川西新场致密砂岩气藏层理缝成因探讨

Causese of bedding fractures of tight sand gas-reservoir in Xinchang, West Sichuan region

根据岩心描述和激光共焦扫描电镜研究 ,得出川西地区新场上沙溪庙组气藏致密砂岩的低角度至水平微裂缝十分发育而垂直缝和构造缝极不发育的认识。根据裂缝特征 ,结合对该气藏的成藏过程和构造形变分析 ,认为低角度至水平微裂缝是层理缝 ,形成的原因是 :①在砂岩致密化和构造隆升之前 ,天然气已相对富集成藏 ,在储集层和烃源层内形成了有效的封闭体系 ;②砂岩致密化和构造隆升达到一定程度后 ,当封闭体内压力梯度升高的值大于致密砂岩薄弱面 (层理面 )破裂成缝的值时 ,自然流体压裂作用很可能沿致密砂岩中的层理面发生 ,形成“层理裂缝系统”。这些层理缝改善了致密砂岩储集层的渗透性 ,层理缝发育区是气井高产能区。图 5表 1参

Based on the studies of core description and electron microscope scanning of laser com focus, the results have been presented that fissures with a low angle to horizontal one of Upper Shaximiao formation are well developed, while its vertical and structural fissures are not well developed yet. The fissures with low angle to horizontal one are defined as the bedding ones. The formed causes of these fissures are: (1) natural gas must have been accumulated into the gas reservoir prior to the compaction of the formation and structural uplifting, and the effective enclosed system must have been formed within the reservoir formation and hydrocarbon source rock beds; (2) the compaction of the formation and structural uplifting must arrive to a certain degree, the value that the pressure gradient of enclosed system should be bigger than the value that is desired when the weak planes (bedding plane) of tight sand are fractured. The fracturing effect of natural fluid might be taken place on bedding plane of tight sand, then, the 'bedding fissure system' could be formed. They will be good for improving and the permeability of reservoir. The area well developed bedding fissure system should be the region with high productivity of gas wells.