湘鄂西地区下寒武统石龙洞组古岩溶发育及储层特征

 四川盆地下寒武统龙王庙组天然气勘探近期取得的重大发现,引起了对同期沉积的石龙洞组的广泛关注。通过对鄂西渝东-湘鄂西地区石龙洞组多个野外露头的研究,观察到在平面上石龙洞组顶部存在典型的风化岩溶现象,推测在该区内存在与早寒武世末兴凯运动有关的区域性岩溶不整合面。以湖北恩施咸丰背斜庙梁子-丁寨剖面为例,石龙洞顶部的表层岩溶带厚度为3 m,主要为风化壳层和垮塌角砾岩层。同时,受大气淡水和裂缝影响,表层岩溶带下部地层在较深范围内都受到大气淡水淋滤作用影响,属于岩溶改造带。根据胶结物类型及溶蚀特征可分为大气淡水垂向渗滤溶蚀区及潜流溶蚀区。此外,初步探讨了表生岩溶与储层发育的关系。研究区整体岩性较致密,但局部存在优质储层,尤其是表层岩溶带孔洞发育十分丰富,连通性良好。而岩溶改造带由于后期埋藏阶段成岩作用的影响,尤其是晚期硅化作用的破坏,现存孔洞相对较少。研究区优质储层的形成主要受表生及埋藏期溶蚀作用和白云化作用控制。

Karst development and reservoir characterization of Lower Cambrian Shilongdong Formation in Western Hunan-Hubei area

Significant gas discovery in the Lower Cambrian Longwangmiao Formation in Sichuan basin has received wide attention on the time-equivalent Shilongdong Formation in Western Hunan-Hubei area. In order to provide guidance to the regional evaluation of reservoir in Western Hunan-Hubei area, detailed research has been carried out with methods including observation of outcrop, thin section, casted thin section, and cathode luminescence analysis. Based on the research of several outcrops in Eastern Hubei-Chongqing and Western Hunan-Hubei area, typical weathering crust karst features have been observed. Miaoliangzi-Dingzhai cross section has been chosen as the typical section in Enshi Xianfeng area for detailed research. The surface karstificated zone is 3 meters in thickness, composed of weathering layer and collapse breccia layer. The underlying strata have been affected by meteoric water leaching and dissolution due to faults and fracture system. And the meteoric water dissolution reaches as far as hundreds meters. According to the cement types and dissolution features, the vertical seepage flow zone and horizontal subsurface flow zone have been distinguished. Lithology is overall dense in the research area, but high quality reservoir with high porosity and permeability exists in some area, especially in the surface karstificated zone. In the vertical seepage flow zone and horizontal subsurface flow zone, pore space is lost due to late silicification, and only minor pore space is preserved. According to the discussion of diagenesis process and reservoir development, high quality reservoir is mainly controlled by supergene karstification, burial dissolution, and dolomitization. It is speculated that regional unconformity karsts are present in the research area, which are related to Xingkai movement during early Cambrian. This research is important to the regional evaluation of reservoir in Eastern Hubei-Chongqing and Western Hunan-Hubei area.