鄂尔多斯盆地西缘石沟驿向斜的形成演化与致密砂岩气成藏模式

Syncline development and tight sandstone gas accumulation model in Shigouyi area at western margin of Ordos Basin

向斜构造是一种基本的褶皱样式,是煤层气、致密砂岩气、页岩气、盐类以及铀矿等矿产资源赋存的主要场所之一。研究向斜的形成机制将为剖析褶皱冲断带的构造演化及矿产资源的分布特征奠立重要基础。以鄂尔多斯盆地西缘的石沟驿向斜为例,应用新的高分辨率地震剖面与深探井资料,解析向斜的深层地质结构及其成因机制。研究表明,石沟驿向斜处于鄂尔多斯盆地西缘北段与中段的过渡部位,形成于中侏罗世末期—晚侏罗世,是在以羊虎沟组—山西组含煤岩系为滑脱层形成的叠瓦冲断系统基础上,下伏的早期阶状正断层组合反转形成的大型构造楔的上层褶皱体系;惠安堡断层为一正反转断层,在该区起构造-沉积分划作用;石沟驿向斜及邻区的上石炭统—二叠系的致密砂岩气具有自生、自储、自封闭成因,于早白垩世成藏。石沟驿向斜的形成期早于油气成藏高峰期,天然气资源丰富,有较好的勘探前景。

Syncline as a basic type of folds is one of the main structures hosting coal-bed gas,tight gas,shale gas,salt and gypsum,and uranium,etc.The study on the formation mechanism of syncline means significantly to analyzing the tectonic evolution and resource distribution in fold zones.The deep structure and forming mechanism of the syncline in Shigouyi area at the western margin of Ordos Basin were analyzed based on recent high-resolution seismic profiles and deep borehole data.It is shown that the syncline was formed during the end of the Middle Jurassic and Late Jurassic and located at the transitional area between the northern and central segments of the western foreland margin of the Ordos Basin.Based upon the large-scale imbricate thrust system detached along the coal measures of the Upper Carboniferous Yanghugou Formation and the Lower Permian Shaanxi Formation,the syncline is the upper folding system of a large-scale tectonic wedge underthrusted by underlying earlier domino-style normal fault blocks.A positive reverse fault named Hui’anpu in the area can be used to define the tectonic and depositional domains.The Upper Carboniferous and Lower Permian tight sandstones in the syncline and adjacent areas were the trinity of source rocks,reservoirs and cap-rocks for gas during the Early Cretaceous.The syncline,coupled with the fact that it was formed prior to gas pooling peak period,is therefore considered rich in gas and a good exploration target.