Origin and accumulation mechanisms of deep paleozoic oil and gas: A case study of the central Tarim basin, western China

Old strata and deep petroleum systems are becoming increasingly important. The Cambrian-Lower Ordovician petroleum reservoirs in the eastern Tazhong Uplift, central Tarim Basin, western China were studied comprehensively using various methods, including biomarkers, carbon isotopes of oil and gas, TOC and Rock-Eval pyrolysis of potential source rocks, as well as fluid inclusions, thin sections of dolomite rocks and Raman spectra of reservoir bitumen. The results show that the content of C28 steranes among the C27-29 steranes in most of the oil samples is greater than 0.25. The original hydrocarbon generation and expulsion potential of Cambrian and Lower Ordovician shale samples from the nearby Manjiaer Depression is considerable, and the oils are considered mainly derived from the Paleozoic shales in the Manjiaer Depression and the surrounding slops. Three notable types of oil and gas reservoirs were recognized in the Cambrian and Ordovician dolomite rocks: 1) the relatively well preserved primary reservoirs; 2) the residual reservoirs after deep oil-gas cracking with participation of thermochemical sulfate reduction (TSR); 3) the reformed reservoirs by the late charged high mature gas. In the vicinity of the No.1 fault zone, the effect of late gas charging and invasion is remarkable. They are mostly the mixture of gas cracked from both crude oil and heavy hydrocarbon gas in high mature stage. Oil and gas-liquid inclusions detected in the Cambrian reservoirs together with the Raman spectral characteristics of reservoir bitumen confirm the existed Cambrian paleo-oil reservoirs and oil cracking process. The gypsum-salt rock developed in Cambrian system is considered as a crucial factor for the primary oil and gas reservoirs, and the nearby areas within or below the strata where these gypsum-salt rock developed are suggested as favorable targets for deep to ultra-deep petroleum exploration.