ABSTRACT

The recent discovery in Central Saudi Arabia of Arabian Super Light oil in 'Usaylah-1 confirms stratigraphic trap potential of the Permian Unayzah Formation. The trap is an updip pinch-out of an upper Unayzah sandstone along the eastern flank of the north-south trending Hawtah anticline. The seal is the basal shales and siltstones of the Khuff and Unayzah formations and the source rock is the Lower Silurian Qusaiba Shale. The well encountered an oil column of 31 feet in an upper Unayzah eolian dune facies. The areal distribution of the prospect was mapped with a 3-D seismic survey. Seismic horizon slices and relative amplitude maps at the target reflection clearly delineate the trap. The oil-productive Unayzah sandstone is imaged as a high-amplitude reflection in an isolated area of approximately 8 square kilometers. Seismic isochron maps are proving effective in focusing future exploration to areas of similar stratigraphic trap potential in the region.

INTRODUCTION

Since 1989, eighteen oil and gas fields have been discovered in Central Saudi Arabia. In contrast to the major producing Jurassic and Permian carbonate reservoirs of Eastern Saudi Arabia, the reservoirs in Central Arabia are Paleozoic clastics; primarily the sandstones of the Permian Unayzah Formation. The Unayzah sandstone reservoir is laterally discontinuous and consists of a variety of continental, braid-plain facies.

The fields in Central Arabia are located along two prominent structural trends: the Hawtah and Nuayyim trends (Figure 1). Of the 18 fields discovered in the region, 16 are four-way anticlinal closures, many of which have a stratigraphic component, and two are stratigraphic traps. The stratigraphic trap potential in the region was postulated because the mapped vertical closure in many fields is insufficient to account for the observed hydrocarbon column.

This paper describes the first purely stratigraphic trap discovered in Central Arabia, 'Usaylah field, located 175 kilometers (km) south of Riyadh (Figure 1). 'Usaylah is the 14th oil and gas field discovered in Central Arabia. The discovery well 'Usaylah-1 tested 49.4° API from the upper part of the Unayzah Formation. The oil column is 31 feet (ft) thick.

In contrast to the structural traps which were mapped with a conventional 2-D seismic grid, the 'Usaylah prospect was mapped from a 3-D seismic survey. This discovery demonstrates another successful application of 3-D seismic in Saudi Arabia (Al-Hauwaj et al, 1993; Saudi Aramco, 1994; Husseini and Chimblo, 1995; Hastings-James and Al-Yahya, 1996).

STRATIGRAPHY

The Unayzah Formation is a complex succession of continental clastics consisting of braid-plain and eolian sands and floodplain silts. The Unayzah Formation rests unconformably on the Qusaiba Shale of Lower Silurian age and is overlain by the Late Permian Khuff Formation. The lowermost part of the Unayzah consists generally of debris flow gravels and coarse clastics which infill the Hercynian erosional unconformity on the underlying Qusaiba Shale (Senalp and Al-Duaiji, 1995). The Unayzah fines-upward into more distal, braided-stream longitudinal bars and channel-fill sandstones and floodplain siltstones.

A marked unconformity (Pre-Khuff Unconformity) separates the Unayzah Formation from the overlying basal Khuff clastics which consist of marine to marginal marine shales and incised channel-fill sandstones (Senalp and Al-Duaiji, 1995). The stratigraphic succession is shown on the log of the discovery well (Figure 2).

The source rock for the Central Arabian oil and gas fields is the Qusaiba Shale, deposited during the Early Silurian sea level rise following the deglaciation of Gondwana (Mahmoud et al., 1992; McGillivray and Husseini, 1992). The oil discovered at 'Usaylah-1 is typical of Qusaiba-sourced oil with a low sulfur content and high gravity.

STRUCTURE

Hawtah and Nuayyim Trends

The Hawtah and Nuayyim structural trends were recognized early in Saudi Aramco’s exploration program when over 1,500 kilometers (km) of 2-D seismic data was acquired to evaluate the area. Following the initial discoveries in 1989 and 1990, over 1,500 square kilometers (sq km) of 3-D seismic data was acquired to better image the laterally discontinuous channelized Unayzah reservoir and resolve the structural complexities in the field areas.

Figure 3 is a schematic cross-section which shows the Hawtah structural trend and illustrates the generalized stratigraphic section in the study area. Prominent, high-angle reverse faulting is apparent in the older Paleozoic section, which does not cut above the Unayzah Formation in the study area. More gentle drape folds occur in the shallower Mesozoic. The 'Usaylah-1 discovery is shown on the east flank of the Hawtah Trend.

Figure 4 is the top Unayzah depth map in the study area. The Ghinah, Umm Jurf and Layla fields are all dominantly structural traps, with a stratigraphic component on the south part of the Umm Jurf field.

Growth History

Following a period of relative quiescence during deposition of Lower Paleozoic Ordovician sandstones and the overlying Lower Silurian Qusaiba Shale, major structural growth occurred during the Late Devonian to Carboniferous Hercynian Orogeny (Husseini, 1991).

During this period of deformation, high-angle reverse faulting occurred in Central Saudi Arabia paralleling and overlying pre-existing zones of weakness established during accretion of the Arabian Plate in the Precambrian (Simms, 1995). The Late Carboniferous orogeny established both the Hawtah and Nuayyim structural trends along these deep-seated crustal lineaments.

Following a period of erosion marked by the Pre-Unayzah Unconformity, the Early to Late Permian continental Unayzah Formation infilled the erosional topography with its source to the west. Although major Hercynian-age structural growth had ceased prior to deposition of the Unayzah, structural deformation continued throughout deposition of the Unayzah and prior to the major marine ‘Khuff’ transgression. This later growth is clearly reflected by both erosion of Unayzah section at the Pre-Khuff Unconformity and non-deposition on paleo-structural highs along the Hawtah Trend.

Evidence for contemporaneous growth during Unayzah deposition is expressed by the expansion or thickening of correlatable intervals throughout the Unayzah Formation interval in both the paleo-structural lows off the paleo-highs and on the ‘downthrown’ sides of major faults. The sandstone/siltstone ratio within these intervals typically increases from ‘paleo-structural’ high wells to ‘paleo-structural’ low wells. Further evidence for growth during deposition of the Unayzah is clearly seen in 3-D seismic horizon slices where thicker, sand-rich meandering braid-channels are imaged flowing around the paleo-highs and confined to the paleo-lows.

More subtle structural growth continued throughout deposition of the Late Permian carbonates of the Khuff Formation, the reservoir seal in Central Saudi Arabia. This growth is clearly reflected in the Khuff Formation seismic isochron map, where isochron thins directly overlie the axes of earlier structural highs which are reflected in older Paleozoic isochron maps.

More pronounced growth resumed along the same, pre-existing structural highs during deposition of the Traissic Sudair Formation and the overlying Jilh dolomite. This reactivation occurred during periods of rifting and subduction along the Arabian Plate boundaries associated with Tethys spreading (Simms, 1995). Again, this later growth is clearly evident on the Jilh to Khuff seismic isochron map where Triassic paleo-structural highs (isochron thins) directly overlie earlier structural highs.

Late-stage, Tertiary regional tilting to the east shifted the axes of these paleo-structural highs to the west so that present-day structural axes along both the Hawtah and Nuayyim trends are west of the paleo-structural highs.

STRATIGRAPHIC TRAP PLAY CONCEPT

Stratigraphic Traps in Central Arabia

Three types of stratigraphic traps have been identified along the Hawtah and Nuayyim structural trends: (1) isolated reservoir sands that pinch-out updip to the west, forming stratigraphic traps along the east flank of the Hawtah and Nuayyim trends; (2) truncation traps of upper Unayzah sandstones below the regional Pre-Khuff Unconformity; and (3) combination structural-stratigraphic traps where reservoir sand facies pinch-out across structural highs.

'Usaylah field is an example of the first type of stratigraphic trap where the productive upper Unayzah sandstone is totally encased in surrounding impermeable shales and siltstones. The south part of the Umm Jurf field, northwest of 'Usaylah, is an example of the second and third types where sands are both truncated below the Pre-Khuff Unconformity and pinch-out across the Umm Jurf structural high.

In all three potential stratigraphic traps, hydrocarbon was sourced from the Lower Silurian Qusaiba shale to the east and migrated updip to the west where both structural and stratigraphic accumulations occur along the Hawtah and Nuayyim trends.

Stratigraphic trap potential is believed to occur in both the laterally discontinuous Unayzah Formation as well as the marginal marine clastics of the Khuff Formation. Geochemical fingerprinting of oils recovered from Khuff Formation clastics at the Nuayyim field indicate a likely stratigraphic trap in that area (M.I. Halpern and M.H. Tobey, personal communication, 1996). In contrast, stratigraphic trap accumulations have been found only in Unayzah Formation clastics along the Hawtah Trend.

Risks

Risks associated with stratigraphic trap plays are substantially higher than purely structural traps, and the stratigraphic trap play along the Hawtah and Nuayyim trends is no exception. Indeed, most four-way structural closures in Central Arabia had been drilled prior to drilling 'Usaylah-1.

The principal risks encountered in Central Arabia include: (1) the stratigraphic complexity of the Unayzah reservoir; and (2) the inability to consistently resolve stratigraphic trap geometry because of limits in seismic resolution. For example, a sand may ‘appear’ to pinch-out on seismic data, yet continue updip as a thin stringer and provide leakage of hydrocarbon to adjoining permeable sands. Minor faults of 30 to 40 feet may go undetected by the interpreter yet allow leakage from a target sand to an adjoining, thin sand juxtaposed updip across the fault. A breach in the trap can easily occur. Seismic tuning effects can also mislead the interpreter in mapping sandstone boundaries.

Although the risks are substantial, the shallow depths to the primary objective Unayzah reservoir (6,000-7,000 ft in the Hawtah Trend and 9,000-10,000 ft in the Nuayyim Trend) and the fact that production facilities are largely in place make this a viable economic play in Central Arabia. The continued use of 3-D seismic data and mapping paleo-structure as a predictive tool in recognizing stratigraphic potential are techniques emphasized in this paper that should lower risks further in this play.

'USAYLAH FIELD DISCOVERY

The 'Usaylah-1 wildcat tested a stratigraphic trap or updip pinch-out of an upper Unayzah sandstone along the east flank of the Umm Jurf field (Figure 5). Drilled as a deviated hole because of land-surface problems, the well encountered three sandstones within the Unayzah Formation and confirmed the 3-D seismic interpretation done prior to drilling this well. The oil reservoir in 'Usaylah-1 is an eolian sandstone consisting of both dune and interdune facies. Reservoir quality is good with an oil column of 31 ft. Slump and dewatering structures were observed in a core cut in the middle sand at 'Usaylah-1 which may indicate gravity sliding on channel margins during deposition (A. Al Duaiji, J. Cocker and J. Filatoff, unpublished Saudi Aramco report, 1996). Figure 6 is a stratigraphic cross-section flattened on a regional stratigraphic marker, the Khuff-D anhydrite, which shows the oil-productive, upper Unayzah sandstone pinching-out to the west.

3-D Seismic Interpretation

Horizon slices from 3-D seismic data illustrate the stratigraphic complexity of the upper Unayzah interval in the 'Usaylah field area. Figure 7 is a horizon slice 12 milliseconds (ms) below the top of the Unayzah Formation seismic event. The high-amplitude, oil-productive dune sand (blue) is clearly imaged at the 'Usaylah-1 location. To the south, a meandering braid-plain channel is evident cutting across the area from northwest to southeast. The overbank silts and clays associated with this channel system effectively compartmentalized the eolian sands to the east and formed the stratigraphic-trap at 'Usaylah-1.

Seismic traverses A-A’ (dip-line, Figure 8) and B-B’ (strike-line, Figure 9) show the productive eolian sandstone (blue, positive amplitude) at the 'Usaylah-1 discovery and the channel axis to the west. An offsetting dry-hole (Well B), drilled 2 km southwest and structurally high to the discovery well, encountered well-developed, stacked channel sandstones which are hydraulically separate from the oil-productive eolian sandstone in the discovery well. This well was drilled on a trap postulated to be a truncation of upper Unayzah sandstone below the Pre-Khuff Unconformity.

Well B was likely dry because the targeted upper Unayzah sandstone appears to be cut by a down-to-the-east fault that juxtaposed the upper sand against a stratigraphically lower sand to the west. A trap breach therefore could have occurred. Another explanation is that oil has moved further updip within the channel sand penetrated by Well B where erosion was incomplete. This explanation seems less likely because the channel sand is so limited areally that any oil accumulation that occurred updip should also have extended downdip to Well B.

Figure 10 shows a horizon slice of the upper Unayzah interval superimposed on the top Unayzah depth map. Different colors are used in this display to highlight the lateral discontinuities of facies within the upper Unayzah section. The oil-productive eolian sandstone at the 'Usaylah-1 discovery is shown in red. The updip, westward limit of the targeted upper Unayzah interval coincides with structural drape over the down-to-the-east fault that flanks the Umm Jurf field. The lack of upper Unayzah sandstone west of this updip pinch-out is likely caused by both non-deposition and erosion as structural growth resumed during upper Unayzah deposition.

To the southwest of the eolian sandstone in 'Usaylah-1, braid-channel meander belts are imaged cutting across the area from northwest to southeast immediately east of the steep dip (drape) over the major down-to-the-east fault trace. Potentially productive, stratigraphic traps, (red, positive amplitudes) on the lower-left part of this figure could extend to the south. Additional 3-D seismic data is being acquired to further evaluate the area.

Figure 11 is a depositional model for the Unayzah Formation in the 'Usaylah field area which schematically illustrates depositional features apparent on the horizon slices. The 'Usaylah-1 well and the offsetting dry hole (Well B) are both shown on the schematic.

FUTURE STRATIGRAPHIC-TRAP POTENTIAL IN CENTRAL ARABIA

Mapping the structural growth history of the Hawtah Trend using seismic isochron maps provides a predictive tool in recognizing additional stratigraphic-trap potential in Central Arabia. Except for late-stage Tertiary regional tilting to the east, different periods of structural deformation from the Late Paleozoic to the Mesozoic were focused along the same structural trends.

Because the base of the primary objective Unayzah Formation is an erosional unconformity, which is very difficult to image seismically, mapping Unayzah Formation thickness variations using seismic data alone is usually not possible. Integrating well control with the seismic data has limited use when predicting Unayzah sand distribution on the flanks of the structural highs because well control is limited to structural highs where most wells have been drilled.

Seismic isochron maps of the shallower Late Permian Khuff Formation (Figure 12) and the Triassic Jilh Formation to Khuff Formation (Figure 13) interval were generated where seismic events are much stronger. These isochron maps were used to show structural growth, i.e., paleo-highs and paleo-lows, from Late Permian to Triassic time. Realizing that structural growth along these same structural trends was active during earlier Unayzah deposition, the interpreter can predict areas of stratigraphic-trap potential within geologically older Unayzah section using isochron maps of the younger events.

The areas of better stratigraphic-trap potential are in the paleo-lows on the east flanks of the paleo-highs where ‘untruncated’ Unayzah section is likely to be preserved. These paleo-lows also are sand depocenters where isolated, potentially thicker oil-productive sand bodies are likely to occur. The eolian sand at the 'Usaylah field discovery is an example.

East-Flank of Ghinah Field

Paleo-structural trends were used to predict stratigraphic-trap potential on the east flank of the Ghinah Field, 20 km north of the 'Usaylah field. Figure 3 shows the present-day structural crest of the reservoir objective Unayzah Formation in black. Figures 12 and 13 are the seismic isochron maps of the Khuff Formation (Late Permian) and the Jilh Formation to Khuff Formation (Late Permian to Triassic), respectively. The paleo-structural axes of both of these isochron maps are coincident and are shown in blue on these figures.

Figure 14 is a horizon slice 12 ms below the top Unayzah Formation extending from the 'Usaylah field in the south to the Ghinah field in the north. Figure 15 is an enlargement of this display in the Ghinah field area. The horizon slice shows both the westward pinch-out of the upper Unayzah sandstone section as well as the lateral distribution of sands within the interval. The seismic amplitude character and the stratigraphic position indicate that these sands are likely eolian dune and braid-plain channel sands.

Superimposed on the horizon slice are the present-day structural axis (black) of the top Unayzah Formation as well as the paleo-structural axis (blue) of the Jilh Formation to base Khuff Formation events. Significantly, the present-day structural axis has shifted about 5 km to the west of the paleo-structural highs due to late-stage Tertiary regional tilting to the east.

Because of the shift in the structural axis to the west, the present-day structural axis does not reflect nor ‘predict’ areas of stratigraphic-trap potential nearly as well as the isochron thin or paleo-structural high.

The upper Unayzah sand pinch-out (positive, red amplitudes) is best developed immediately east of the axis of the paleo-high.

Figure 16 is a seismic traverse through the northern part of the Ghinah field showing present-day structure. The prospective upper Unayzah sand pinch-out (2 km east of the present-day, eastern flank of the Ghinah field) is a much greater distance than current structure would suggest. In contrast, the flattened section (Figure 17) clearly shows a more ‘predictable’ updip sand pinch-out immediately east of the isochron thin or paleo-high.

CONCLUSION

The recent 'Usaylah-1 oil discovery confirms stratigraphic-trap potential in the Permian Unayzah Formation in Central Saudi Arabia. The well was the first purely stratigraphic trap test drilled along the Hawtah and Nuayyim structural trends and encountered 31 ft of oil in an upper Unayzah eolian dune facies. Stratigraphic traps in Central Arabia include both updip pinch-out of reservoir sands and truncation traps below the regionally extensive Pre-Khuff Unconformity.

Mapping paleo-structure is a predictive tool in focusing future exploration in Central Arabia where additional stratigraphic potential exists. 2-D seismic data has been shot extensively throughout Central Arabia, with distances of 5 to 10 km between dip lines. The widely-spaced 2-D seismic grid has been sufficient to map regional paleostructure and identify general areas of stratigraphic trap potential, but is not adequate to map stratigraphic trap prospects in the area. Because of rapid lateral changes in Unayzah sand facies, 3-D data is necessary to mature drillable stratigraphic trap prospects in the region.

Although the risks of this stratigraphic trap play are clearly greater than a conventional structural play, the use of 3-D seismic data, the fact that production facilities are already in place, and the relative shallow depths to the target horizon make this play viable economically.

ACKNOWLEDGEMENTS

The authors would like to thank the Saudi Arabian Oil Company (Saudi Aramco) and the Saudi Arabian Ministry of Petroleum & Mineral Resources for their support and permission to publish this paper. The authors appreciate J.A. Al-Hajhog, C.J. Heine and M. Senalp of Saudi Aramco for sharing their understanding of the Unayzah Formation stratigraphic complexity and would like to thank J.R. Wilkins, P.S. Gregorio, and M.A. Zarea for their contribution in preparing maps and cross-sections. Appreciation also is extended to Scott Mussett, M.A. Qureshi, R.D. Petsef and V.O. Tegelan for graphics design. Finally, special gratitude is extended to Gulf PetroLink and the reviewers whose helpful comments and suggestions significantly improved the original manuscript.

ABOUT THE AUTHORS

Daniel S. Evans is currently Geological Specialist in the Central Area Exploration Division of Saudi Aramco developing prospects along the recently discovered Hawtah Trend in Central Arabia. Dan worked as a groundwater geologist between 1970 and 1975 and joined Aramco in 1975 where he worked on a variety of assignments for the Exploration Department until 1981. He then joined Tenneco Oil Co. as an Exploration Geologist from 1981 to 1984. From 1984 to 1989, he worked for an independent oil operator based in San Antonio. He rejoined Saudi Aramco in 1989. Dan holds BA and MA degrees in Geology from the University of Texas at Austin.

Ahmed M. Al-Otaibi received a BSc degree in Engineering Geology from King AbdulAziz University in Jeddah in 1990 and a MSc degree in Geology from Keele University, UK in 1993. Ahmed has been working on various projects including reservoir characterization, geostatistical reservoir modeling and exploration since he joined Saudi Aramco in 1990. He is currently a member of the Stratigraphic Trap Exploration Team, Central Saudi Arabia.

BassamH.Bahabri is currently the Chief Geologist of Northern Area Reservoir Geology Division in the Exploration Organization of Saudi Aramco. For the last 16 years, Bassam had been involved in several geological and exploration projects before his current position. As part of his career development, he participated in exploration assignments with Exxon, Texaco and Chevron. Bassam graduated with a BSc in Geology from King Saud University in Riyadh in 1981. He also obtained his DIC/MSc from Imperial College in Petroleum Exploration in 1993.