川东地区龙潭组页岩岩性序列及勘探意义

Lithologic sequence and exploration significance of Longtan Formation shale in eastern Sichuan Basin

川东地区龙潭组具有较大的页岩气勘探前景,该地层沉积于海陆过渡环境,沉积相变化快,岩性复杂,存在难以通过常规的矿物成分方法划分岩相以及当前岩相划分尚不精确的问题,制约了该地区龙潭组页岩有利岩相带识别及勘探。本文基于岩心观察、野外剖面实测和薄片鉴定,结合钻井的泥地比、单层岩性厚度,建立了川东地区龙潭组7种特征岩性组合模式(泥页岩型、富泥含砂型、富泥含灰型、灰泥互层型、富灰含泥型、致密灰岩型和火成岩型);综合TOC、孔隙度、含气性、脆性指数等页岩储层特征参数,明确了不同岩性组合在龙潭组各段的平面分布规律。研究表明,纵向上优势岩性组合主要为龙潭一段富泥含灰型和泥页岩型,龙潭二段主要为泥页岩型、灰泥互层型和富泥含灰型。平面上龙潭一段优势岩性组合富泥含灰型主要分布在开州—云阳一带,泥页岩型主要分布在重庆—綦江一带;龙潭二段优势岩性组合泥页岩型分布在巴南—綦江一带,灰泥互层型分布在达州一带,富泥含灰型分布在万州—梁平—石柱一带。该项研究成果为川东地区龙潭组页岩气勘探提供了重要依据。

Longtan Formation in eastern Sichuan Basin has large potential for shale gas exploration and development.This formation deposited under marine-continental environment has complex rock types and rapidly changing sedimentary sub-facies,which leads to the problems on how to identify and classify the rock-combinations by normal means.However,the present classification of rock-combination is not precise enough.These problems set limits and troubles on identifying the favorable rock-combinations and exploration activities for Longtan Formation in this area.On the basis of core observation,field profile measurement and thin section identification,combined with the mud-to-ground ratio and single-layer lithology thickness of the well,seven lithological combination types were established for the first time,namely:shale,rich shale containing sandstone,rich shale containing limestone,shale-limestone interbedding,rich limestone containing shale,dense limestone and igneous rock types.Based on the characteristic parameters of shale reservoirs such as TOC,porosity,gas content and brittleness index,the planar distribution of different lithological combinations in each section of the Longtan Formation was studied.The results show that the longitudinal dominant lithology combinations are mainly rich shale containing limestone and shale in Longtan section,and the second section of Longtan is mainly shale,shale-limestone interbedding and rich limestone containing shale.On the plane,the dominant lithological combination of mud-rich and mud-containing types in Longtan is mainly distributed in the Kaizhou-Yunyang area,and the shale is mainly distributed in the Chongqing-Qijiang area.The dominant shale type of Longtan SectionⅡis distributed in the Banan-Qijiang area,the shale-limestone interlayer is distributed in the Dazhou area,and the mud-rich limestone is distributed in the Wanzhou-Liangping-Shizhu area.The research results provide an important basis for the exploration of shale gas in the Longtan Formation in eastern Sichuan.