الفهرس 
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Abstract
Libya is one of the largest oil and natural gas producing country in Africa with average production rate about 1.74 million barrels of oil daily. However, this rate can be improved with more detailed examination of the structural framework and investigation of the reservoir characterizations. The Sirt Basin is located in the north-central part of Libya, with an inshore area of approximately 375,000 km², associated with the opening history of Atlantic Ocean and the convergence between African and Eurasian plates. The basin comprises a broad NW trending embayment that is bounded to the south by the Tibisti Massif and to the west by the Al Gargaf Uplift, the Ghadames and Murzuq Basins. Eastwards it is bounded by the Cyrenaica Platform and Al Bottnan Basin, and to the north by the Gulf of Sirt. The tectonic activities associated with a series of subsidence events formed a number of E-W directional faults during the Early Cretaceous and NW-SE oriented faults of Late Cretaceous. These events caused the Sirte Basin to be distinguished by a series of northwestern basins separated by a series of platforms extending 600 km west-east.The area of study is located in the south-eastern part of the Sirt Basin, where the exploration activity began in concession No. 6 in the onshore part. The Cl-6 discovery well is considered the first commercial well in Libya. The Sheghega Formation is widely developed and composed of a series of shallow, nummulites-bearing marine mudstones that locally evolved into highly bio-limestones nummulites. It is the most economically gas-bearing intervals especially the reservoir intervals in the Sahl gas field located in eastern portion. The current work presented the formation evaluation and reservoir haracterization of the Middle Eocene Sheghega Formation, Sirt Basin-Libya. The gas–bearing intervals in the Sahl and Assumood fields are characterized by limestone lithological nature with a minimum shale content that exhibits laminated-dispersed mode of distribution. The deduced cementation exponent (m) value equals (2.6) and is influenced by the shale content, fluid saturation and effective porosity. The petrophysical characterizations aid in better assessment of the gas reserves of the Sheghega Formation, Sirt Basin Libya.Wireline log readings are employed to discriminate the lithological nature and delineate the reservoir intervals and its petrophysical properties. Gamma-ray log was used as a single shale indicator to estimate the shale index and corrected using Steiber formula to determine the shale volume. The porosity calculation was performed applying the density-neutron model and corrected to obtain the effective porosity values.The deduced effective porosity values, shale volume and its mode of distribution, water saturation and lithological discrimination are used to delineate the hydrocarbon-bearing intervals. The interpretation used the following criteria: shale (high GR), crossover of RHOB and very high NPHI associated with cutoff parameters Vsh>25%, PHIE<10%; gas-bearing zones: low GR, negative separation of the NPHI and RHOB, relatively high resistivity with remarkable separation (permeability indicator) established the cutoff parameters: Vsh<25%, PHIE>10’%, SW<50% and swirr<50%). The saturation crossplot and the lithe-facies distribution illustrate changes in the limestone lithological nature of the reservoir intervals. The lateral and vertical variations in reservoir facies of the Sheghega Formation and their related petrophysical properties have been recognized by log response, quantitative well-log analysis, dia-crossplot and special distribution maps of various parameters. The litho-saturation crossplots, dia-porosity crossplots, water saturation crossplots (Pickett and Buckle plots), pay zone facies, and cementation factor proven that the Sheghega Formation is a gas-bearing reservoir. The gas-filled zone is characterized by the following parameters: a=0.85, n=2.7, m=2.6 and Rw=0.0116 ohm-m with Swirr equals 0.18. The delineation of the cementation exponent and saturation exponent enhanced the Petrophysical evaluation of the reservoir intervals. The deduced cementation exponent (m) value equals 2.6 and is influenced by the shale content, fluid saturation and effective porosity. The petrophysical characterizations aid in better assessment of the gas reserves of the Sheghega Formation, Sirt Basin Libya.The gas–bearing intervals in the Sahl and Assumood fields are characterized by limestone lithological nature with a minimum shale content that exhibits laminated-dispersed mode of distribution. The pay zone is characterized by excellent effective porosity values (15-26%), relatively low water saturation (19-34%), thick net pay thicknesses (67-189ft) and low dispersed shale content. The petrophysical analysis of 6 well logs from Sahl and Assumood fields, divided the Middle Eocene Sheghega Formation into three zones according to its fluid content (gas-filled, transition and water –filled zones). This zonation is in accordance with the lithofacies and microfacies that point the different depositional environments (back-bank, fore-bank, and back-back lagoon outer-ramp open marine). The fore-bank facies have proven to be gas productive with effective porosity values rang between (0.15 to 0.26) and characterized by relatively high water saturation (0.19- 0.39), thick net thickness (67-189ft), early diagenesis (leaching) enhanced the reservoir properties and improve hydrocarbon bearing capacity. The transition zone was saturated with either irreducible water saturation or gas saturation. The net thickness increases towards the Assumood field. The most productive area located within the overlain gas-filled zone at Sahl field and the less productive area correlate to Assumood field westward. The transition zone is characterized by increased shale volume and deposited in lagoon conditions. The water- filled zone was saturated with irreducible water and free movable water. This zone is characterization by open- marine depositional environment.The gas-filled zone is discriminated into three lithofacies and pay zones (A, B and C) based on the deduced petrophysical parameters. Zones A, B and C have proven to be gas productive composed of limestone deposited in fore-bank environment, gas-wet system, owing to its petrophysical parameter, is limited to the NW-SE structure called Assumood Ridge (up-thrown fault blocks). The petrophysical evaluation reflects the vertical and lateral variations in fluid composition and revealed compartmentalization and reservoir architecture. The post depositional tilting of the Sirt Basin formed up-thrown structural traps favorable for gas accumulations when prominent leached or vugy limestone facies dominate that enhanced the reservoir characterization sufficiently.The significant variations in the petrophysical characteristics of the Middle Eocene Sheghega Formation are interpreted in terms of differences in their mode of deposition (fore-bank conditions) and the geological events (vugy and leaching diagenesis) that subsequently affected the reservoir quality. These reservoir characterizations are important in the exploration program and drilling of a new prospects (gas-bearing intervals) where it is closely related to gas-filled intervals (A, B, and C zones). Zone A is considered the most productive and promising, subsequently comes zone B of average productivity, and level C is the least level of the lowest uality.RECOMMENDATION-Especial core analysis of the Middle Eocene Sheghega Formation should be performed on cores for each litho-facies (zones A, B and C) -Integration of the litho-facies and petrophysical parameters improved the understanding of the net-pay, reservoir characterizations and quality.-A proper examination of the log response is highly recommended to define the lithofacies and construct maps using the core data available for the particular limestone zone of interest.-Implement of the well-log readings and analyses with RFT data into lithofacies model is exceptionally suggested