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
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
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DOI：
10.1016/j.marpetgeo.2006.10.001
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发表时间：
2007
影响因子：
4.2
通讯作者：
H. Tian;X. Xiao;R. Wilkins;Xianqing Li;Huajun Gan
中科院分区：
文献类型：
--
作者：
H. Tian;X. Xiao;R. Wilkins;Xianqing Li;Huajun Gan
塔里木盆地东部YN2气藏被认为是石油裂解形成的天然气聚集。但该气源模型不能解释天然气碳同位素值较重（δ13C1= - 37.5‰~ - 36.2‰；δ13C2= - 34.7‰~ - 30.9‰）、氮气含量较高（平均15.5%）等地球化学特征。本研究对盆地典型烃源岩和原油样品进行了热解实验，评价了其生气和甲烷碳同位素分馏的动力学参数，并对其地质条件进行了外推。结果表明，干酪根裂解气和石油裂解气均不能作为YN2气藏的单一气源。55%干酪根裂解气与45%石油裂解气混合模型与气藏地球化学特征吻合较好。YN2气藏的凝析气主要来源于中、上奥陶统地层原油的裂解，氮气主要来源于英吉苏坳陷中心寒武系—下奥陶统泥岩中干酪根的裂解。YN2气藏的形成经历了3个阶段：(1)寒武系—下奥陶统烃源岩油在中奥陶统—寒武系和奥陶系储层中聚集，晚奥陶世—早泥盆世寒武系—下奥陶统的干酪根和古油藏同时生气；(2)该区晚泥盆世—三叠纪因隆升作用或晚侏罗世因断裂活动强烈而发生天然气泄漏；(3)寒武纪—下奥陶统地层干酪根晚期裂解和白垩纪后快速沉降的中上奥陶统储层原油裂解是天然气再生的主要原因。两种气体在侏罗系圈闭中混合聚集，形成了现今含氮量高、甲烷碳同位素值较重的湿性气藏。
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 (δ13C1=−37.5‰ to −36.2‰; δ13C2=−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.