Structural and Stratigraphical Significance of Reservoirin Heglig Field (Sudan)

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1 Journal of Science and Technology 11 (2) March 2010 ISSN X Sudan University of Science and Technology Structural and Stratigraphical Significance of Reservoirin Heglig Field (Sudan) A. M.Yagoub Sudan University of Science and Technology-College of Petroleum Engineering and Technology ABSTRACT: The Greater Heglig area encompasses some 380 sq.km. The area forms part of the Cretaceous-Tertiary Muglad Basin in south-central Sudan. The basin was initiated as an extensional graben to the immediate south of the Central African Shear Zone. An early phase of extensional tectonics led to rapid subsidence and lacustrine basin fills comprising the rich source rocks of the Lower Cretaceous Barremian-Neocomian Sharaf Formation and Albian-Aptian Abu Gabra Formation. The reservoir section in the Late Albian to Cenomanian Bentiu Formation accumulated as widespread sheet sandstones in response to a cessation of active extension and during a period of regional sag marks a fundamental change from an internally draining lake basin to larger scale sediment dispersal patterns transporting sediment out of the basin in the north and south. A sudden change from the sandstone-dominated successions of the Bentiu Formation to the shale-dominated interval of the Aradeiba Formation marks the onset of a second phase of extension and increases subsidence during the early Turonian. The Bentiu Formation comprises the main reservoir interval in the study area and is characterized by stacked successions of thick, amalgamated cross-bedded sandstones and intervening extensive laterally, thinner mudrock intervals. Sandstones of the overlying Aradeiba Formation are characteristically isolated within an otherwise mudrock-dominated succession. No significant thickening of stratigraphic units across faults is evident in the study area. It is likely that subtle difference in subsidence due to differential compaction across buried grabens and halfgrabens will have influence on sediment dispersal patterns during Bentiu and Aradeiba times. INTRODUCTION The study area is located in the southwestern Sudan, in the boarder between the Western and Southern Kordfan States Fig (1). It extends from lat 9 57 N to 10º 10 N and long E to E, and occupies an area of about 380 km 2. Heglig field is located on the northeast flank of the intracratonic Muglad rift basin. The northwest-southeast trending Muglad basin complex covers an area of 750km (465mi) long and in excess of 150km (95mi) wide. Fig (1) showing the influence of African Shear Zone on the Sudanese interior basins. from Fairhead (1988) Previous Study: Southern Sudan was essentially unexplored prior to 1974, and no surface indications of oil or gas were known. Chevron Explorationists (1975) postulated that deep sedimentary basins related to Central and East Africa rifting might exist in the area. The study area is a part of the Late Jurassic to Early Cretaceous Sudanese Muglad Rift Complex (Schull, 1988). The area had been considered to be shallow intracratonic sag containing Tertiary and Cretaceous sediments overlying possible shallow basement (Norman R. Giedt 1990). There are three rifting phases occurred during the Early Cretaceous, Late Cretaceous and Early-Middle Tertiary respectively (McHargue et al., 1992). Regional Geology and Tectonics: The Muglad basin is one in a series of Cretaceous-Tertiary failed rifts, that trend across the Central African Craton from the Benue trough in Nigeria through Chad and the Central Africa Republic into Sudan. The right-lateral movement on the Central Africa rift system (Fairhead, 1988) is interpreted to have translated to northeast-southwest extension in Sudan, beginning by Barremian-Neocomian time, resulting in basins with an overall northwest-southeast trend that is nearly perpendicular to the shear zone Fig (1). The evidence for further southeast extension has been destroyed by Tertiary uplift associated with recent rifts in East Africa, namely in Ethiopia and Kenya. All basins of the Sudanese rift-related system, such as the Muglad, White Nile, Blue Nile, Khartoum and the Atbara basins terminated northwards at the Central African Shear Zone as shown in Fig (1). The development of the rift basins of the southern Sudan is related to processes that operated not only within central Africa, but also along the western and eastern continental margins. The three episodes of rifting resulted in the deposition of sedimentary section of up to 13 km thick in the deep troughs (Schull, 1988). 77

2 Muglad basin evolution has been divided into pre-rift and rift phases (Norman, R. Giedt (1990). Schull, (1988); Kaska, (1989); Giedt, (1990) Mohamed et al. A.Y, (2000) generalized the stratigraphc column in the area Fig (2). This classification comprises the following three major phases of rifting. 1-Phase I- Late Jurassic Early Cretaceous ( Ma). 2-Phase II - Late Cretaceous (95-65 Ma). 3-Phase III Paleocene (63-30Ma). The first rift phase contains primarily lacustrine shales and claystones of Sharaf and Abu Gabra Formations in the deeper parts of the basin, these lacustrine deposits are overlain by Bentiu Sandstone, which consists mostly of stacked channel and bar deposits, as suggested by the high sand-to-shale ratio characteristic of the braided and meandering streams. They were deposited during the first rifting cycle of Muglad Basin ( T. Pletsch et al., 2001; Gain Battista Vai, 2003) The second rift phase began with deposition of the Darfur Group, a coarsening-upward interval consisting primarily of interbedded marginal lacustrine and fluvial-deltaic sandstone and claystones. It contains the most important reservoirs in the field (Aradeiba, Zarqa and Ghazal formations) and includes all of the hydrocarbon seals. It is overlain by the Paleocene Amal Sandstone, representing the last phase of the second rift cycle. The third rift phase initiated deposition of the thick Eocene Nayil shale that grades upward into an increasingly sandy interval of upper Nayil and younger Tertiary units. These three rifting periods resulted in the accumulation of up to 5400m of sediments in Muglad rift basin (Abdalla Y. Mohamed et al. A.Y, (2001). Fig (2) Generalized Stratigraphic Column of the Study Area (modified from Schull, 1988; Kaska, 1989; Giedt, 1990; Mohamed et al. A.Y, 2000) METHODS A total of 1075 seismic inlines and 319 seismic crosslines were distributed in the study area, from which 270 inlines and 78 crosslines were interpreted in this work. In order to define structure configuration of the interested sequences in the study area, four horizons were defined and interpreted.. The Seis2DV/Seis3DV viewing and interpretation seismic sections supported by the GeoFrame software, was used to visualize and interpret these seismic lines. The target sequences to be interpreted in the study area are the Bentiu-1 Sequence which consists of (Bentiu 1A, Bentiu 1B, Bentiu 1C and Bentiu 1D) subsequences and Aradeiba Sequence which consists of (Aradeiba A, Aradeiba B Aradeiba C and Aradeiba D) subsequences. The actually picked horizons in this study are the tops of Bentiu 1A, Bentiu 1D, Aradeiba A, and Aradeiba D. RESULTS AND DISCUSSION Structural Framework: Figs (3) and (4) show the structural contour maps of Bentiu-1A and Aradeiba-A Formations respectively. On the seismic sections the Bentiu top Formation is characterized by positive peak amplitude through the whole area. The units of Bentiu Formation are separated by laterally extensive intervals of mudrock. Thinner mudrock units are locally correlatable, but are not of regional extent. The Bentiu Formation is overlain by the Aradeiba Formation, with varying thicknesses. Aradeiba Formation was deposited and distributed laterally in the whole area. The Heglig area comprises a series of tilted halfgrabens, stepping down towards the southwest. Fig (5) shows that the main structural elements of the area include the down-to the west extensional faulting dominantly on the eastern side, and much more complex faulting, possibly with a strike-slip component in the west. Regionally, the area is situated near the intersection of two major structural trends. A NNW structural trend dominates much of the Muglad Basin, but in the Heglig area an additional fault trend strikes approximately north-south. The eastern part of the study area features relatively simple fault geometries dominated by NNE-SSW extension. The western part of the study area exhibits greater complexity in the fault patterns including fault dip reversals of adjacent faults along strike (scissor faults). Fault density is also significantly higher in the west of the study area, and fault dips are steeper. It is likely that the western part of the study area has a stronger strike-slip component than the east. All of these fault systems are of Late Jurassic to Early Cretaceous age. The faults present planar normal faults and listric normal faults in rotated fault zones. Faults combine into different assemblage 78

3 patterns such as Y-shaped, fan-shaped and parallel step-wise could also be observed in Fig (6). These faults totally control structure evolution, generat oiltrapping fault blocks. The tensional movements resulting in a tilted fault-block, hence, these styles of structure mainly include rotated tilted fault blocks, faulted anticlines and horst blocks. The faults usually juxtapose the Bentiu A1 against Aradeiba D. Fig (3) Bentiu-1A Formation depth structure map Fig (4) Aradeiba A Formation depth structure map Fig (5) Depth-structure map of the Bentiu-1 ( the main structural elements of the area are including the down-to the west extensional faulting dominantly on the eastern side, and much more complex faulting, possibly with a strike-slip component in the west) 79

4 Fig (6) T he fault styles and there significance distributed through the study area Stratigraphic Framework: The stratigraphic framework of the reservoir studied in this paper comprises in ascending stratigraphic order the Albian-Cenomanian Bentiu Formation and the overlying lower Toarcian Aradeiba Formation. The Bentiu Formation is divided into four component intervals within the Greater Heglig area (Bentiu -1, Bentiu-2, Bentiu-3, and Bentiu-4) in descending stratigraphic order. Each of the Bentiu units is further divided into sub-units, which are named using capital letter suffixes in descending stratigraphic order (Bentiu-1A, 1B, 1C and 1D). Fig (7) provides an example of wireline log section and stratigraphic subdivision for parts of the units studied. Correlation was conducted using sequence stratigraphic principles. This comprises initially the identification of marker horizons relating to significant changes in subsidence or sediment supply (accommodation). Marker horizons may comprise laterally extensive shales resulting from lacustrine flooding, laterally extensive mature palaeosols, systematic changes in sediment stacking patterns (prograding, retrograding or aggradational stacking patterns), or surfaces of incision. A significant change in sediment stacking patterns occurs at the boundary of Bentiu and Aradeiba Formations, with a much greater proportion of mudrock preserved in the Aradeiba Formation, and more isolated sand bodies, rather than the extensive sand sheets of the Bentiu Formation. This suggests the onset of another phase of subsidence in the basin, after times of relatively quiescent, regional sag during accumulation of the Bentiu fluvial unit. The contact between the two units is often cryptic and often several possible correlation points seem to correspond to the top of the Bentiu Formation. In the Greater Heglig area marker horizons were identified within the Aradeiba Formation and within the Bentiu Formation. Marker horizons closely approximate time lines, and thus divide the succession into depositional episodes. The interpretation of the depositional environments forms the basis for stratigraphic correlation and for subsequent estimates of reservoir architecture and heterogeneity. The Bentiu Formation comprises Interbedded and amalgamated fluvial sandstones interbedded with moderately mature palaeosols, the depositional environments of Aradeiba Formation comprise deposits of high sinuosity, mixed-load fluvial channels and their associated overbank deposits in the lower Aradeiba Formation. Additional information about sediment body geometries of channel deposits can be obtained from cores and wireline logs. For pointbar deposits channel belt width can be estimated from channel thickness, based on a number of empirical and experimental relationships. Lateral migration of meandering river courses results in the classical fining-up profile of pointbar deposits (Allen 1963). Each fining-up profile records the accumulation of successively finer grained and shallower deposits on a pointbar surface as both channel and pointbar migrate laterally Fig (8). The thickness of a full fining-up profile is therefore a measure of bankfull channel depth. The width of the active channel belt and the depth of channels are genetically linked (Leopold and Wolman 1960, Fielding and Crane 1987, Bridge and Mackey 1993), and are also proportional to a series of other parameters such as wavelength and radius of meanders Fig (9). Fig (7) Idealized reference section for upper reservoir intervals in Heglig area (Well -6 and well-2 both are located in Heglig East field) Fig (8) Depositional Environment Pointbar from Allen, J.R.L. (1963) 80

5 Fig (9) Depositional Environment Channel Parameters Relationships from Allen, J.R.L. (1963) REFERENCES 1. Abdalla Y. Mohamed, A.Y, Ashcroft,W.A, and Whiteman A.J(2001). Structural development and crustal stretching in the Muglad Basin, Southern Sudan, Journal of African Earth Sciences, 32(2): Allen, J.R.L. (1963). the classification of crossstratified units, with notes on their origin. Sedimentology, 2, Bridge, J.S and Mackey, S.D (1993). A theoretical study of fluvial sandstone body dimensions. In: Flint, S. & Bryant, I.D., eds., Quantitative description and modeling of clastic hydrocarbon reservoirs, and outcrop analogues. IAS Spec. Publs, 15, p Fairhead, J. D., (1988). Mesozoic plate tectonic reconstruction of the central South Atlantic Ocean: the role of the West and Central African rift system: Tectonophysics, v. 155, p Fielding and Crane (1987). An application of statistical modelling to the prediction of hydrocarbon recovery factors in fluvial reservoir sequences in: Ethridge et al (eds) Recent developments in fluvial sedimentology S.E.P.M special publication Gain Battista Vai, (2003). Development of palaeogeography of Pangaea from Late Carboniferous o Early Permian, Palaeogeography, Palaeocolimatology, Palaeoecology, 196(1-2): Giedt, N. R., (1990). Unity Field-Sudan, Muglad Rift Basin Upper Nile Province: in E. A. Beaumont and N. H. Foster, eds., Structural traps III: AAPG Treatise of Petroleum Geology, Atlas of Oil and Gas Fields, P Kaska, H. V., (1989). A spore and pollen zonation of Early Cretaceous to Tertiary non-marine sediments of central Sudan: Palynology, v. 13, p McHargue, T. R., T. L. Heidrick, and J. E. Livingston, (1992). Tectonostratigraphic development of the interior Sudan rifts, Central Africa-Geodynamics of Rifting, Volume II. Case History Studies on Rifts: North and South America and Africa: Tectonophysics, v. 213, p , Amsterdam. 10. Mohamed. A.Y., Iliffe.J.E, Ashcroft,W.A, and Whiteman A.J., (2000). Burial and maturation history of the heglig field area, Muglad Basin, Sudan. Jour. Petr. Geol. 23(1): [ ]. 11. Leopold and Wolman (1960). River meanders. Geological Society of America Bulletin vol. 71. No Schull,T. J., ( 1988). Rift basins of interior Sudan: Petroleum exploration and discovery, American Association of Petroleum Geologists Bulletin, v. 72/10, p T. Pletsch J.Erbacher, A.E.L., Holbourn, (2001). Cretaceous separation of Africa and South America: the view from the West Africa margin ( ODP Leg 159), Journal of South America Earth Sciences, 14(2):