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)

Abstract The basaltic Intrusions of the Central Jabal Nafūsah Foothills which extend from NW Wādi Zāret to NE Wādi Ghān Dām are part of the latest stage of Gharyān Volcanic Province (GVP). These intrusions are classified on basis of their shape and mechanism of emplacement into four kinds of volcanic bodies. These are; sheets (dykes and a sill), dykes associated with volcaniclasts, volcanic cone and lava mounds. Generally, the dyke emplacements are restricted only to the area between Wādi Zāret to Abū Ghaylān, while the other forms extend from Rās al Mazūl Dome to Wādi Ghān Dam. The more differentiated rocks are restricted to the area between Rās al Mazūl Dome and Wādi Ghān. Farther west towards Wādi Zāret, ultramafic xenoliths and magnesium-number increase, suggesting closer proximity to the source. The rocks are essentially alkaline with within Intraplate signatures. They straddle the composition from picrites, basanites, alkali basalts through hawaiites, mugearite to benmoreites with a compositional gap between the last two types. Generally, the phenocrysts are represented by foresteritic olivine, Cadiopsidic pyroxene, magnesiotaramitic amphibole, plagioclase, K-feldspar and titanomagnetite. The chemical composition of the mafic minerals indicates that they are high pressure phenocryst phases. The most primitive picrites satisfied the criteria of primary mantle melts. The rocks are generally, enriched in LILE suggesting an enriched mantle source. The studied rocks were grouped into five groups based on incompatible trace element ratios; Group-A includes picrite, basanites and hawaiites, and Group-B includes picrite (Z-3), basanites , alkali basalts, and hawaiites, while Group-C is formed of hawaiites, Group-D is composed of mugearite and Group-E is made up of benmoreites. Picrites and basanites of these rock have high Mg-number (>0.64), high Cr and Ni contents and strong light rare earth element enrichment, but systematic depletion in Rb, K and Ba relative to trace elements of similar compatibility in anhydrous mantle. Alkali basalts and more differentiated magmatic rocks have lower Mg-number and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe–Ti oxide, amphibole and plagioclase. The variation in the concentrations of major, trace, rare earth elements, and incompatible element ratios in the rock samples demonstrate the heterogeneous character of their source region. Such heterogeneity can be interpreted by the involvement of a heterogeneous mantle reservoir to different degrees of partial melting. The REE data require residual spinel stability peridotite field in the source and constrain the melting process of Group-C and Group-D to 2% to 3.5% degrees of melting respectively, Group-A and Group-B both to 5% degree of partial melting while Group-E to 10% degree of partial melting of spinel lherzolite xenoliths of Al Ourban area. Mass balance modelling of the major suggests two possible FC scenarios; Derivation of basanites and hawaiites of group-A from G-3 picritic parental magma. Derivation of Group-D and Group-E was also possible from these basanites. Derivation of basanites of Group-B from Z-3 picrite parental magma and simultaneous derivation of G-4 and QJ-1 alkali basalts from Z-3 picrite parental magma. V Simple mass-balance calculations suggest that the melting assemblages of picrites and basanites consisted of forsteritic olivine, diopsidic clinopyroxene, Ti-magnetite. While the alkali basalts and more differentiated magmatic rocks, mass-balance calculations suggest that the melting assemblages consisted of sodic plagioclase, magnesiotaramitic amphibole, diopsidic pyroxene, Ti-magnetite, K-feldspar with sub amounts of apatite and sphene.
سمية عون (2015)

Abstract Seven geological sections have been examined and sampled with emphasis on sedimentological & diagenetic processes within Abu Ghaylan Formation and the contact relationships with the underlying Abu Shaybah and overlying Kiklah Formations. Based on current detailed field loggings and microscope investigations, 10 facies are recognized within Abu Ghaylan Formation. The lowermost succession characterised by an overall transition from continental fluvially dominated deposits of the Abu Shaybah Formation into transitional to marginal marine, tidal flat deposits of the Abu Ghaylan Formation upward. The transitional nature of this lowermost part succession is demonstrated by interbeddings of claystone, sandstone and fossiliferous facies arranging in coarsening up cycle, in which most probably deposited in estuary / beach environment.The Abu Ghaylan Formation generally wedges laterally eastward and exposes intermittently east of Wadi Ghan area resulting of syngenetic uplift of Wadi Ghan area followed by erosion (ElHinnawy & Cheshitev, 1975). A slightly earlier uplift and greater erosion in a westward sloping basin of deposition near the close of the Early Jurassic and Early Cretaceous times is suggested by Fatmi & Sbeta (1991). The current study reveals that several exposure surfaces are demonstrated within Abu Ghaylan Formation suggesting uplifting and erosion episodes interrupted Abu Ghaylan Formation, where a restricted distribution of Abu Ghaylan may suggest a local tectonic overprint.Although the overall impression is that the base beds of Kiklah Formation starts with sandstone and red clay beds as channel infill deposits above Abu Ghaylan carbonate unit with surface of unconformity, however, based on current close investigation, a sequence characterized by interbeddings of carbonate and greenish clay overlying Abu Ghaylan Formation with surface of unconformity and gradationally overlain by sandstone beds should be introduced as separate unit attributes to Kiklah Formation.The current study emphasizes and supports a solution collapse origin for the breccias and introduces better insight into architecture and geomorphology of the breccias bodies that are exposed in the area of study. The dissolution of evaporites is considered responsible for all breccias development in this stratigraphic interval. According to the occurrence mode of breccias, the breccias intervals have been divided into two main parts; a lower section of strata that contains collapsed paleocaves and an upper section of strata that is deformed to varying degrees due to the collapse and compaction of the section of paleocave-bearing strata. The sharp flat base, inverse grading, V-shaped/ sag structures and irregular undulating top of brecciated bodies are recognized within Abu Ghaylan Formation and typically characterize solution collapse processes. Based on field study and architecture relationships, five distinctive karst facies are recognized in the area of study.The early diagenetic genetically related processes probably interrupted the deposition of Abu Ghaylan Formation is a most possible assumption for the origin and timing of breccias formation.
أحمد أبوبكر الحلو (2015)