川东地区石炭系典型古岩溶剖面露头表征

Outcrop Characteristics of the Typical Paleokarst Section of Carboniferous in Eastern Sichuan Basin

石炭系黄龙组古岩溶储层是川东地区重要的天然气储集类型之一,位于华蓥山地区的溪口剖面出露完整,岩溶现象明显,为川东地区石炭系难得的岩溶型露头剖面。通过野外精细观测、室内薄片鉴定分析,该剖面岩溶岩系发育,主要见有网缝镶嵌状、角砾支撑状和基质支撑状岩溶角砾岩,分别代表不同成因的岩溶改造,角砾成分均为云质,角砾间及角砾内的溶蚀孔洞缝发育,常充填地表水下渗携带的砂质、碳泥质充填物和大气淡水胶结物。根据溪口剖面岩溶角砾岩类型等古岩溶标志的识别,结合大量钻井取芯资料观察,石炭系黄龙组可识别出地表残积岩溶相、洞穴跨塌堆积相、地下暗河搬运充填相、基岩溶蚀破裂相、基岩溶蚀孔洞相和基岩溶蚀交代相6种岩溶相类型,不同岩溶相由不同类型岩溶岩及充填物构成。结合古岩溶储层与岩溶旋回期次和溶蚀特征的关系,该剖面可划分地表溶蚀段、下部溶蚀段和底部溶蚀段,其中下部溶蚀段为主要的古岩溶储层发育段。

Paleokarst reservoirs of Carboniferous Huanglong Formation are regarded as important reservoirs for natural gas accumulations in eastern Sichuan Basin. Xikou section in the Huayingshan region has been considered as the most typical outcrop section due to the developed karst rock series. It is demonstrated by field observation and indoor thin section analysis that karst rock series are significantly developed, which primarily consist of breccias of net-seam mosaic form, breccia-supported form, and matrix-supported form, corresponding to karst structures with dif-ferent causes. Breccias are commonly dolomitic, and there develop large amounts of solution holes, caves and seams, which are usually filled with sandy and carbonaceous fillings brought by the surface water infiltration as well as cements of atmospheric fresh water. Based on identification of paleokarst symbols such as the karst breccia types in Xikou section combined with comprehensive observation and analysis of numerous drilling cores, this study classifies paleokarst facies into six categories, namely surface residual karst facies, cave collapse and accumulation facies, under-ground river transporting and filling facies, bedrock solution holes and caves facies, and bed rock solution and replacement facies, respectively. They are composed of different types of karst rocks and fillings. Then, three segments could be divided in Xikou section according to the relationships of paleokarst reservoirs, karst cyclicity, and dissolution characteristics, corresponding to the surface, lower and bottom dissolution parts, respectively. Among them, the lower dissolution part is the main part where develop paleokarst reservoirs.