Source rock appraisal of Taqrifat Shale-Mallegh formation and the origin of crude oils of Cyrenaica region, NE Libya

Abstract

The Cyrenaica consists of two main tectonic provinces that influence the area: the Cyrenaica Platform in the south and the Al Jabal Al Akhdar Uplift and Fold Belt in the north, and it is considered one of the smallest basins in Libya. The goal of this work is to attempt to correlate the geochemical characteristics of the source rocks with the samples of crude oils and infer their organic matter richness, thermal maturation, and depositional environment. The following four wells yielded samples of fifty-nine cutting samples of source rocks: nearly fifteen cutting samples from each well, Mallegh Formation in wells A1-NC120 and B1-NC152, whereas Taqrifat Shale in wells T1-41 and A1-46. Four crude oil samples were examined and subjected to Gas Chromatography-Mass Spectrometry (GC–MS) in this work: one sample of oil from each well, the Daryanah Formation in wells A1-NC120 and B1-NC152, and the Antenlat Formation in wells A1-46 and T1-41. Rock-Eval pyrolysis and GC–MS were utilized with these materials to determine the specific type of organic matter, thermal maturation, and depositional environment. The relationship between TOC and S₂ showed the Mallegh Formation in wells A1-NC120 and B1-NC152 and Taqrifat Shale in well A1-46 had poor to fair organic matter, while the TOC of Taqrifat Shale in well T1-41 had good to excellent organic matter contents. TOC vs. S₁+S₂ revealed the source rock of the Mallegh Formation in wells A1-NC120 and B1-NC152, and the source rock of the Taqrifat Shale Formation in well A1-46 was revealed as non-potential hydrocarbon, while the source rock of the Taqrifat Shale in well T1-41 showed potential hydrocarbon. TOC and S₂ were used to identify the kerogen types, the Mallegh Formation in wells A1-NC120 and B1-NC152 and Taqrifat Shale in well A1-46, represented by two types of kerogen (III and IV), while Taqrifat Shale in well T1-41 is represented by three types of kerogen (II/III, III, and IV). The ratio of CPI vs. Pr/Ph and Pr/Ph vs. C₃₁ 22S/(22S+22R) revealed that the oil samples from the Antenlat Formation in wells A1-46 and T1-41 and the oil sample from the Daryanah Formation in well B1-NC152 were deposited in a marine environment under oxidizing conditions, whereas the oil sample from the Daryanah Formation in well A1-NC120 was deposited in a marine environment under an anoxic condition. Based on the plot of Ph/n-C₁₈ vs. Pr/n-C₁₇ for the origin of organic matter in oil samples, the organic matter from the oil sample of the Daryanah Formation in well A1-NC120 was derived from algal marine type, and the oil samples from the Daryanah Formation in well B1-NC152 and the oil samples from the Antenlat Formation in wells T1-41 and A1-46 were derived from mixed organic matter (marine type and terrestrial type). According to organic matter type and thermal maturity level for the source rock samples and oil sample results, the source rock of the Mallegh Formation in well B1-NC152 was match correlated with the oil sample from the Daryanah Formation (reservoir rock) in well B1-NC152; the source rock of the Taqrifat Shale in wells T1-41 and A1-46 was match correlated with the oil samples from the Antenlat Formation (reservoir rock) in wells T1-41 and A1-46, while the source rock of the Mallegh Formation in well A1-NC120 was not correlated with the oil sample from the Daryanah Formation (reservoir rock) in well A1-NC120 due to differences in the origin of organic matter.