鄂尔多斯盆地中东部奥陶系马家沟组米探1井天然气成因与来源

Genesis and source of natural gas in Well Mitan⁃1 of Ordovician Majiagou Formation,middle⁃eastern Ordos Basin,China

鄂尔多斯盆地中东部米探1井在奥陶系马家沟组四段钻获高产工业气流,实现了奥陶系盐下天然气勘探的重大突破,但目前对其成因来源存在争议。实测结果表明,米探1井天然气以烷烃气为主(95.18%),气体干燥系数(C_(1)/C_(1-5))为0.947,非烃气体中H_(2)S含量为3.49%,还有少量的N_(2)和CO_(2)。天然气中甲烷、乙烷、丙烷的碳同位素值分别为−45.5‰、−26.4‰、−24.3‰。基于区域地质背景、潜在烃源岩特征和天然气地球化学特征,认为米探1井天然气为奥陶系盐下碳酸盐岩自生自储的油型气,但其存在甲烷碳同位素组成偏轻和乙烷碳同位素组成具有煤型气特征等地球化学异常。结合生烃热模拟实验和岩石残余气特征认为米探1井特殊的地球化学特征与普遍存在的膏岩关系密切:一方面,普遍存在的膏岩提供了良好的盖层使得很多早期生成的天然气得以留存;另一方面,膏岩的存在促进了H_(2)S和乙烷等重烃类气体的生成。此外,小于5%的H2S含量和较高的重烃气体(C_(2^(+)))含量说明即使发生了硫酸盐热化学还原作用(TSR)也应该不太强烈,此时甲烷不会受到TSR影响,乙烷碳同位素组成可能是识别TSR的敏感指标。

The Well Mitan-1 obtained high-yield industrial gas flow in the fourth member of the Majiagou Formation of Ordovician(O_(1)m_(4))in the mid-eastern Ordos Basin,which has achieved a major breakthrough in the exploration of Ordovician subsalt natural gas.However,there are disputes on the origin of natural gas in Well Mitan-1 at present.The measured results show that the natural gas in Well Mitan-1 is mainly composed of alkane gas(95.18%),the gas drying coefficient(C_(1)/C_(1-5))is 0.947,the content of H_(2)S is 3.49%,and there is also a small amount of N_(2)and CO_(2)in non-hydrocarbon gas.The carbon isotopic compositions of methane,ethane and propane in the natural gas are−45.5‰,−26.4‰and−24.3‰,respectively.Based on the regional geological background,the characteristics of potential source rocks and the geochemical characteristics of natural gas,it is considered that the natural gas in Well Mitan-1 is self-generated and self-accumulated oil-associated gas in Ordovician subsalt carbonate rocks.However,there are some geochemical anomalies,such as methane carbon isotope value(δ^(13)C_(1))is lighter and ethane carbon isotope(δ^(13)C_(2))has the characteristics of coal-type gas.Combined with the thermal simulation experiment of hydrocarbon generation and the characteristics of residual gas in rocks,it is considered that the special geochemical characteristics of Well Mitan-1 are closely related to the gypsum rocks.On the one hand,the ubiquitous gypsum-rock provides a good caprock,which makes retainment of the early-generated natural gas.On the other hand,the existence of gypsum rock promotes the generation of heavy hydrocarbon gases(C_(2^(+)))and H_(2)S.In addition,less than 5%H_(2)S content and higher C_(2^(+))content indicate that the thermochemical sulfate reduction(TSR),if any,should not be so strong that methane is not affected by TSR.The δ^(13)C_(2)may be a sensitive parameter for identifying TSR.