Restoration of paleokarst geomorphology of Sinian Dengying Formation in Sichuan Basin and its significance, SW China

Abstract

Based on newly drilled well data in the Gaoshiti area in the central Sichuan Basin, profile data of more than 150 field outcrops of the regional geological survey, and stratigraphic division and correlation of more than 30 wells in the Sichuan Basin and its adjacent areas, combined with regional seismic data, moldic methods are comprehensively used to restore the karst paleogeomorphy of the Dengying Formation, and thus studying the paleogeographic pattern and the significance of oil and gas exploration. The Sichuan Basin was surrounded by paleo-lands/underwater highlands in the late Sininan Dengying period, including Kangdian paleo-land in the west, Songpan paleo-land in the northwest, Hannan paleo-land in the north, Qianjiang-Zheng'an, Zhenba and Wuxi-Jianshi underwater highlands in the southeast and northeast. The Sichuan Basin was adjacent to Jiangnan Basin southeastwards and Qinling paleo-ocean northeastwards respectively. Affected by the separation of NS-striking Zitong-Junlian Aulacogen, NE-striking Langzhong-Tongjiang and Chongqing-Kaixian depressions in this basin, the Sichuan Basin presents the NS-trending framework of “three uplifts (Zhenba, Chuanzhong and Qianjiang-Zheng'an) and two depressions (Langzhong-Tongjiang and Chongqing-Kaixian)”, and is divided into two relatively isolated EW-trending paleo-uplift systems (NS-striking Mianyang-Leshan-Xichang paleo-uplift and nearly NE-striking Chuanzhong paleo-uplift). Controlled by karst paleogeomorphy of the Sinian Dengying Formation, the pattern of karst landscape consists of five secondary geomorphic units, such as karst highland, karst platform, karst slope, karst depression and karst basin, of which the karst platform and karst slope are the favorable zones for the development of karst reservoirs, providing advantages for the formation of large gas fields. Key words: paleogeomorphologic restoration, moldic method, weathering crust paleokarst, karst reservoir, paleogeographic framework, Sinian Dengying Formation, Sichuan Basin