渤海湾盆地天然气聚集差异性探讨

A DISCUSSION ON DIFFERENTIAL GAS ACCUMULATION IN BOHAI BAY BASIN

渤海湾盆地内各坳陷天然气探明储量、天然气成藏深度、天然气富集层位明显不同,文章从气源、盖层、断层和古构造背景等4个方面进行了分析。认为主力气源岩沙三段泥岩、油页岩进入不同的热演化阶段是天然气富集程度差异的内在原因。区域性、地区性盖层与断层活动和古构造背景联合作用,控制了天然气的空间展布,盖层厚度越大,其延伸范围就越远,连续性越好,即使受断层破坏,其连续性比薄盖层要稳定,从而减少了因断层破坏引起的天然气散失。在盖层封闭能力相同条件下,构造活动较弱地区的盖层厚度对天然气藏长期保存起关键作用,断层活动影响到的层位即是天然气最终聚集位置,各坳陷中的古构造带是天然气运聚的有利指向。烃源岩生成的天然气在沙一段区域性盖层和沙三段、明化镇下部—馆陶组上部地区性盖层遮挡下,形成了现今的天然气分布格局。

The proved gas reserves, depth of gas accumulation and horizons of gas enrichment are different in each depression in Bohai bay basin. Based on analyses of source rocks, cap rocks, faults and palaeotectonic setting, it is believed that the internal cause of differential gas enrichment is that the mudstone and oil shale in the 3~ rd member of Shahejie Fm, the main gas source rocks, are in different thermal evolution stages. The spatial distribution of gas was controlled by regional and local cap rocks, fault activities and palaeotectonic setting. The expanded range of cap rock enlarges and its continuity improves along with increasing thickness. Even if a thick cap rock is damaged by faults, its continuity is still better than thin cap rocks, thus the amount of gas escaped due to damage of faults is reduced. When sealing abilities of cap rocks are similar, the thickness of cap rocks in areas with weak tectonic activity would have played a key role in long term preservation of gas accumulations. Horizons that have been involved in tectonic action are the place where gas has finally accumulated. The palaeostructural zones in each depression were favorable directed areas of gas migration and accumulation. The present distribution of gas has been formed due to screening of the regional cap rocks in the 1~ st member of Shahejie Fm and of the local cap rocks in the 3~ rd member of Shahejie Fm and the lower Minghuazhen Fm-upper Guantao Fm.