鄂尔多斯盆地古构造演化在气田形成中的作用及意义

SIGNIFICATION AND EFFECT OF PALAEOSTRACTURAL EVOLUTION ON GAS FIELD FORMATION IN E′ERDUOSI BASIN

鄂尔多斯盆地中部大气田是发育在奥陶系顶碳酸盐岩古风化壳中的地层油气藏。由于储层的今构造面貌是一个角度不到 1°的西倾单斜 ,因而人们一直认为该气田形成与构造无关 ,而与上倾方向不整合面侵蚀谷中充填的太原组—山西组泥质岩对储层封堵有关。因而在天然气勘探中一度注重寻找侵蚀谷。虽然找到了许多侵蚀谷遮挡 ,但是并非所有侵蚀谷下倾方向都会形成气藏。古构造演化研究表明 ,天然气大量生成后的运移聚集和散失一直都受古构造形态的控制 ,虽然气田在今构造图上是一个平缓的大单斜 ,但在古构造图上往往是长期稳定发育的大型鼻隆。今天多个气藏的位置正好位于古鼻隆与今鼻隆轴线之间或轴线的交叉地带 ,说明非构造油气藏并非与构造无关 ,而是它的形成、演化、定型与破坏始终与古构造演化密切地联系着。

The giant central gas field in E′erduosi Basin is a stratigraphic gas reservoirs developed in the ancient weathered crust of carbonate rocks on the top of the Ordovician.Owing to the fact that the existing structure of the reservoir is a west-tilted monocline with dip angles of less than 1°,it was always thought that the gas field formation was out of relation to the structural condition and in connection with the reservoir′s being sealed by the silty carbargilites of the Taiyuan Formation-Shanxi Formation filled in the erosion valleys on unconformity surface of updip direction top,therefore a upsurage in surveying for the erosion valleye was once sprang up in natural gas exploration.Although many barriers of the erosion valleys were found,the gas reservoirs wouldn′t be formed in all the downdip parts of the erosion valleys.Through analyzing the palaeostructural evolution it is shown that the natural gas migration,accumulation and dissipation after its being largely generated were always controlled by palaeostructural shape;and although the gas field is a gentle huge monocline on present structural map,it is often a huge nose structure with long-term stable development in palaeostructural map.At present,the positions of many gas reservoirs are just located in the places between the ancient nose structure axial line and the present one or in the crossed zones of the two axial lines,which indicates that the non-structural reservior isn′t with no relation to the structure,its formation,evolution,finalization and destruction,however,are closely connected with the palaeostructural evolution from beginning to end.