Abstract The Sirt Basin is one of the youngest sedimentary basins in Libya and covers an area of approximately 600.000 km² in north central Libya. It is located on the northern margin of the African plate, with approximate coordinates 14°00`- 20°00`E and 28°00`-31°00`N. The Late Jurassic-Early Cretaceous as Sarir Sandstone in the Sirt Basin evolved as a consequence of the interplay between global eustasy and regional tectonics. As Sarir Sandstone of the eastern Sirt Basin is composed mainly of sandstones and shales resting unconformably on a basement complex of igneous and metamorphic rocks. It is unconformably overlain by the Upper Cretaceous sediments of Maragh, Lidam and Etel formations. It has been subdivided into three members. The upper member consists mainly of sandstones with intercalations of siltstones and shales of variable thickness. The middle member consists of shale and silty shale. The basal part of the formation, which rests directly on the crystalline basement represents the lower member. It is comprised of sandstone with subordinate intercalations of siltstones and shale. Two main facies have been recognized on the basis of lithological features and types of stratifications: which consist mainly of shale and intercalation of silt and sand. The Middle Shale Member consists mainly of shallow lake deposits, containing black shale facies. Continental Fluvial Deposits represent most of the cored interval and it belongs to the Upper as Sarir Sandstone. These facies have been subdivided into braided and meandering lithofacies: The braided depositional system, has been recognized in three different intervals of the studied cores; classified as subfacies (1), subfacies (2) and subfacies (3). The meandering depositional system, represents three sub-environments including point bars, abandoned channels and over-bank deposits. The reservoir characteristics of these sediments are studied on the basis of porosity, permeability and reservoir zonation. The reservoir quality of As Sarir Formation is largely controlled by primary sedimentary features associations, which were shaped by the depositional environments. Meandering Lithofacies are formed from continuous sand successions and show good porosities, up to 18.51%, and permeabilities up to 125.5mD. The Braided Lithofacies also show good reservoir quality, but the effective porosity and permeability are lower, because of associated shales. The most important diagenetic features responsible for a reduction in reservoir quality are those resulting from compaction, cementation, and the diagenesis of clay minerals.
صلاح الدين محمد الوحيشي (2014)

Abstract As part of the NE-trending Mourizidie Shear Zone in south-central Libya (south of Mourizidie pass), four units representing Preccambrian basement rocks (metasediments) were mapped during field work: phyllites (metaclaystone); metapelites (metasiltstone); pasmmites (metasandstone) and Quartzites. Granitic bodies of various sizes (few meters to hundreds of meters) are found throughout the study area. The metasediments and granites are both intruded by veins and dikes having an overall NE trend. Four Palaeozoic sedimentary units were mapped within the study area. The concidence of the S1 foliation with the S0 of the original bedding of the protolith is a proof that the S1 foliation is caused by deep burial. The petrographic description of the minerals in thin section shows an assemblage of sericite, muscovite and biotite, all representing a sub-greenschist facies to greenshcist facies. This low grade metamorphism helped in preseving the original sedimentary structures of the protolith, thus helping in identifying the original bedding plane S0. Faults observed in the field or traced on aerial photograhs belong to three major trend: NE, NNE, and ENE. Folds are extremely diverse in shape, attiude and tightness, thus their classification is equally diverse. This diversity is observed in the field even within a distance of no more than few meters. The isostatic rebound of the basement could have caused this great difference in the attitude of the folds, especially at considerable depths characterised by a kinematically ductile, restricted and contained settingAt least two phases of deformation are present in the area. D1 is marked by the generation of S1, while D2 is marked by the first folding F1 caused by the isostatic rebound he generation of S2 foliation as fan cleavage is directly related to the F1 folding. Some folds underwent refolding, which lead to an F2 phase of folding.Sequential schematic structural model is proposed to explain the structural history of the study area. This model should be tested through intensive detailed field work large scale map in nearby areas.
مسعودة محمد حنبولة (2015)

Abstract The area of study is located in the southeastern part of Block NC98 and is informally named C-Structure area, and it is completely situated in the center of Al Hameimāt Trough, South East Sirt Basin, Libya. The main reservoir rock in this structure is the Upper Nubian Sandstone Member, whereas the Reworked Sandstone Unit and Transitional Beds are considered as secondary reservoir rocks. These reservoirs are mainly composed of sandstone to argillaceous sandstone. The Upper Nubian Sandstone reservoir quality is adversely affected by volcanic rocks especially the thick intercalation of volcanoclastics within the Upper Nubian Sandstone. The Upper Nubian Sandstone Member was deposited during Early Albian times. The Reworked Sandstone Unit was possibly deposited in Cenomanian? to Turonian times and may be related to the overlying Transition Beds.Up to date, four wells have been drilled in this structure (C1-NC98, C2-NC98, C3-NC98 and C4-NC98) and were targeting the clastic reservoirs of the Upper Nubian Sandstone and/or overlying Upper Cretaceous Transitional beds and Reworked Sandstone unit. Only C1-NC98 and C2-NC98 wells have tested significant hydrocarbon in the Upper Nubian and Reworked Sandstone unit whereas C3-NC98 and C4-NC98 wells have tested no hydrocarbon in both reservoirs and are abandoned as dry wells. This study is aimed to validate the stratigraphical aspects, depositional environment and reservoir quality assessment in order to decipher the reasons of drilled dry holes. The obtained results are anticipated to provide valuable geological information for the current and future prospect evaluation in the area. All available wire line logs of C wells (C1-NC98, C2-NC98, C3-NC98 and C4-NC98) and adjacent wells have been used to correlate the stratigraphy of the Pre-Upper and Upper Cretaceous silisiclastic sediments in the area of study.Representative core samples across the reservoir rocks of the candidate wells have been laid down for core description. The acquired results have been integrated with conventional core analysis and petrographic results to investigate the lithofacies characterization and depositional environment in order to sub-divide them into different lithofacies.Different types of geological maps have been prepared in this research (e.g. stratigraphical/structural cross sections, subcrop maps, isobach maps, structure contour maps and etc.) to delineate the formation extension and correlation. Burial history diagrams across all depositional sequences in the candidate wells were also prepared to identify the depositional scenario and to estimate the time span of tectonic subsidence along the depositional sequences.The results of all available geological studies which recently have been carried out by Waha Oil Company in the area of study and adjacent areas have been reviewed, elaborated and integrated with the results of this study to assess the discussion and final conclusion. The final results of sedimentological study indicates that there is no major effect of diagenetic processes on the reservoir rocks during and after the deposition. As a consequence, C3-NC98 and C4-NC98 wells were drilled in an area of poor reservoir quality of Reworked Sandstone and Upper Nubian reservoirs due to the existence of thick sequences of clay and siltstone which were mostly deposited in levee and over bank sub-environments of likely combined braided and meandering river. Whereas, C1 and C2-NC98 oil wells were drilled in an area of good reservoir quality of Reworked Sandstone and Upper Nubian Sandstone reservoirs where the sand bars sub-environment are deposited as a thick sequence of clean sands carrying the genetic of good reservoir properties. Therefore, the reservoir rocks in this area have different properties due to different sub-environment of deposition. This new results will probably assess the prospect generation and evaluation for the future drilling activity in the area of study.The encountered volcanoclastic sediments in well C2-NC98 reveals that this sediment is likely transported by river from the area of volcanic eruption to the area of deposition rather than in situ eruption. This type of volcano is likely applied to cinder cone volcano type where the pyroclastic fragments are not cemented together and thus easy to erode. Therefore, the intercalation of volcanoclastic sediment in Upper Nubian Sandstone has no marked effect in the reservoir rocks.Burial history curves of wells C1-NC98, C2-NC98, C3-NC98 and C4NC98 indicate three major subsidence events took place during Early Cretaceous time (Early Aptian - Early Albian), Late Cretaceous time (Turonian - Maastrichitian) and the Palaeocene - Eocene subsidence. However, Plaeocene - Eocene subsidence shows the highest subsidence rate with comparison to the other two subsidence rates.
محمد عمار هامان (2015)