Characteristics and differential origin of Qiongzhusi Formation shale reservoirs under the“aulacogen-uplift”tectonic setting,Sichuan Basin

The Qiongzhusi Formation,following the Wufeng-Longmaxi formations,has been recognized as a promising target for future shale gas exploration and exploitation in the Sichuan Basin. Presently,notable achievements have been made in shale gas exploration in wells Z201 and WY1,drilled at the center and margin of the Deyang-Anyue aulacogen, respectively. However,there is a lack of clear understanding of the Qiongzhusi Formation shale reservoirs occurring in the aulacogen. Focusing on wells Z201 and WY1,coupled with other data on shale gas exploration and exploitation,we systematically analyze the mineral and organic geochemical characteristics,reservoir and storage space characteristics, and gas-bearing properties of each shale in the Qiongzhusi Formationin the study area. Key findings are outlined as follows. (1) The Qiongzhusi Formation shales in the study area can be divided into eight layers,predominantly composed of brittle minerals overall. This formation generally exhibits total organic carbon(TOC)content exceeding 1 %,suggesting high-quality source rocks. Furthermore,the TOC content is higher within the aulacogen than on its margin,indicating favorable gas generation conditions. aulacogen(2)Both organic and inorganic pores are found in the Qiongzhusi Formation shales,more prevalent within the aulacogen,contributing to extremely high gas content. Black shale reservoirs in layers 1,3,5,and 7 exhibit high quality,especially the layer 5. (3)The quality of shale reservoirs in the Qiongzhusi Formation is governed by the Deyang-Anyue aulacogen. Specifically,reservoirs encountered in drilling well Z201,situated within the aulacogen,exhibit superior characteristics compared to those in well WY1 located at the margin. (4)The degree of organic matter evolution in the Qiongzhusi Formation shales is significantly influenced by the Leshan-Longnvsi paleo-uplift. The organic matter generally tends to be less mature within than outside. The moderate-degree organic matter evolution within the paleo-uplift creates conditions favorable to large-scale gas enrichment. Therefore,high-quality Qiongzhusi Formation shale reservoirs are identified as a major successor play for future shale gas exploration and exploitation. © 2024 Editorial Department of Oil and Gas Geology. All rights reserved.