Structural evolution of JG and JD fields, Abu Gharadig basin, Western Desert, Egypt, and its impact on hydrocarbon exploration
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The Abu Gharadig basin in the Western Desert (NW Egypt) is a mature hydrocarbon province with over 95% of the oil and gas fields in the Upper Cretaceous reservoirs. JG and JD fields are producing fields; JG is producing from the Jurassic reservoirs, while the JD field is producing from the shallower Tertiary reservoir. Both fields lie on the footwall of the main bounding fault of Abu Gharadig basin trending E–W and NW–SE. Four dry wells have been drilled into the deeper Jurassic reservoir, and this paper will try to figure out the possible reasons for the failure of Jurassic reservoir in JD compared to JG field 2002. Two main geological cross sections have been created. The first cross section ties the JD field to the Abu Gharadig basin and the second one ties the JG field to same depocenter. The restoration has been carried out using 2D Move (Midland Valley software) to restore the two sections to their original tectonic status. Since the Eocene–Miocene is the common time for the hydrocarbon expulsion in Abu Gharadig basin, both JG and JD traps were formed at that time. The restoration clearly showed the juxtaposition of the Jurassic reservoirs against Abu Roash and Khoman (shale, limestone and chalk) in JG. In contrary, the Jurassic reservoir is juxtaposed with Kharita and Alam El Bueb (Sandstone) in JD. Fault juxtaposition has been carried out also and concluded the juxtaposition of Abu Roash F member and Masajid flip East of JD well so no chance of Abu Roash F member charging JD.
KeywordsStructural evolution Hydrocarbon exploration JG field JD field Western desert
The Abu Gharadig basin in the Western Desert (NW Egypt) is a mature hydrocarbon province with over 95% of the oil and gas fields in the Upper Cretaceous Abu Roash, Bahariya and Kharita sandstone reservoirs. Many wells had penetrated the thick Lower Cretaceous Alam El Bueib sandstone reservoir, but no significant discoveries were made. This led most operators to believe that there was very little prospectivity at deeper levels, in particular in the Jurassic Safa sandstones overlying the Paleozoic basement. JG filed is one of the main oil-producing fields from the Jurassic section in Abu Gharadig basin and JD is the closest one for it; both of them are on the main bounding fault for Abu Gharadig basin, but JD wells were dry. This study will answer this mystery. The structure of the JG field is an elongated E–W fault dip closure bounded to the south by the Cretaceous Abu Gharadig basin-bounding fault. It is bisected by a series of smaller ENE–WSW and NW–SE trending. The JD structure consists of an elongated NE–SW-trending fault/dip closure mapped at the Jurassic Masajid level. To the SW, the JD closure is bounded by the Abu Gharadig basin-bounding faults. This fault system consists of fault throw transfers, a complex of relay ramps and hard fault linkages all in an area with poor seismic data quality. Toward the SE, the main AG fault terminates and the main part of the fault throw is transferred to the southern fault and a relay ramp is developed in between the two faults which are well developed at the Base Khoman level in the basin. Another fault is developed E of the main AG fault, and this fault also takes over part of the throw of the main AG fault. This fault is linked to the main AG fault by a relay ramp at Alamein level and by a hard-linked fault at the Masajid level. The main purpose of this paper is to evaluate and compare the reservoir intervals of JG and JD fields to investigate the structural evolution and its impact on the petroleum potentiality of the fields. The seismic interpretation and the 2D restoration are the main workflow items to figure out why JD is dry at the Jurassic level and JG is producing from the same level while they are very close to each other and lies on the same main bounding fault.
Location of study area
Structural phases in Abu Gharadig basin
According to Smith (1971), Africa moved eastward relative to Eurasia in the Early Jurassic and westward during the Late Cretaceous. The westward movement of Africa most probably caused rejuvenation of the ENE- to E–W-oriented preexisting normal faults by right-lateral wrenching. Local convergence in some parts of the basin is attributed to strike–slip movement at restraining fault bends, which causes local development of positive structural areas controlled by folding or reverse faulting. Geologic field data in the north Eastern Desert (Moustafa et al. 1985) indicate post-Late Eocene rejuvenation of the faults by oblique extension (major dip-slip normal components and subordinate right-lateral strike–slip components). This post-Eocene movement can be related to the Late Tertiary period of extension in northern Egypt. Later deformation in northern Egypt during the Oligocene seems to have been caused by ENE–WSW extension and was accompanied by the extrusion of basalts. This extension rejuvenated the old ENE- to E–W-oriented faults as normal or diagonal-slip faults and, in addition, formed NNW–SSE-oriented normal faults parallel to the Gulf of Suez trend. These sets (ENE to E–W and NW–SSE) controlled the deposition of the Miocene rocks in the basin. The three rhomb-shaped sub-basins of the Abu Gharadig basin are “right stepped” and indicate that they were perhaps formed as pull-apart (rhomb) grabens by right-lateral oblique-slip movements on three ENE- to E–W-oriented, preexisting right-stepped, en-echelon normal faults and these part will be covered by separate chapter titled by structural evolution of JG and JD fields.
Seismic data interpretation
The synthetic seismograms were carried out using two key wells Sheba 42-1 and JD4 using NDI (SHELL property package) as follows:
SW–NE-oriented faults mainly.
Growth faults at the Jurassic–Late Cretaceous time.
The Semblance cube over the survey has been carried out, and it shows clearly the major faults and some minor faults, especially the faults that are away from the main bounding faults due to the fault shadow effect.
Cross-sectional restoration is a potentially powerful tool for structural analysis. The objective of the restoration process is to unfold and unfault actual geologic data starting with the present state of deformation. The total deformation of a sequence can be described as the sum of deformation due to folding, faulting and compaction; therefore, all effects should be considered to restore profiles correctly (Novoa et al. 2000). The method used in this study for restoration combines structural and back-stripping techniques and consists of three major steps.
The previous figures show the steps of the restoration; especially at the time of sedimentation of Apollonia (time of hydrocarbon expulsion), these cross sections show that the juxtaposition at that time in JD area was Bahariya–Kharita (Sandstone mainly) against the Jurassic reservoirs and it means the hydrocarbon will leak to shallower level.
JG shows largest separation between FW Masajid and HW Abu Roash F member.
Juxtaposition of Abu Roash F member and Masajid flip East of JD well and no chance of Abu Roash F member charging Masajid.
Khoman chalk HW juxtaposed Jurassic reservoir FW on JG.
Bahariya–Kharita (sandstone mainly) juxtaposed Jurassic reservoir FW on JD.
Apply 2D and 3D restoration exercises on any prospect, especially they are located on the main bounding faults and the major faults.
Build a fault juxtaposition diagram (Allan diagram).
The authors thank the Egyptian General Petroleum Corporation (EGPC) for releasing the data of this study. Also deep thanks to Tarek Kamel, Shell Egypt, for advice during writing this manuscript.
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