乌连戈伊气田形成机制及其启迪

FORMATION MECHANISM OF URENGOY GAS FIELD IN THE WEST SIBERIAN BASIN AND ITS IMPLICATION

西西伯利亚盆地是世界上最大的含油气盆地,其中乌连戈伊气田储量达10.2×1012m3。对于该气田的形成机制,迄今仍众说纷纭。该盆地北部地区95%的天然气都储集于白垩系波库尔组中,而当中的2/3又储于波库尔组顶部的赛诺曼阶。这些天然气的δ13C1分布在-54‰^-46‰,其中在乌连戈伊气田δ13C1集中在-49‰左右,它不应当属于纯生物气的范畴。基于该气田天然气的组成和碳同位素特征认为,该气田的天然气也不可能是生物气与下部热成因天然气混合的产物。塞诺曼阶天然气的分布与成熟度处于低演化阶段的波库尔组源岩在时空分布上有着极好的对应关系,前者的δ13C1分布特征和波库尔组热演化指标Ro之间的关系与煤型气碳同位素二阶模式的低演化回归曲线吻合极好。因此,可以用腐殖型有机质在低演化阶段能规模性成气的机理合理解释该气田的成因。最后认为,乌连戈伊气田塞诺曼阶天然气属于低熟气的范畴。

The West Siberian Basin is the largest petroliferous basin in the world. Its northern part contains the proved natural gas reserves of 40 trillion cubic meters, of which 10.2 trillion cubic meters are distributed in Urengoy gas field. Until now, there are various opinions about the forming mechanism of this gas field. Almost 95% of the gas reserves in the northern part of the basin are contained in Cretaceous Pokur Formation, two thirds of which are contained in the top Cenomanian. The δ 13 C_1 of the natural gas in this basin ranges from-54‰ to-46‰, while the δ 13 C_1 of gas in Urengoy field concentrates around-49‰, thus it should not be in the category of pure biogas. Based on the characteristics of carbon isotope and components of gas in this field, it is believed that the gas is unlikely a mixture of biogas and the lower thermogenesis natural gas. The distribution of the Cenomanian natural gas is well correlated in time and space with the distribution of the low mature Pokur Formation source rocks. The curve of relation between δ 13 C_1 distribution of the former and R_o of Pokur Formation matches well with the regression line of two-stage model of carbon isotopic fractionation of coal related gas in low-mature phase. Therefore, the mechanism that humic-type organic matter can generate gas on a large scale in low-maturity phase can be used to reasonably explain the genesis of the gas in this field. It is believed that the Cenomanian gas in Urengoy field belongs to the category of low-mature gas.