Gas sources of the YN2 gas pool in the Tarim Basin—Evidence from gas generation and methane carbon isotope fractionation kinetics of source rocks and crude oils

Abstract

The YN2 gas pool in the eastern Tarim basin has been believed to be an accumulation of gases resulting from oil-cracking. However, this gas source model cannot explain such geochemical characteristics of the natural gases as heavier carbon isotope values ( δ 13C 1=−37.5‰ to −36.2‰; δ 13C 2=−34.7‰ to −30.9‰) and high content of nitrogen gas (15.5% on average). In this study, we performed pyrolysis experiments on typical source rock and crude oil samples selected from the basin, evaluated their kinetic parameters of gas generation and methane carbon isotopic fractionation, and extrapolated them to geological conditions. The results show that neither kerogen-cracking gas nor oil-cracking gas can act as the single gas source of the YN2 gas pool. By contrast, a mixing model of 55% kerogen-cracking gas and 45% oil-cracking gas results in an excellent match with the geochemical characteristics of the gas pool. The gas condensate from the YN2 gas pool originated from the cracking of oils in mid-upper Ordovician strata, and the nitrogen gas was mainly derived from the cracking of kerogen in Cambrian–lower Ordovician mudstones in the Yingjisu depression center during the very high level of maturation. The YN2 gas pool went through three stages of formation: (1) oils from Cambrian–lower Ordovician source rocks accumulated in Cambrian and Ordovician reservoirs during the middle Ordovician, and gases were generated from both kerogen and paleo-oil pools in the Cambrian–lower Ordovician strata during late Ordovician to early Devonian; (2) gases leaked during either late Devonian to Triassic due to uplifting, or late Jurassic due to intense faulting activity in this area; (3) gases were regenerated from the late cracking of kerogen in the Cambrian–lower Ordovician strata as well as from oil-cracking in mid-upper Ordovician reservoirs with rapid subsidence after the Cretaceous. The two types of gas mixed and accumulated in the Jurassic traps to form the present wet gas pool with high nitrogen content and a heavier methane carbon isotopic value.