鄂尔多斯盆地东南部延长探区上古生界天然气成藏特征

Natural gas accumulation characteristics in the Upper Paleozoic in the Yanchang exploration block of southeastern Ordos Basin

鄂尔多斯盆地东南部的天然气勘探进展缓慢,缘于对其上古生界天然气成藏方面的相关研究较少。为此,利用大量钻井、地球化学薄片鉴定、压汞试验、流体包裹体测试等资料,研究了该区上古生界天然气的成藏地质条件和成藏期次。结果表明:①与盆地北部一样,东南部延长探区具有"广覆式生烃、大面积成藏"的特点,其主力生烃层系为下二叠统山西组2段、中石炭统本溪组暗色泥岩和煤系地层;②储层砂体发育,多层系砂体垂向叠置、横向连片,加上构造不发育,具备形成大面积复合岩性圈闭的重要条件;③上石盒子组—石千峰组发育的巨厚泥岩具有物性和压力双重封闭性能;④流体充注时期发生在晚三叠世、中晚侏罗世和早白垩世。依据生储盖的空间配置关系,将该区上古生界气藏划分为源内成藏、近源成藏和远源成藏3种组合类型,并在分析圈闭类型、输导体系、运聚特征等的基础上,建立起了上古生界天然气的成藏模式。结论认为:该区上古生界具有良好的天然气成藏地质条件和勘探潜力,是鄂尔多斯盆地下一步天然气储量增长的新领域。

No big breakthrough has been made in gas exploration in southeastern Ordos Basin for lack of detailed study of gas accu- mulation in the Upper Paleozoic there. In view of this, various data concerning drilling, geochemistry, thin section authentication, mercury injection test and fluid inclusion test were integrated to study the geologic conditions and stages of gas accumulation in this study area. Like the northern part of this basin, the southeastern Yanchang exploration block is also featured by ＂pervasive hydrocar- bon generation and accumulation＂. The major source rocks there are dark shale and coal measure strata in the 2nd member of Lower Permian Shanxi Fm and the Middle Carboniferous Benxi Fm. Vertically-superimposed and laterally connected well-developed sand- bodies in combination with the underdeveloped structures provide necessary conditions {or the formation of large compound lithologic reservoirs. The thick mudstones in the Upper Shihezi - Shiqian{eng Fm have a capacity of physical property and pressure. Hydro- carbon charging occurred in the Irate Triassic, Middle-Late Jurassic and Early Cretaceous periods. According to the source-reservoir- caprock assemblage, the Upper Paleozoic gas reservoirs are divided into three groups, namely inner source rock accumulation, proximal accumulation, and distal accumulation, the models of which were also built based on the analysis of trap types, carrier systems and hydrocarbon migration and accumulation. In the end, it is concluded that the Upper Paleozoie in this study area has favorable geologic conditions for gas accumulation and is high in exploration potential, thus it is a new domain for gas reserves growth in the future in the Ordos Basin.