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Upper Cretaceous Source Rock Distribution, Richness, Thermal Maturity, and Petroleum Generation History, Sirt Basin, Libya


Two marine petroleum source rocks were analyzed, characterized and mapped in order to define the distribution, quality, thermal maturity and petroleum generation history in the largest petroliferous sedimentary basin in Libya. These two Upper Cretaceous marine petroleum source rocks, from oldest to youngest are Coniacian Rachmat and Campanian Sirte shale. Rock-Eval pyrolysis analysis of 2566 samples in 141 wells penetrated the Cretaceous ssection in Sirt basin were used to identify the quantity, quality and distribution for each source rock. Regional geochemical correlation was conducted to identify the lateral and vertical variation of the organofacies. Maturity levels based on 1D burial history models and 2D modeling approaches were calibrated with %VRo data in order to establish regional maturation maps for each source rock. Mapping the present-day kerogen transformation ratio (TR), total organic carbon (TOCo) and hydrogen index (HIo) were employed to carry out the geographic extent of the source kitchen throughout the basin. We used basin modeling techniques to define the time of the petroleum generation, amount, and the expelled phase from Rachmat and Sirte shale. Both marine source rocks with kerogen type I/II showed good geochemical characteristics (generally TOCo >3 wt.% and HIo > 550) with rapid lateral and vertical variation in their organic content. However, Rachmat and Sirte shale are dominated by two organofacies (B and D/E). These two organofacies, show direct influence on the generated petroleum in Sirt basin (type and amount). The maturity levels ranged from mid to late-mature and show progressive increase from the basin platforms toward the troughs as they entered the late maturity levels in the deepest part in Sirt basin (Ajdabiya trough). The petroleum generation history models suggest that Rachmat formation started to generate petroleum in the middle Paleocene (64 M.y). In contrast, Sirte shale started to generate petroleum during the middle Eocene time (48 M.y) and the generation peaked during Oligocene time (30 M.y). The expelled phase from both the source rocks is oil and gas phase based on the organic content and maturity levels. The predictions that were acquired from the modeling of the physical properties GOR in range of 300-700 scf/bbl and API gravity in rang of 30-45 for the expelled petroleum were obtained simultaneously with the modeled generation history. Keywords: Petroleum Generation History; Rachmat; Sirt basin; Sirte shale; Source rocks