The geochemical characteristics, distribution of marine source rocks and gas exploration potential in the northwestern Sichuan Basin, China

摘要

In recent years, the northwestern part of the Sichuan Basin has become a popular location for natural gas exploration. However, up to thispoint, the geochemical characteristics and distribution of marine source rocks in this area have not been thoroughly investigated. In thenorthwestern Sichuan Basin, there are eight sets of marine source rock, including the Lower Sinian Doushantuo Formation, the third Member ofUpper Sinian Dengying Formation, the Lower Cambrian Qiongzhusi Formation, the Lower Silurian Longmaxi Formation, the Middle Devonian,the Middle Permian Maokou Formation, the Upper Permian Longtan Formation/Wujiaping Formation, and the Upper Permian Dalong For_mation, from which eight outcrop profiles were selected for systematic organic geochemical analysis. The Qiongzhusi Formation has the thickesthigh-quality source rocks, up to 80 m thick, followed by the Upper Permian Longtan/Wujiaping and Dalong formations, which have up to20e30 m thick high-quality source rocks, whereas the thickness in other layers is relatively thin. By the abundance of organic matter, theDoushantuo, Qiongzhusi, Longmaxi, Longtan/Wujiaping, and Dalong formations developed high-quality source rocks with total organic carboncontent (TOC ) greater than 3, while the third Member of Upper Sinian Dengying Formation, Middle Devonian and Maokou formations hadlow TOC. The kerogen of source rocks gradually enriched 13C from the type I organic matter of the Lower Cambrian Qiongzhusi Formation tothe Upper Permian Dalong Formation, and the d13C values of Doushantuo Formation, the third Member of Dengying Formation, and QiongzhusiFormation are less than 30.0‰. The d13C values of Longtan Formation or Wujiaping Formation and Dalong Formation with type II2 organicmatter ranges from f 28.0‰ and 26.0‰. Others lie somewhere in between type I and type II2. Except for Permian source rocks in Guangyuanand Chaotian areas, which are in the mature stage, source rocks in the northwestern Sichuan Basin are in the over-mature stage and largely in thethermal stage of dry gas. High abundance source rocks have provided rich marine natural gas resources. As a result, future gas exploration in thenorthwestern Sichuan Basin will focus on the gas-bearing system associated with the Qiongzhusi Formation and Upper Permian source rocks.