Gas-in-place (GIP) model and resource potential of the ultra-deep Lower Cambrian gas-rich shales in the southern Sichuan Basin, China

Abstract

The deep and ultra-deep Lower Cambrian Qiongzhusi (DUDQ) shales have abundant gas resources. However, the strong heterogeneity of the DUDQ shale makes it complex to accurately predict the gas-in-place (GIP) content and resource potential. The first high-yield DUDQ shale well Z201 in the southern Sichuan Basin, offering a crucial opportunity to investigate their gas-bearing characteristics. This study focuses on the Qiongzhusi Formation Q1₂ submember (Q1₂) of well Z201, analyzes their geological characteristics, and measures its adsorption parameters through high-temperatures (60−150 °C) and high-pressures (0.01−35 MPa) methane adsorption experiments, and a geological GIP model is established to predict the shale gas resources of the DUDQ shales. The maximum absolute adsorption gas content (n₀) and adsorbed phase methane density (ρ_ads) of the DUDQ shales range from 1.90 to 4.91 cm³/g and 0.28−0.47 g/cm³, respectively. Both n₀ and ρ_ads are positively correlated with total organic carbon (TOC) content and geopressures, but negatively correlated with temperature. The average adsorption capacity of inorganic matter (IM) in the DUDQ shales is 1.99 cm³/g, which, together with high TOC content, are the main important factors contributing to the high GIP contents. With increasing burial depth, the formation temperature and pressure gradually increase. Meanwhile, the adsorbed gas content (n_ads) and total gas content (n_tot) initially tend to increase followed by a subsequent decrease, whereas the free gas content (n_free) tends to increase. Moreover, n_tot is also affected by the TOC content and effective porosity. The most promising areas for the DUDQ shale gas exploration and development are the central area of the intracratonic sag and the surrounding regions of the Weiyuan anticline, with estimated shale gas resources exceeding 2.52 × 10¹² m³.