鄂尔多斯盆地上古生界天然气成因新探索

PROBING THE GAS GENESIS OF UPPER PALEOZOIC IN E′ERDUOSI BASIN

鄂尔多斯盆地上古生界天然气一直被认为是油型气和煤成气二源混合气。通过区域构造特征、盆地演化、地震活动和地球化学等分析 ,发现上古生界天然气藏沿深大断裂分布 ,具有生烃期与构造活动时间相吻合的特点 ,以及部分井天然气中CO2 、N2 、He含量非常高、甲烷同系物碳同位素发生反转、CO2 碳同位素和3 He/4He高异常等特征 ,从而认为上古生界天然气是油型气、煤成气和深源无机气三源混合而成。中生代晚期盆地深大断裂的强烈活动导致深源流体上涌并与古生界天然气发生混合 ,并且其中的H2 对烃源岩生烃起到加氢作用 ,促进了其生烃演化 ,扩大了天然气的丰度。

The Upper Paleozoic natural gas in E'erduosi Basin has been always regarded as a two-source mixed gas of the oil-formed gas and coal-formed gas. Through the investigation on regional tectonic features, basin evolution, seismic activity and geochemistry.etc. it was found that the Upper Paleozoic gas reservoirs in the basin are of these characteristics, such as the distribution along discordogenic faults, the identity of hydrocarbon generation period with structural activity time and the very high CO2-,N2- and Heconent.CH4 series carbon isotope reversion, high CO2 carbon isotope and high 3He/4 He ratio in natural gas in partial wells, etc. For this reason, it is considered in the paper that the Upper Paleozoic natural gas is a three-source mixed gas of the oil-formed gas, coal-formed gas and deep-source inorganic gas. In Late Mesozoic, the strong activity of the discor-dogenic faults in the basin caused the deep fluids to be moved up and to be mixed with the Upper Paleozoic natural gas,and the hydrogen in the fluids had played a role of hydrogenizing in the hydrocarbon generation in source rocks, which could promote the evolution of source rocks and increase natural gas abundance. Such an idea is of great importance to probing the mechanism of gas genesis of the tectonic activity regions in E'erduosi Basin and to the natural gas exploration in the future.