地表油气地球化学勘探中轻烯烃形成机理探讨

Study on light alkenes forming mechanism in surface geochemistry exploration for oil and gas

油气藏烃类能够垂向微渗漏到地表是油气地球化学勘探的理论基础。在油气田地表轻烃检测中,高浓度C_1—C_5烷烃常常伴有烯烃(C_2H_4,C_3H_6),众所周知,地下的石油和天然气中是不存在轻烯烃的。目前地表油气地球化学勘探中,土壤中的烯烃普遍被认为是地表原位有机物在一定条件下由微生物生成。油气藏地表烯烃异常的客观存在,如不给出合理的解释,将导致地质界对油气藏地表轻烷烃异常的质疑,影响地球化学勘探技术的应用。通过渤海湾盆地济阳坳陷惠民凹陷南坡临南油田、美国怀俄明州Teapot Dome油田地表地球化学勘探实例发现,油田区地表土壤轻烃异常中含有烯烃,且烯烃与烷烃相关性较好,并与油气区吻合度较高,说明这些烯烃并非来自地表原位有机质的生物作用。为了进一步说明油气藏地表烯烃的来源,对油气藏烷烃类垂向微渗漏到地表的地球化学特征进行了实验模拟,实验结果发现,模拟的地表土壤轻烃异常中含有较高浓度的烯烃,且烯烃与烷烃相关性较高,说明烯烃是在地表环境中由微渗漏的烷烃演化而来,具有油气指示意义。上述研究结果初步阐明了油气田地表轻烯烃异常形成机理,证实了地球化学勘探中轻烃异常对油气藏指示的可信性,同时为轻烯烃作为地表地球化学勘探新指标提供了理论和实践依据。

Though no light alkenes existed in oil and gas reservoirs, light alkenes including C2H4 and Ca H6 often occur in the soil gas over oil and gas fields,accompanied with high concentration of C1-C5 alkanes.The exploration cases in Linnan Oilfield, Huimin Sag of Jiyang Depression,China and Teapot Dome Oilfield, Wyoming,USA suggest that these light alkenes are not derived from in-situ surface organic matters.In order to explain the origins of these light alkenes in the soil gas to reinforce the foundation of Surface Geochemistry Exploration （SGE）,a simulation experiment was conducted to simulate the surface geochemical characteristics of hydrocarbons micro-seeping vertically from a hydrocarbon reservoir.The experimental resuits are in accord with that in oilfield.It is proved that these light alkenes come from the surface microbial evolution of the light alkanes micro-seeping from the hydrocarbon reservoirs. The above results illustrate the forming mechanisms of the surface light alkenes anomalies over oil and gas fields and enhance the credibility of the light hydrocarbon anomalies in SGE.It is also provided that the light alkenes can be a new in- dicator in SGE on the basis of theory and practice.