Geochemical segregation of early Permian, Paleocene and Eocene sediments of Potwar Basin, Pakistan: I Geophysical and isotopic analysis for source and depositional environment

Segregation of sediments for oil and gas potential is a complex task. In this study three oil wells i.e. Dhurnal, Pariwali and Pindori were analyzed for oil and gas potential using geophysical well logs and isotopic analysis. 50 sediments (Eocene to Early Permian) from three productive oil wells were analyzed for source and depositional environment. The Eocene Chorgali limestone and Eocene Sakesar limestone showed good permeability (Spontaneous Potential log) with interbedded shale (Gamma Ray log) while Paleocene Patala and Early Permian Sardhai formation indicated low permeability and good shale presence. Isotopic (13δC and 15δN) and elemental analysis suggested that the source of organic matter was C3 plants with anoxic carbonate depositional setting. Total organic carbon and Rock Eval parameters (S1 and S2) indicated a good quantity of organic matter. Ratio based on Rock Eval parameters indicated the dominant potential for oil expulsion in Paleocene Patala and Early Permian Sardhai formation. Hydrogen index and oxygen index indicated the Type-II/III kerogen as a source of organic matter. The sediments were thermally mature and within the oil expulsion window. Based on results, Eocene Chorgali limestone has a good reservoir while Paleocene Patala shale has a good source rock character.