鄂尔多斯盆地天环坳陷北部奥陶系天然气成藏期次

Hydrocarbon charging history of Ordovician gas reservoirs in north Tianhuan depression,Ordos Basin

以鄂尔多斯盆地天环坳陷北部奥陶系含气储层为研究对象,进行天然气成藏期次研究。采用流体包裹体岩相学观察及均一温度测试、气烃包裹体激光拉曼和包裹体碳同位素分析技术,结合埋藏热演化史分析,确定了天然气成藏期次,并根据已有天然气地球化学特征及成藏地质条件,建立了成藏流体演化特征及成藏过程。结果表明,奥陶系含气储层发育早、晚两期烃类包裹体,早期以沥青包裹体和液烃包裹体为主,反映早期原油成藏过程,包裹体均一温度峰值122~142℃对应中侏罗世成藏过程;晚期发育气液烃和气烃包裹体为主,包裹体均一温度峰值154~168℃对应侏罗纪末—早白垩世成藏过程。包裹体气烃成分主要为CH4,其甲烷碳同位素平均值在-38.84‰,表明包裹体中CH4来源于早期原油裂解成因气。结合研究区天然气成藏地质特征分析,侏罗纪末-早白垩世为天然气关键成藏期,在天环坳陷斜坡深部及深凹陷区可以形成源自奥陶系气源岩的内幕气藏。

Combined with analysis of thermo-burial history,hydrocarbon charge phases of Ordovician gas reservoirs in north Tianhuan depression of Ordos Basin were determined by microscopic observation of petrography,as well as measurements of homogenization temperature,laser Raman spectroscopy and gaseous hydrocarbon isotopic of alkane on fluid inclusions.Also,the history of reservoir fluids was recovered on the basis of gas geochemistry characteristics and geological conditions of the study area.The results show that two generation order of hydrocarbon inclusions were determined according to the relation between mineral diagenesis and distribution of hydrocarbon inclusions,suggesting the occurrence of two main phases of oil and gas migration and accumulation.The early charging phase consists of bitumen and liquid hydrocarbon inclusions and the homogenization temperature range of 122~142℃corresponds to middle Jurassic charging period.The late charging phase develops liquid-gas bearing hydrocarbon and gaseous hydrocarbon inclusions and the homogenization temperature range of 154~168℃corresponds to end of Jurassic and early Cretaceous charging period.The main content of gaseous alkane in inclusions is CH4 and the average value of carbon isotopic of methane in the inclusions is-38.84,suggesting possible generation from thermal from oil cracking.Based on the geologic conditions of Ordovician gas reservoirs analysis,the key charging phase was end of Jurassic and aerly Cretaceous period and gases originated from Ordovician source rocks exist in the deep burial area of Tianhuan depression such as in deep slope and secondary depression areas.