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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。
Hydrocarbon reservoirs can vertically microleak to the surface of the earth, which is the theoretical basis of the geochemical exploration of oil and gas. High-concentration C 1 -C 5 alkanes are often accompanied by olefins (C 2 H 4, C 3 H 6) in surface-light hydrocarbon testing of oil and gas fields. It is well-known that the presence of light olefins is absent in underground petroleum and natural gas. At present, in geochemical exploration of surface oil and gas, the olefins in the soil are generally considered to be formed by microorganisms under certain conditions on the surface. The objective existence of surface olefin abnormalities in oil and gas reservoirs, if not given a reasonable explanation, will lead to the geological field questioning the abnormality of light alkanes on the surface and affecting the application of geochemical exploration techniques. A case study of the surface geochemistry of the Teapot Dome oil field in the southern slope of the Huimin Sag in the Jiyang Depression of the Bohai Bay Basin and the Wyoming Basin in the United States found that the light hydrocarbon anomalies in the surface soil of the oilfield area contain olefins and the correlation between olefins and alkanes is good. And high degree of agreement with the hydrocarbon area, indicating that these olefins are not from the biological effects of surface organic matter in situ. In order to further explain the source of surface alkenes in oil and gas reservoirs, the geochemical characteristics of microleakage of alkanes to the surface of the reservoirs are experimentally simulated. The experimental results show that the light hydrocarbon anomalies in the simulated surface soils contain higher concentrations of olefins, The correlation between olefins and alkanes is high, indicating that olefins are evolved from microleakage of alkanes in the surface environment and have the significance of oil and gas. The above results preliminarily clarify the mechanism of the formation of light alkenes on the surface of the oil and gas fields, and confirm the credibility of light hydrocarbon anomalies in the geochemical exploration, and provide the theoretical and practical evidence for the light alkenes as a new indicator of surface geochemical exploration.