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Abstract This study was carried out to evaluate the source rock and to determine the organic richness of the studied unit, as well as, the effects of structural and stratigraphic implications on the subsurface inferences, in order to conduct more activities at area. The area of study is located in East Bahariya area which lies at the northeastern part of the Western Desert, Egypt. It is bounded by latitudes 29° 45´ - 29° 48´ N and longitudes 29° 24´ -29° 30´ E which belong to Qarun Petroleum Company and represents the development leases of Yomna and “X” oil fields. Four wells (Yomna-1X, Yomna-7a, Yomna-22 & Yomna-25) were used in this work. The data used in the source rock evaluation of ARF Member were mud logs, geochemical analysis and wire line logs in addition to twenty 3-D seismic sections. The geochemical results included (TOC wt.%, Rock-Eval pyrolysis, kerogen composition and vitrinite measurements), and the available wire line logs used in determining the organic richness of the fore-mentioned rock unit included resistivity logs, porosity logs (Sonic, Neutron and Density) and gamma-ray logs. A geological review of the study area including stratigraphy, structure and tectonic implications that operated in East of Abu Gharadig Basin, northern Western Desert is exhibited. The stratigraphic succession was defined from the drilled wells in the area, which exhibited rock units ranging in age from the Jurassic Khattatba Formation, which unconformably overlain the Pre-Cambrian Basement complex, to the Miocene Moghra Formation. The constructed isochore maps for the study area revealed a general thickening in the southwestern part toward “X” oil field, especially in Khoman Formation due to the growth thickness on the major WNW-ESE fault. ARF Member showed a widespread correlatable rock unit all over the study area. The description of the rock composition, from the mud logs and wireline logs, suggested that, it was formed mainly of limestone with locally some shale streaks and organically rich with free gases. It represented a maximum flooding surface (MFS) and the suggested depositional environment was slightly outer shelf marine environment. The seismic interpretation of the faults and horizons was conducted on twenty 3D seismic sections, and some of these horizons were mapped which included Abu Roash “G”, “D” Members and Khoman Formation. The interpreted seismic sections and structure maps showed that, the area was affected by many faults which trending (WNW-ESE, NW-SE and E-W). Also, they showed a right lateral strike movement in Summary and Conclusions 128 | P a g e the late Cretaceous time where the WNW oriented deep-seated fault was rejuvenated by this movement which could be due to a regional shear couple in the north Western Desert. Detailed source rock analyses were carried out on representative cutting samples collected from three wells in the study area. The geochemical results revealed that, the ARF Member is a source rock, with good to very good potential to generate mainly oil and is considered as a strongly oil-prone source rock (Type I- II Kerogen) for most of the studied area. Also, the kerogen microscopy investigation showed predominant of oil-prone sapropelic materials associated with little humic debris (unstructured lipids 85-100% and vitrinite up to 10%). The top of the oil window (defined by 0.6% Ro) appeared to be encountered close to the top of the ARF Member at approximately 8100 feet. The unstructured lipids TAI values appeared to be in agreement with the Ro% values and suggested a slightly lower maturity than that derived from the Ro% values. Moreover, a geochemical analysis was carried out on one gas sample and one crude oil obtained from Yomna-22 well within the study area which recovered from the Upper Cretaceous ARE Member at (2662 – 2677 & 2713 - 2717) ft. The oil sample from Yomna-22 well has 24.1o API. The whole oil gas chromatograms showed a normal crude oil with an n-alkane distribution pattern in the range of iC4 to n- C35+. In addition to, the oil sample suggested to be generated at moderately thermal maturity close to 0.6% Ro and an expulsion temperature of " ~ "125o C. Besides that, the oil was generated from a predominantly marine organic source rock which deposited under mixed environmental conditions. The discussions of geochemical characterization, Biomarker characteristics of oil and source extract shows oil–source rock correlations as the following: 1) The Eastern trough of the Abu Gharadig Basin is located in the Northern part of the concession. This sub-basin comprises a Well-developed Jurassic shale section with excellent source rocks in the Khatatba Formation which deposited in a continental to inner-middle shelf environments. Several intervals within Khatatba have oil and gas prone source rocks (Kerogen type’s Ⅱ & Ⅲ) also the late cretaceous Abu Roash “F”, carbonate on inner-middle neritic environment has outstanding oil source characterization (Kerogen type’s Ⅱ). Summary and Conclusions 129 | P a g e 2) The Development of Yomna field structure on level Abu Roash is thought to be pre-oil migration that took place after Khoman deposition. This evidenced by the angular unconformity between the Abu Roash and the overlying Khoman beds, where the almost flat lying Khoman layers are on-lapping onto the underlying, relatively steeply dipping, Abu Roash “A” Surface. This in-turn suggests that the Mubarak structure has been formed after Abu Roash but before Khoman, i.e. before oil migration. 3) The source rock extracts possess a degree of maturation level similar to that of oil samples. The extracts of the ARF derived from mixed marine inputs with a limited terrestrial contribution and Khatatba formations are seen to be derived from mixed organic sources in which terrestrial dominates marine sources, and deposited in transitional environments under less anoxic conditions. 4) The time of oil generation from Khatatba sources started most likely during the middle to late santonian (Abu Roash deposition) or about 90 million years ago. However expulsion did not commence until 75 million years ago during Campanian (Base Khoman Deposition) and stopped expelling oil 34 million years ago during the Miocene. Thus timing of oil migration relative to the structure trap formation. 5) Generation and expulsion of gas from the Khatatba continue to the present day. 6) The late cretaceous Abu Roash “F”, carbonate on inner-middle neritic environment has outstanding oil source characterization (Kerogen type’s Ⅱ). Oil generation and expulsion from the Abu Roash “F” carbonate started during the Masstrichtian (about 80 million years ago) or 10 million years after the lower Safa generation and expulsion 7) ARE oil sample are slightly more mature than the extracts which deposited in transitional environments with mixed organic source input. 8) The gross composition and biomarker analyses of oil and source rock extracts support the indigenous mixed source of the ARE oil which are related to different sources including the ARF, Khatatba formations. Summary and Conclusions 130 | P a g e Thermal and geochemical modeling is one of the most widely used techniques used to predict the validity of an area for future and upcoming oil and gas discoveries. The integration between the available geochemical data of the sedimentary units led to the construction of basin models to evaluate the burial and thermal histories of the sedimentary sequence. In addition, determination of the favorable zones for the generation of hydrocarbon from ARF source rock was defined based on the deposition and the thickness of the overburden rocks. Moreover the timing of the hydrocarbon generation and expulsion by using the 1-D Petro-Mod software was accomplished. The thermal modeling showed that, the oil window in the study area was started during the Late Cretaceous at about 30 Ma and the end of hydrocarbon generation not reached up-till now. The maturity distribution throughout the study area reflected the presence of immature ARF sediments occupied the northern and southern parts of the map in X1X well area, whereas the analysis is affected by the unstructured lipids Also, the hydrocarbon transformation of Abu Roash “F” Member X-1X show that the generated hydrocarbon from the early mature source rock reached about 0.25 Mtons Kg/m2 and 2.73 & 1.68 Mtons for Yomna-1X U.Safa & L.Safa respectively, and the migration pathways were assumed. Only if sufficient hydrocarbons are generated in the source rock, primary migration (i.e. expulsion of hydrocarbon from the source rock) will occur whether diffusion or pressure-driver mechanisms operate. As might be expected, there is a lag between the onset of petroleum generation and expulsion because a minimum degree of petroleum saturation within the source rock is required. However, petroleum can only migrate from a coarse-grained rock into a fine-grained rock if the capillary potential is overcome (or if fractures are present). where the sediment sorting is poor or the mudstones are particularly silty, only a very small capillary barrier may need to be overcome before petroleum can migrate from coarse-grained into fine-grained rocks. A small petroleum accumulation may build up in each layer of coarse-grained rock, and then the seal may be breached, allowing further flow. The modeled wells together with the depth structure contour map at the top of ARF Member, and the permeability were used as the input parameters to locate the possible pathways for hydrocarbon transportation (migration pathways) from the expulsion area (kitchen) to the possible hydrocarbon accumulation areas which located at the shallower depths. On the other hand, some of the petroleum system in the study Summary and Conclusions 131 | P a g e area such as reservoir, source rock with its maturation stages and the proposed migration. Moreover, by integrating the subsurface information, the results of source rock evaluation and the basin modeling of the studied area, the present study can stated that, the ARF Member, Khatatab and AEB formation is rich with organic matter and both could be considered as an effective source rock for the oil presented in Yomna 7a & 22 wells. Detecting the migration pathway of the hydrocarbon from source areas to Reservoir in the study areas will open new rooms of prospect new locations of shallow reservoir. from the previous discussion it should be clear that petroleum flow is predominantly vertical in fine-grained, low-permeability mudstones and laterally updip in coarse-grained, higher permeability rocks. The present study confirm the importance of tectonic evolution of the far field and field scale areas in hydrocarbon migration, which it should be considered in any forthcoming prospect generation. It is recommended to test the selected proposed oil accumulations along the paths of oil migration to enhance the productivity of this field & adjacent fields which will add more new reservoir prospects. Accordingly we have a good opportunity to drill more development wells to enhancement the productivity of the study area. |