西昌盆地上奥陶统五峰组—下志留统龙马溪组优质页岩层序沉积特征及页岩气聚集模式

Sedimentary sequence characteristics and shale gas accumulation pattern of high-quality shale in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, Xichang Basin

西昌盆地多口钻井于上奥陶统五峰组—下志留统龙马溪组获得页岩气显示,其中,五峰组—龙马溪组一段(龙一段)的含气量较好,表明该层段具有良好的页岩气勘探开发潜力。为进一步做好西昌盆地龙一段的前期评价工作,综合利用岩心观察、岩心样品薄片等资料,结合测井解释,对优质页岩进行划分并对优质页岩的地层层序、沉积相、沉积模式等进行了研究。结果表明:(1)西昌盆地五峰组—龙马溪组优质页岩为(1)—(5)小层;(2)优质页岩可划分为两个三级层序,SQ1发育潮坪相中的生屑滩微相及深水陆棚相中的硅质泥棚微相,SQ2发育深水陆棚相中的灰质泥棚和含灰粉砂质泥棚;(3)五峰组—龙一1亚段为深水陆棚沉积,该沉积环境控制了优质页岩的发育,优质页岩有机碳含量高、硅质含量高、富含笔石;(4)断裂带发育程度、优质页岩埋深、优质页岩距断裂带的远近是影响西昌盆地五峰组—龙马溪组优质页岩层段页岩气聚集的关键因素。

Shale gas shows have been observed from the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in several wells in Xichang Basin. The drilling results indicate that gas content of Wufeng Formation-first member of Longmaxi Formation(Long 1member) is relatively good, showing promising potential for shale gas exploration and development. By comprehensively applying core observation and thin section data, and integrating with logging interpretation, the high-quality shale is classified and the stratigraphic sequence,sedimentary facies and depositional pattern are studied, so as to primarily evaluate shale gas potential of Long 1 member in Xichang Basin.The study results show that:(1) The high-quality shale of Wufeng-Longmaxi Formation is composed of five thin layers(1)-(5);(2) The highquality shale is divided into two third order sequences. Deposits of bioclastic beach microfacies of tidal flat facies and siliceous mud shelf microfacies of deep-water shelf facies were developed in SQ1, while SQ2 was composed of lime mud shelf and limy sandy mud shelf deposits of deep-water shelf facies;(3) The development of high-quality shale was controlled by the deep-water shelf environment during the deposition period of Wufeng-first sub member of the first member of Longmaxi Formation(Long 11 sub member), which had high organic carbon content,high silica content and rich graptolite;(4) The shale gas accumulation in high-quality shale of Wufeng-Longmaxi Formation is affected by three key factors, i.e., development degree of fault zone, burial depth of high-quality shale and distance between the high-quality shale to fault zone.