鄂尔多斯盆地榆林气田天然气成藏地质条件

GAS RESERVOIR FORMATION GEOLOGICAL CONDITIONS OF YULIN FIELD IN E’ERDUOSI BASIN

榆林气田目的层系位于上古生界二叠系山西组底部。通过大量的化验分析样品数据,采用数理统计的方法对生烃条件、储层物性、储盖组合配置、岩性圈闭类型等成藏地质条件进行了较为深入的研究,认为榆林气田处于鄂尔多斯盆地最大生烃强度区,其烃源岩为石炭—二叠系煤系地层,具备形成大型气田的物质条件。山西组山2段储集层为三角洲分流主河道沉积的中粗粒石英砂岩,具有低孔低渗特征,在低孔低渗背景上,受沉积环境、岩性和粒度等因素影响,发育高效储层,成为该区主要工业产层。气藏类型为南北向条带状发育的大型岩性圈闭。根据形成机理,将其岩性圈闭进一步划分为原生和次生两种类型。其天然气成藏具有独特的浅水三角洲成藏模式。近期钻井勘探证实榆林气田储集砂体继续向西南方向延伸,呈现出广阔的勘探前景。

The target strata in Yulin gas field lie in the bottom of Shanxi Formation of Permian, Upper Palaeozoic. On the basis of analyzing and chemically examining a large numble of samples, through comprehensively studying the reservoiring geological conditions as hydrocarbon generation, petrophysical property, reservoir-caprock assemblage and lithologic trap type, by use of mathematical statistics, it was considered that Yulin gas field is located in the greatest hydrocarbon-generating intensity area in the basin and its hydrocarbon source rocks are the Permo - Carboniferous coal measure strata, thus possessing the material conditions of forming large-sized-gas fields. The P1 s 2 (the second member of Shanxi Formation, Lower Permian) reservoir is mainly composed of the medium-coarse quartzose sandstones of delta distributary main channel that have the features of low-porosity and low-permeability but it can become a main productive zone in the field because of the high effective reservoir being developed on the background of low-porosity and low-permeability owing to the effects of these factors as sedimentary environment, lithologic property and grain size, etc. The type of trap is the large-sized lithologic trap showing a north-south extent band. The lithologic traps can be further divided into two types, i. e. primary and secondary lithologic traps, according to forming mechanisms. The gas reservoir formation is of an unique reservoiring pattern of shallow water dalta. Through drilling currently, it is proved that the reservoir sand bodies in Yulin gas field have been continuously finding out in its southwest, thus opening up a vast range of prospects for gas exploration.