Abstract:

Fractures in the low-permeability reservoirs are usually high dip-angles. But there exist low dip-angle fractures formed by tectonism in the thrust zones of the western basins of China. These low dip-angle fractures are mainly distributed in the medium-granular sandstones and grit stones nearby the thrust or sliding layers. These fractures are often oblique to bedding plane with tiny angle and have equal spacing. Some fractures are filled with calcite, quartz and asphalt.Some fractures are characterized by dissolution and cut by the high dip-angle fractures. The geological analysis and numerical simulation show that the low dip-angle fractures are mainly formed by shearing due to thrusting or interlayer gliding under tectonic compression. These low dip-angle fractures are widely distributed along the thrust or sliding layer and have good lateral connectivity and high permeability in the horizontal direction. They can be also connected with high dip-angle fractures to form a good network system,which is advantageous to the enrichment for oil and gas in the low-permeability formation and to generation of high production wells.

Key words: low-permeability reservoir, low dip-angle fracture, thrust zone, distribution characteristics, origin, geological significance

摘要：

低渗透储层裂缝通常以高角度构造裂缝为主,但在中国西部盆地挤压逆冲构造带的低渗透储层中,还发育一类低角度构造裂缝。这类裂缝主要发育在逆冲断层或滑脱层附近的中砂岩和粗砂岩中,裂缝与层理面呈小角度斜交,具有较好的等间距性。部分裂缝被方解石、石英、沥青或碳质充填,部分裂缝具有明显的溶蚀现象,并与高角度构造裂缝呈相互切割关系。地质分析和数值模拟研究表明,这类低角度裂缝的形成主要与构造挤压作用下断层的逆冲作用或层间滑动造成的近水平剪切作用有关,而非卸载成因。这类低角度裂缝的发育范围较大,横向连通性好,水平渗透率高,并且和高角度裂缝相互交织在一起,可以构成良好的裂缝网络系统,有利于油气在低渗透地层中的富集和形成高产油气井。

关键词: 低渗透储层, 低角度裂缝, 逆冲构造带, 分布特征, 成因, 地质意义