The Sichuan Basin serves as a key arena for ultra-deep natural gas exploration. The Nanhuan System Gucheng Formation, characterized by its ancient geological age and great burial depth, lacks almost any drilling data within the basin interior, and its sedimentary features and natural gas potential remain unstudied. Based on outcrop sections of the Nanhuan Gucheng Formation along the northern margin of the Sichuan Basin, sedimentological and hydrocarbon reservoir characteristics were analyzed. The study reveals: ① The lower Gucheng Formation at the Chenkou Yuyang section comprises three lithofacies: deformed-bedding conglomeratic sandstone, massive-bedded medium sandstone, and dark-gray horizontally-bedded mudstone, interpreted as deposits of turbidity channels, turbidite fan lobes, and deep-water shelf mud, respectively; ② The turbidity channel and fan sandstones exhibit dissolution pores, with porosities ranging from 8% to 12%, representing favorable reservoirs with a cumulative thickness exceeding 40 m. The deep-water shelf mud shows TOC values between 0.8% and 1.5%, serving as favorable source rocks with a cumulative thickness over 30 m. These two units are interbedded, forming an effective source-reservoir assemblage; ③ Based on the west–east outcrop transect (Zhenba Xiaoyangba, Chenkou Yuyang, and Mahuang Gou sections), the thickness of the Gucheng Formation displays a thin–thick–thin variation, interpreted as reflecting a sedimentary transition from shallow-water shelf delta to deep-water shelf/turbidite systems and back to shallow-water shelf deposits. A rift depositional model with a gentle western slope and steep eastern slope is proposed. In deep-water shelf areas, turbidite sandstone reservoirs are vertically interbedded with shelf mudstone source rocks, while in shallow-water shelf areas, deltaic sandstone reservoirs are laterally connected to source rocks. Spatially, this constitutes a hydrocarbon distribution pattern characterized by “vertical stacking and lateral connectivity,” providing valuable insights for ultra-deep natural gas exploration in the Sichuan Basin.
Deng et al. (Tue,) studied this question.