川南及邻区下志留统龙马溪组下段沉积相与页岩气地质条件的关系*

在川南及邻区下志留统龙马溪组下段沉积相详细研究的基础上,结合偏光显微镜、X衍射及元素地球化学分析等,提出沉积相对页岩气地质条件的影响特征。研究发现:川南及邻区龙马溪组下段主要发育潮坪相和浅海陆棚相。局限滞留的缺氧还原环境、快速海侵形成的分层水体、适宜的沉积速率及较高的生物产率,造成了龙马溪组下段有机质富集,并以硅质型页岩为主,利于页岩气的富集与开发。隆起边缘的潮坪相沉积,为页岩气的非有利区;沉积中心的深水陆棚沉积区,主要发育碳质硅质页岩、碳质页岩与含粉砂含钙碳质页岩,为页岩气的有利区;砂泥质浅水陆棚含有较高的有机质,应为页岩气发育的次级有利区;灰泥质浅水陆棚主要发育“钙质页岩+含碳泥质灰岩”,有机质含量相对较低,为页岩气的较不利发育区。

Relationship between sedimentary facies and shale gas geological conditions of the Lower Silurian Longmaxi Formation in southern Sichuan Basin and its adjacent areas

On the basis of detailed study of sedimentary facies of the Lower Longmaxi Formation of Silurian in southern Sichuan Basin and its adjacent areas,combining with the results of experimental methods such as polarizing microscope,X-ray diffraction and element geochemistry,the relationship between sedimentary facies and shale gas geological conditions were studied. Tidal flat and shallow shelf facies are the two main sedimentary facies in the study area. The lower section of the Longmaxi Formation is rich in organic matters and siliceous shales,which were conducive to the enrichment and development of shale gas. The formation of siliceous shale riched in organic matter of the lower member of Longmaxi Formation was due to the anoxic retention environment,layered water caused by a rapid transgression,appropriate deposition rate and relative high biological yield. The tidal flat deposits are unfavourable for shale gas formation. Deep-water shelf facies in the depocenter are dominated by carbonaceous siliceous shale,carbonaceous shale and calcium carbonaceous silty shale,which are favorable for shale gas formation. Sandy and muddy shallow shelf is the secondary favorable area for shale gas development containing more organic matters;marlite and muddy shallow shelf is composed mainly of “calcareous mudstone and carbonaceous argillaceous limestone”,which are not favorable for shale gas formation with relative low organic matter content.