廊固凹陷浅层天然气成因类型及机制

GENETIC TYPES AND MECHANISMS OF SHALLOW GAS IN LANGGU SAG

廊固凹陷是一个晚期构造运动强烈 ,具有残留盆地性质的新生代沉积盆地。多年勘探证实 ,浅层天然气 (生物气 ,埋藏深度小于 2 0 0 0m)资源丰富 ,且发现了具一定规模的天然气藏。由于沙河阶组 (Es)气源岩沉积以后发生了强烈的构造运动 ,使成岩早期形成的生物气难以保存下来。文章在对国内、外煤层气型生物气的地化分析数据和形成的地质条件分析的基础上 ,与廊固凹陷地质条件和油气分布状况相对比 ,结合微生物模拟实验数据 ,提出廊固凹陷的浅层天然气类型为生物改造气和后期式生物气 ,并初步阐明了其形成机制。特别是后期式生物成因气形成机制 ,对具有类似地质条件地区的天然气勘探 。

Langgu Sag is a Cenozoic sedimentary basin suffered a strong tectonic movement in the late stage, being of the properties of remnent basin. Through exploration for many years it is proved that the resources of shallow gas (i. e. biogas with buried depths of less than 2 000 m) are abundant and the gas reservoirs with a certain scale have been found. The biogas origined in eodiagenesis was difficult to be preserved, because the strong tectonic movement occurred after the gas source rocks of Shaheije Formation (Es) had been deposited. On the basis of analyzing the geochemical indicators of the biogases of coal-bed gas type found at home and abroad and analyzing the geological conditions of forming these biagases, in comparison with the geological conditions and oil and gas distribution in Langgu Sag and in combination with microbial analogic experimental data, it is proposed that the types of the shallow gas in Langgu Sag should be biogenic transform gas and telogenetic biogas. Their origin mechanisms are primarily expounded in the paper; especially, the origin mechanism of the telogenetic biogas has a certain value for reference in natural gas exploration at these regions with the same geological conditions.