Sedimentary Evolution Model of Upper Ordovician Wufeng-Lower Silurian Longmaxi Organic-rich Shale in the Sichuan Basin and Its Surrounding Area

Key words: 
• Wufeng Formation-Longmaxi Formation /
• shale /
• graptolite biostratigraphy /
• spatial-temporal distribution /
• sedimentary evolution model

Abstract: The Upper Ordovician Wufeng-Lower Silurian Longmaxi organic-rich shale in the Sichuan Basin and its peripheral regions,whose thickness is 20-60 m,is the main focus for the exploration and development of shale gas in China. According to our research in recent years,the temporal and spatial distribution of the organic-rich shale varies greatly. Based on the observation of key sections,core drilling,lithosphere biostratigraphy,geochemistry and miner-al analysis,the spatial-temporal distribution characteristics of the organic-rich shale in the Sichuan Basin and its sur-rounding areas were clarified. Three types of organic-rich shale depositional evolutionary models were established, which are described as follows:(1) The "two sags sandwiched between three uplifts" model in the Changning-Weiyu-an area:the organic-rich shale deposits at WF2-LM5 in Changning and at LM1-LM8 in Weiyuan. The thickness of the organic-rich shale in the uplift is much thinner than that in the sags. (2) The "stage-progressive distribution pat-tern" model in the Chongqing and Qianjiang area:The closer the area is to the interior of the basin,the later the sedi-mentation of the organic-rich shale tends to end. The Jiaoshiba area is the depositional center of the shale,while the thickness of other areas is relatively thin. (3) The "stable uplifting" model in northeastern Sichuan:The organic-rich shale in Well Wuxi 2 in the center of the basin has a large thickness and a long depositional duration (WF2-LM9). The thickness of the shale in the marginal area of the basin is significantly thinner,and the region presents an overall upward trend. With the passage of time,the organic shale depositional area becomes smaller and smaller. In general, the deposition of these shales is mainly influenced by the Guangxi movement. The organic-rich shale sedimentary cen-ters tend to move northward gradually,and the shale only deposits in the Weiyuan and Wuxi areas after the Aeronian stage in the Sichuan Basin. The research results have an important guiding significance for the micro-layer division and fine reservoirs contrast of the Wufeng-Longmaxi shale in this area.