鄂尔多斯盆地靖边气田奥陶系马家沟组热液活动特征及油气地质意义

Characteristics of hydrothermal activity and petroleum geological significance of the Ordovician Majiagou Formation in Jingbian Gas Field,Ordos Basin

含油气盆地中热液流体对油气成藏的影响已经成为油气地质研究的热点。基于钻井岩心观察、岩石薄片鉴定、流体包裹体测温、碳氧同位素、元素地球化学及地层水分析等实验技术方法,分析鄂尔多斯盆地靖边气田马家沟组五段(马五段)碳酸盐岩储层中热液活动特征,探讨热液活动对油气的地质意义。结果表明:靖边气田马家沟组热液活动特征主要体现在热液黄铁矿充填溶孔、流体包裹体的均一温度异常增高、脉体方解石的碳氧同位素和天然气中H_(2)S硫同位素均指示了高温热液异常、白云岩稀土元素显示明显的δEu正异常和δCe负异常,且样品全部分布在Ya/La—Yb/Ca交会图的热液成因区域,部分主量元素和微量元素因热液作用显著增加。地层水特征显示出多种地质因素综合作用的结果。因此推断深部热液流体通过基底断裂和古断裂上升,沿着马五段顶部不整合面和古断裂进入马家沟组。前期可对岩石中的不稳定组分进行溶蚀,形成大量的次生溶蚀孔洞,对储层起建设性作用。后期随着热液流体温度的降低及压力系统的改变,热液流体形成的黄铁矿、石英和方解石开始大量沉淀,充填溶蚀孔洞和裂缝,对储层起破坏性作用。

The influence of hydrothermal fluid on hydrocarbon accumulation has become a hot spot in the study of petroleum geology in petroliferous basins.Through drilling core observation,rock thin section identification,temperature measurement of fluid inclusions,carbon and oxygen isotopes,element geochemistry,formation water analysis and other experimental technical methods,the characteristics of hydrothermal activity in the carbonate reservoir of Majiagou Formation,Jingbian Gas Field,Ordos Basin are analyzed,and the geological significance of hydrothermal activity for oil and gas is discussed.The result shows that the characteristics of hydrothermal activity of Majiagou Formation in Jingbian Gas Field are mainly reflected in the hydrothermal pyrite filling dissolution pores,the abnormal increase of homogenization temperature of fluid inclusions,the carbon and oxygen isotopes of vein calcite and the H_(2)S sulfur isotopes in natural gas indicate the high temperature hydrothermal anomaly,and the rare-earth elements in dolomite show obvious δEu positive anomaly and δCe negative anomaly,and all the samples are distributed in the hydrothermal origin region of Ya/La-Yb/Ca intersection diagram,and some major and trace elements are significantly increased due to hydrothermal action.The characteristics of formation water show the result of the comprehensive action of various geological factors.Therefore,it is inferred that the deep hydrothermal fluid rises through basement faults and paleofaults,and enter Majiagou Formation along the unconformity surface at the top of the Ma 5 Member and paleofaults.In the early stage,the unstable components in the rock can be dissolved,forming a large number of secondary dissolution pores,which plays a constructive role in the reservoir.In the later stage,with the decrease of the temperature and the change of the pressure system of the hydrothermal fluid,the pyrite,quartz and calcite formed by the hydrothermal fluid began to precipitate in a large amount,filling dissolution pores and fractures,which played a destructive role in the reservoir.