四川盆地长宁西部地区上奥陶统五峰组—下志留统龙马溪组龙一1亚段页岩岩相划分及意义

四川盆地长宁地区上奥陶统五峰组—下志留统龙马溪组页岩作为目前页岩气勘探开发的重点目标已取得重大突破。为明确长宁西部地区岩相差异、纵横向分布规律、储层发育特征及其影响因素，基于岩心、X衍射测试、扫描电镜以及各类分析测试资料对长宁西部地区五峰组—龙马溪组龙一₁亚段的页岩岩相进行了细致研究。研究区主要发育硅质页岩相、混合硅质页岩相、含灰硅质页岩相、含黏土硅质页岩相、混合质页岩相、含黏土/硅混合质页岩相以及含灰/硅混合质页岩相7种岩相。其中主要分布在龙一₁亚段底部的硅质页岩相因其具有高TOC含量、高孔隙度和高含气性，是研究区最为优质的页岩岩相；其次五峰组沉积时期和龙一₁亚段沉积晚期发育的混合硅质页岩相、含灰硅质页岩相、含灰/硅混合质页岩相和含黏土/硅混合质页岩相，在储层条件上稍差于硅质页岩相，为研究区中等的页岩岩相。综合分析认为，五峰组—龙马溪组龙一₁亚段页岩优势岩相储层形成可能受2个因素共同控制，一是上层富氧而下层缺氧—还原的沉积环境，该类型的沉积环境能够提供大量富有机质的硅质矿物；其次，在后期成岩作用中，充足的富有机质硅质矿物能为储层提供优异的孔隙类型。研究结果将为长宁西部地区页岩气富集区提供岩相学支撑，有利于四川盆地五峰组—龙马溪组海相页岩气的下一步勘探。

Classification of shale lithofacies from Ordovician Wufeng Formation to first section of first member of Silurian Longmaxi Formation,western Changning area,Sichuan Basin,and its significance

Shale gas exploration has been successfully carried out in the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation of the Changning area of Sichuan Basin. To clarify the lithofacies differences, vertical and horizontal distribution signatures, reservoir development characteristics and influencing factors in the western part of Changning area, based on core X-ray diffraction test, scanning electron microscopic observation and various analytical test data, the shale lithofacies of the Wufeng Formation (O₃w) to the first section of the first member of Longmaxi Formation (S₁l₁1) were studied in detail. Seven lithofacies in the study area were recognized including siliceous shale, mixed siliceous shale, carbonate-rich siliceous shale, clay-rich siliceous shale, mixed shale, argillaceous/siliceous mixed shale and calcareous/siliceous shale. The siliceous shale facies developed at the bottom of S₁l₁1 is the best shale lithofacies in the study area since its TOC content, porosity and gas-bearing capacity are all higher than those of others. The mixed siliceous shale, carbonate-rich siliceous shale, calcareous/siliceous mixed shale and argillaceous/siliceous mixed shale in O₃w and the upper S₁l₁1 are slightly poorer than siliceous shale in terms of reservoir conditions, which are the sub-optimal lithofacies in the study area. Comprehensive analyses showed that the formation of dominant shale lithofacies reservoirs in O₃w and S₁l₁1 may be controlled by two factors. One is the sedimentary environment rich in oxygen in the upper layer and hypoxic and reducing in the lower layer, which provided a large amount of organic-rich siliceous minerals. Secondly, abundant organic-rich siliceous minerals provided excellent pore types for reservoirs during late diagenesis. The results will provide lithographic support for the studies of shale gas enrichment zones in the western Changning area, and are conducive to the further exploration of marine shale gas in O₃w and S₁l₁1 in the Sichuan Basin.