鄂尔多斯盆地东部上古生界储层热液蚀变作用

Hydrothermal alteration and its influence on quality of the Upper Palaeozoic gas reservoirs in eastern Ordos Basin

鄂尔多斯盆地东部上古生界为典型的致密储层,为阐明储层致密化成因,应用岩相学和地球化学方法,对砂岩成岩作用进行研究。结果表明,上古生界储层砂岩除了正常埋藏成岩作用外,还经历了强烈的热液蚀变作用,形成了早、晚2期蚀变矿物组合。早期蚀变矿物组合为高岭石+细鳞片状伊利石+绢云母+石英等,代表了早成岩阶段煤系有机质热演化形成的有机酸流体进入储层发生的蚀变作用。晚期蚀变矿物组合为伊利石+绿帘石+铁白云石+自形晶黄铁矿+晚期硅质胶结物,其中铁白云石的δ13CPDB值介于-1.18‰^-11.97‰之间,平均值为-5.57%;δ^(18)O_(PDB)值介于-2.28‰^-15.10‰之间,平均值为-12.57%。晚期伊利石和高岭石表现为LREE富集、HREE亏损,稀土配分模式为右倾型,δEu和δCe异常特征明显。晚期成岩矿物流体包裹体均一温度峰值为170℃,远高于地层处于最大埋深时的成岩温度。结合盆地构造演化过程分析认为,早白垩世时鄂尔多斯盆地发生区域性快速沉降的同时,盆地出现了一次大规模热异常事件,导致上古生界煤系烃源岩大规模生烃,形成的含烃流体与热异常事件伴随的热液流体进入储层,使储层砂岩发生热液蚀变作用,形成大量晚期热液矿物组合,充填孔隙使储层致密成为超低渗储层。热液蚀变作用对油气储层的影响值得关注。

The Upper Palaeozoic gas reservoirs in the eastern Ordos Basin are typical tight sandstone reser- voir.Petrographic observation and geochemical analysis were used to study diagenesis and diagenetic fluid of sandstone.Results show that two periods of altered mineral assemblages were formed after strong altera- tion.Early period of altered mineral assemblage includes kaolinite, flaky illites, sericite and quartz, which were altered by organic acidic fluid produced by thermal evolution of coal-measure source rock.Late period of altered mineral assemblage includes illite, epidote, ankerite, and idiomorphic pyriteetc.The δ^13 CpDB values of carbonate cement are from --1.18‰ to --11.97‰, with average of --5.57‰, and the δ^18 OPDB values are from --2.28‰ to --15.1‰ with average of --12.57‰.Illite and kaolinte are enriched in LREE but depleted in HREE characterized by downwards sloping and some extent of 3Eu and 3Ce anomaly.The peak homoge- nization temperature of fluid inclusions in late siliceous cements is 170℃ ,which is much higher than tem- perature of the maximum burial depth of strata.In early Cretaceousthe, Ordos Basin subsided.Meanwhile,a large-scale thermal anomaly event happened, which had facilitated the large scale hydrocarbon-generation of the Paleozoic coal-measure source rocks.The generated hydrocarbon fluid and hydrothermal fluid migrated into reservoir sandstone beds to alter the reservoir sandstone.Abundant of hydrothermal altered minerals were formed to fill pore space of reservoir sandstone which make reservoir compact.