四川盆地南部中二叠统茅口组碳酸盐岩岩溶特征:古大陆环境下层控型早成岩期岩溶实例

Palaeokarst characteristics of carbonate rocks of the Middle Permian Maokou Formation in southern Sichuan Basin: Example of strata-bound eogenetic karst in palaeo-continental settings

综合钻井、录井、岩心、薄片等资料,研究四川盆地南部中二叠统茅口组碳酸盐岩的岩溶特征,并分析讨论其机理与模式。茅口组为开阔海台地沉积,以发育高能滩相颗粒灰岩与非滩相低能泥晶灰岩的不等厚互层为特征。岩溶主要发育在茅口组上部,表现为大量溶蚀缝洞发育,且多被离解的碳酸盐岩碎屑、方解石晶粒、碳质泥屑等不同来源的物质混合充填,但缺乏典型的岩溶角砾。茅口组岩溶属于大陆环境下早成岩期岩溶类型,一个显著特征是岩性与岩相对溶蚀-充填作用的控制:非滩相泥晶岩类岩性致密,岩溶欠发育;高能滩相颗粒岩类岩溶发育良好,并可以识别出基岩带、半离解带和混合充填带3种溶蚀-充填带。根据上述特征,认为泥晶灰岩致密层与颗粒灰岩高渗层在纵向上的多旋回叠置,以及先期的孔渗系统和断裂裂缝体系为岩溶水提供输导体系,导致形成了同期多层的颗粒滩相控岩溶系统,并在岩溶台地和斜坡岩溶型储集层皆较发育。该研究成果不仅可为相关研究提供补充与参考,还对研究区茅口组油气勘探具有一定的指导意义。

By use of drilling, logging, core, and thin section data, the karst characteristics of car- bonate rocks of the Middle Permian Maokou Formation in southern Sichuan Basin are studied. Maokou For- mation, a deposition of open-sea platform, is characterized by the interbedding of grainstone of bank faci- es and micrite limestone of non-bank facies with unequal thickness. The karst is mainly developed at the upper part of Maokou Formation, with a large amount of dissolved fissures and caves overwhelmingly filled by substances from various sources including dissociated carbonate fragments, calcite crystals and carbon- ous clays, and no sign of karst breccia. The karstification belongs to eogenetic karst in palaeo-continental settings, which is significantly featured by the dissolution-filling controlled by its lithology and lithofacies： Micrite limestone within non-bank facies is compact, and shows little sign of dissolution-filling, while grainstone within bank facies is characterized by good karst development, and three zones of dissolution- filling can be recognized including matrix, half-dissociated and mixed-filling zones. Based on the above characteristics, it is suggested that high porosity-permeability layers of grainstone and compact ones of mi- crite limestone were multi-cycle superimposed vertically, and primary porosity-permeability system accom- panied by faults and fissures provided sufficient channels for karst water, which resulted in the whole karst system featured by contemporaneous muh-layer and grain-bank facies controlled. The karst reservoir, there- fore, favored to develop in both karst platform and karst slope. The research result will not only provide supplement and reference for the existing studies of the subject, but also is of meaning to the oil and gas exploration of Maokou Formation.