3.1. Representative sample selection
Three core samples of coal-measure dark mudstone, the main type of source rock, were collected from the Upper Pinghu Formation in Wells K1, K3, and K4 from 4,204, 3,236.7, and 4,350 m, respectively (Fig. 1). These samples were used for total organic carbon (TOC) content, Rock-Eval pyrolysis, and vitrinite reflectance (Ro ) measurements. Only the sample from Well K4 was suitable for the formation thermal simulation experiment. Thus, the dark mudstones used in this study were collected from the upper Pinghu Formation in the K4 well (4,350–4,352 m). The basic geochemical data were: TOC: 0.68%; hydrogen index (HI): 49 mg/g TOC; Ro : 0.75%; and organic matter was type Ⅲ, which also corresponds to the typical dark mudstone in the Xihu Sag. To ensure sample uniformity, the columnar core used in the simulation experiment was a cylindrical core column formed by further pressing after crushing the samples to 40–60 mesh.