Facies Analysis, Organic Geochemical Characteristics, and Hydrocarbon Potential of Ordovician Units in the Sichuan Basin, Southwest China

ABSTRACT Organic matter types, abundance, and thermal maturity were assessed based on organic geochemical data. Then, we provided insights into the source rocks' hydrocarbon‐generating capacity and described the favourable reservoir types in the Ordovician units of the Sichuan Basin. The cross‐plot of hydrogen Index (HI, mg HC/g Total Organic Carbon (TOC)) versus temperature of maximum generation ( T max , °C) shows that samples from the Houtan and Sanhui sections have very low HI values (12.05 < HI < 7.75 mg HC/g TOC). These samples suggest kerogen type IV, which does not generate hydrocarbons and is considered inert. Outcrop and core samples from the southeastern part of the Sichuan Basin have HI values ranging from 450 to 68.29 mg HC/g TOC and suggesting type II b kerogens, which are oil and gas‐prone. Tongzi‐Honghuayuan section samples have TOC, vitrinite reflectance (VRo), and T max values ranging from 0.76 to 1.54 wt.%, 0.98% to 1.96%, and 398°C to 559°C, respectively, which are above the defined lower TOC limit of effective source rock and in the dry gas window. Samples from the Late Ordovician Wufeng Formation have TOC values varying from 0.85 to 3.16 wt.%, in the dry gas generation window. The potential source rocks are mainly type II b kerogens with TOC values higher than 0.5 wt.% and are in the oil window and dry gas window. In the investigated region, favourable source rocks are mainly from the Early/Middle Ordovician Meitan and the Late Ordovician Wufeng formations. Favourable reservoir areas are developed in three facies zones of the Tongzi, Honghuayuan, and Baota formations, including intra‐platform shoal, platform marginal shoal, and palaeokarst areas around the Central Sichuan Uplift. The main reservoir spaces comprise intergranular pores, residual intergranular pores, dissolution pores, intergranular dissolution pores, and karst caves.