foraminiferids and dasycladacean algae. The depositional environment is interpreted as a protected inner shelf to lagoon.

A shelf edge trending north-northwest to south-southeast was mapped from the seismic data. It is onlapped from the east by Sequence II (Figure 8). Reflection patterns consisting of sigmoidal clinoforms, sometimes locally-mounded (time interval thickening) were recognized near the shelf edge. These indicate buildups in relatively high energy shallow water conditions (Figures 5, 6 and 8).

Sedimentological data from the wells located near the margin (wells E, H, I, J, K, S and T) show that this interval is composed of dolomitized oolitic grainstones in the lower part, grading upward to mainly undolomitized oolitic grainstones with low angle, planar cross-bedding. The interval is characterized by very low gamma ray response, except in the basal part (possibly part of the transgressive section) reflecting a shoal/oolitic barrier depositional environment (Figure 12).

Towards the eastern and northeastern parts of the study area (to the east of the shelf edge), Sequence I is represented by a reduced time interval (starved section) with continuous reflections parallel to the underlying Top Jurassic seismic event. The top of the sequence is downlapped by the overlying Sequence II, particularly near the shelf edge of Sequence II (Figures 10 and 11). Well data from the eastern region indicates that Sequence I mainly consists of bioclastic wackestones and packstones and a few grainstone beds that are locally rich in glauconite.

The fauna includes echinoderms and miliolids. This section is overlain by partially dolomitized interbedded lime mudstones and packstones, containing shell fragments, echinoderm debris, ostracods and calpionellids. The lower part of Sequence I in the eastern region may have been deposited during a transgression while the upper part was deposited in a deep, open marine environment, possibly during a highstand.

Habshan Sequence II

Habshan Sequence II, as defined from the seismic data, is only found east of the Sequence I shelf edge (Figures 5 to 9). The western limit of the sequence is defined by a distinct onlap of top Sequence II onto the top of Sequence I along a trend running in a north-northwest to south-southeast direction (Figure 13).

In the central part of the study area, the top of the sequence is marked by clear onlaps of the overlying Sequence III from the shelfal and basinal directions indicating possible exposure and erosion (Figure 8). In the eastern region, the top of Sequence II becomes parallel/concordant to the underlying Sequence I, and is downlapped by the high angle progrades of the overlying Sequence III (Figures 10 and 11).

The overall seismic reflection geometries of Sequence II are marked by the progradational nature of the unit toward the east that indicate possible deposition during a period of a lowstand. Subtle downlaps onto the underlying top Sequence I characterize these progradations which become very pronounced along a belt running in a north-northwest to south-southeast direction marking maximum progradation coupled with maximum time thickening - near the shelf edge (Figures 10 and 11). The Sequence II shelf edge was found to be about 15-18 kms to the east of the underlying Sequence I shelf edge (Figures 8 and 13).

Wells drilled to the west of the shelf edge of Sequence II (e.g. wells F, L and M) indicate that the upper part of this sequence mainly consists of coarse-grained, porous, oolitic grainstones and packstones becoming highly to completely dolomitized in the lower part. To the east of the shelf edge, towards the basin (well-G), Sequence II consists of packstones and wackestones. However, at well-R (Figure 6), a core in the upper part of the sequence consists of fine- to medium-grained, massive oolitic and peloidal packstones interbedded with wackestones.

The overall depositional pattern, distribution and seismic character suggests that Sequence II was mainly deposited during a lowstand period and only to the east of the shelf edge of Sequence I. The diagenetic effect of the lowstand (sea level fall) is recognised by the occurrence of strong dolomitization in the lower part of the underlying Sequence I. Based on regional correlation, Sequence II could be the equivalent of the lower part of the Salil Formation.

Habshan Sequence III

Habshan Sequence III is the youngest section of the Habshan Formation and is present throughout Abu Dhabi. It unconformably overlies Sequence I in the western region (to the west of the shelf edge of Sequence I) (Figures 5, 6 and 8). The contact with the overlying Zakum formation in the same area may represent a depositional surface with local exposure. The top of the Sequence III reflector was picked slightly below the top of the Habshan Formation and is concordant with the overlying Thamama reflectors. An exception occurs in the eastern area where the top of the Sequence III is onlapped by the transgressive section of the overlying Sequence IV (Zakum formation) (Figures 7, 9 and 11).

Sequence III exhibits two different seismic reflection geometries. In the west, where it directly overlies Sequence I (Figures 8 and 9), it is continuous and parallel indicating deposition in shallow water conditions over a broad platform. Gradual eastward onlapping onto the top of the underlying Sequence I was observed from the detailed well log correlations (Figure 6). This onlap was seen on seismic data only in the southern part, indicating possible exposure of the underlying Sequence II near its shelf edge (Figures 9 and 10). The core coverage of this part of Sequence III is very limited. However, the cuttings and wireline log data suggest that this section consists of several fining-upwards sedimentary cycles. These are dominated by dense lime mudstones and wackestones with thin porous pelletic packstones and dolomite interbeds. The depositional environment is interpreted as a shallow water lagoon (subtidal-intertidal) corresponding to a fluctuating sea level highstand (Figure 12).

The geometry of the internal seismic reflections of Sequence III, east of the shelf edge of Sequence II, are oblique progradational. These terminate updip (toplapping) to the top of the sequence, and downlap to the underlying Sequence II (Figures 10 and 11). The overall direction of progradation is toward the east-northeast (basinward). The internal reflection geometry of this section indicates rapid deposition in a high energy shoal environment with possible erosion of the top part. This part of the sequence may represent a late highstand period.

Further east-northeast, toward the basin, Sequence III shows parallel reflection geometry with reduced time thickness indicating more distal, relatively deeper water setting. The shelf edge of Sequence III was defined seismically to the east of well G (Figures 7, 9, 11 and 13) by the last high angle progrades. These are onlapped by lower angle reflectors of the lower part (transgressive section) of the overlying Zakum formation.

To the present, no well has been drilled near the shelf edge and across the steeply-dipping clinoforms described above. However, wells Q and R are the most easterly wells that penetrated Sequence III, to the east of the shelf edge. Cutting samples are interpreted to be from thinly-bedded lime mudstones and wackestones with scattered oolitic grainstone and packestone interbeds, indicating a relatively deeper-water environment. The occasional oolitic horizons may represent transported shelfal sediments. On a regional scale, Sequence III could be the time equivalent of the upper part of the Salil Formation.

ZAKUM FORMATION: SEQUENCE IV

The Zakum formation (informal name, equivalent to Zakum Member of the Lekhwair Formation; Hassan et al., 1975) was also examined in this study. The top of the Zakum formation is characterised by a strong amplitude peak throughout the study area, except in a narrow belt where it overlies the north-northwest to south-southeast trending shelf edge of the underlying Habshan Sequence III (Figure 11). The reflector is concordant with the overlying younger Thamama reflectors in the area to the west of the shelf edge of Sequence III (Figures 7 to 11).

Further eastward, the top of the Zakum formation is marked by a low angle shelf break which is onlapped from the east by the oldest unit of tangential oblique prograding clinoforms. These clinoforms are referred to as the “younger clinoforms” (Figures 6, 7 and 14). These pass laterally into a gently dipping bottomset strata that show apparent downlap onto the underlying top Zakum reflector. These clinoforms might have been deposited either during a late highstand period belonging to the Zakum formation (Sequence IV), or during a sea level fall in a ramp setting. In either case, these clinoforms represent the final fill of the Rayda Basin in the east.

Cores and log data from the wells drilled in the study area indicate an abrupt deepening at the contact between the Zakum formation and the overlying section with no obvious hiatus seen from the microfossils. The interpreted regional seismic lines and published work from outcrops suggest that the reported oolitic-peloidal grainstones of the Habshan Formation in the Oman Mountains are time equivalent to the younger clinoforms in the northeastern part of the study area. Furthermore, there may be no time break between the Habshan Formation and the overlying Lekhwair Formation in the Oman Mountains.

Wells Q and R are the easternmost wells in the study area which might have penetrated the younger clinoforms. A core in the upper part of the section consists of peloidal, bioclastic grainstones with a low angle, planar cross-bedding. The grainstones display a marine fauna including benthonic forams, calcareous algae and occasional stromatoporoids, with coral and bryzoan fragments at the base. The sediments and the faunal content suggest a shallow shelf-shoal environment.

BIOSTRATIGRAPHY OF HABSHAN SEQUENCES

The microfossils and biofacies associations identified within the Habshan Formation in Abu Dhabi indicate depositional environment ranging from very shallow marine to deep shelf basin. A variety of environments including lagoonal, shoal, shelf barrier and slope are also recognized (Figure 12).

Several biostratigraphic studies on the Lower Thamama section were carried out for ADNOC by several consultants. The results of one of these studies (unpublished reports, British Petroleum 1989 and 1992) utilizing benthonic and planktonic foraminifera, calpionellids and calcareous algae are shown in Figure 15.

Detailed high resolution biozonation classification for the Habshan Formation could not be achieved, particularly in the platform sediments. This was due to the facies control on microfossils distribution and the occurrence of a high degree of dolomitization that destroyed both the original facies and the microfossils content at several stratigraphic levels.

The biostratigraphic studies revealed the occurrences of the following major biozones within the Habshan and Zakum formations in the study area.

Calpionella alpina Zone

This zone is defined in the lowermost part of the Habshan Formation and only in the easternmost wells A, B, Q and T. It is dominated by the presence of the calpionellid species Calpionella alpina indicating deep open marine (upper Bathyal) condition. The pelagic crinoid Saccocoma sp. and radiolarians are also present. The presence of Calpionella alpina suggests an early to middle Berriasian age. Similar fauna have been identified in the lower part of the Habshan basinal facies (Rayda Formation) in well Q by de Matos (1994), also in the Oman Mountain outcrops and in the northern Emirates (Simmons and Hart, 1987; Toland and Peebles, 1993; Simmons, 1994).

In wells B and Q, and in the field section, a glauconitic-rich, bioclastic packstones - grainstones (60 feet thick) with abundant echinoderm debris together with rare benthonic foraminifera (e.g. Lenticulina) was found below the Calpionella alpina zone. A similar bed was also identified in the outcrop, and is considered to represent the initial transgressive and maximum flooding surface of the Lower Thamama sediments above the Jurassic succession.

Globuligerina hoterivica Zone

This zone overlies the Calpionella alpina zone in east onshore Abu Dhabi. The zone contains a sparse microfossil assemblage including the planktonic foraminiferal species Globuligerina hoterivica, radiolarian, ostracods and pelagic crinoids. The microfossils of this zone indicate an open marine slope to basinal environment of deposition with an age of Berriasian to Valanginian. A similar zone (G. hoterivica subzone I) was identified in the Salil and Rayda formations in the outcrops of the Oman Mountains by Simmons (1994). This zone passes laterally to Salpingoporella annulata and Pseudocyclammina lituus zones.

Salpingoporella annulata Zone

This zone overlies the barren or unzoned interval in the western part, but eastward, it may pass into the Globuligerina hoterivica zone and be underlain by that zone and the Calpionella alpina zone. This complex relationship probably relates to the progradational nature of the Habshan Formation. The microfossils recorded from this zone include Salpingoporella annulata, Clypeinia parvula, Macroporella enbergeri, Cylindroporella arabica and the foraminifera Nautiloculina oolithica. The presence of this zone would be indicative of a Late Berriasian age and was found to be developed at the topmost part of Habshan Sequence I in well N.

Pseudocyclammina lituus Zone

This biozone is present in all studied wells and covers the lower part of the Nasr Formation, the entire Zakum formation and the upper part of the Habshan Formation (Figure 15). Within the Zakum and Habshan formations, two subzones were identified.

Salpingoporella pygmaea Subzone

This subzone overlies the Salpingoporella annulata zone described above and represents the upper part of Habshan Formation and the lower part of Zakum formation. Microfossils recorded from this subzone include Salpingoporella pygmaea, Pseudocyclammina lituus, Pseudocyclammina vasconica, Trocholina sagittaria, Trocholina spp. The presence of Salpingoporella pygmaea is believed to indicate a Valanginian age. In eastern Abu Dhabi, where more basinal facies exist, this subzone is replaced by the Globuligerina hoterivica zone.

Lenticulina “zakumensis” Subzone

This subzone corresponds to the upper part of Zakum formation. It is characterized by the occurrence of a large and distinctive species of Lenticulina, named herein as Lenticulinazakumensis”. Microfossil assemblages include Dukhania arabica, Lithocodium aggregatum, Nautiloculina bronnimanni, Trocholina spp. and Cayeuxia sp. Based on the age of the underlying and overlying zones, this subzone is regarded as being of Hauterivian age.

AGE OF THE HABSHAN FORMATION SEQUENCES

The biozones and published work from Oman establishes the time transgressive nature of the Habshan Formation associated with eastwardly prograding carbonates. This evidence supports the time equivalence between the basinal Rayda and Salil formations, in Oman, to the shallow carbonate facies of the Habshan Formation in Abu Dhabi.

In Oman, the term Habshan Formation refers only to the dominantly grainstone facies representing shoal sedimentation that overlie the basinal slope sediments of the Salil Formation. Here the Habshan is as young as Hauterivian (Hughes Clarke, 1988; Haan et al., 1990; Simmons, 1994) (Figure 2). By integrating the seismic-stratigraphic interpretation, wireline log correlation, biostratigraphic zonation (although limited) and correlation with the global sea level curve (Haq et al., 1987; Figure 14), the following can be concluded:

  1. Habshan Sequence I is Berriasian and corresponds to the “unzoned” and S. annulata zone in the west. To the east it corresponds to the transgressive, glauconite-rich bed, Calpionella alpina and possibly the lower part of the G. hoterivica zones.

  2. Habshan Sequence II is early Valanginian and corresponds to the middle Globuligerina hoterivica biozone.

  3. Habshan Sequence III is middle to late Valanginian and corresponds to the Salpingoporella pygmaea subzone in the west and the upper part of the Globuligerina hoterivica zone in the east.

  4. Zakum formation (Sequence IV) is early Hauterivian and corresponds to the Lenticulinazakumensis” subzone.

HYDROCARBON POTENTIAL

Within the Habshan and Zakum formations in east onshore Abu Dhabi, several prospective reservoirs are recognised. Good reservoir development occurs in the shallow-water shoal oolitic facies of the Habshan sequences, particularly near the shelf edges and within the prograding clinoforms. Medium quality reservoirs are also developed in the platform area of Habshan Sequence III. So far, hydrocarbon accumulations in the Habshan Formation occur in simple anticlinal closures. This suggests that a good source rock might have been deposited to the northeast of the study area (Figure 16) in the central parts of the shelf basin into which the Habshan sequences prograded. This inferred source rock has yet to be identified by drilling and analytical work.

Recent exploration drilling in the under-explored eastern part of onshore Abu Dhabi encountered hydrocarbons in the Habshan Formation. As a result, ADNOC is actively seeking significant stratigraphic traps by using 3-D seismic data. These subtle traps require detailed seismic-stratigraphic and lithofacies studies.

CONCLUSIONS

The study of the Lower Thamama in east onshore Abu Dhabi revealed the presence of three sequences in the Habshan Formation and one in the overlying Zakum formation. These sequences prograde and become younger toward the east-northeast with their shelf edges trending in a north-northeast to south-southwest direction.

Habshan Sequence I was deposited during transgressive and highstand periods. This conclusion is based on the occurrence of a condensed section in its lower part which is overlain by high energy sediments in the upper part. The sequence is Berriasian in age.

Habshan Sequence II is early Valanginian and was deposited during a lowstand period to the east of the shelf edge of Sequence I. Habshan sequence III is middle to late Valanginian and was deposited during transgressive and highstand periods that flooded the entire platform covering all Abu Dhabi.

Sequence IV (Zakum formation) is early Hauterivian and was deposited during a highstand period. Its lowermost part may be tracts of a transgressive system. Thick prograding clinoforms in the northeastern areas indicate that this sequence provided final fill of the Rayda Basin.

ACKNOWLEDGEMENTS

The authors thank Abu Dhabi National Oil Company (ADNOC) for permission to publish this paper. We also thank Mr. A.O. Belfaqeh who provided the prints for all the seismic lines, Mr. J. Esteves de Matos for his useful discussions and comments and many colleagues in ADNOC’s Exploration Division for their useful discussions. We appreciate the contribution of British Petroleum Exploration for the studies conducted for ADNOC on which the biostratigraphic results are based.

ABOUT THE AUTHORS

Sabah K. Azizis a Senior Review Geologist with Abu Dhabi National Oil Company (ADNOC) in UAE. He has 20 years of experience in the field of exploration Geology in UAE. He holds a BSc in Geology from Baghdad University.

Mohamed M. Abd El-Sattaris a Senior Exploration Geologist with Abu Dhabi National Oil Company (ADNOC), UAE. He has 21 years of petroleum and exploration experience in the Middle East, of which 11 years were with Gulf of Suez Petroleum Co., Egypt. He holds a BSc and MSc in Geology from Ain Shams University, Egypt. He is a member of many professional societies and has published several papers on structural geology and basin evaluation.