Dynamic conditions and processes of three-dimensional accumulation of Sinian-Permian natural gas in Penglai gas area, central Sichuan Basin, SW China

Abstract

Considering the complexities of gas-water relationships in the gas reservoirs, unclear natural gas distribution and difficult exploration expansion of the Sinian–Permian natural gas in the Penglai gas area of the central Sichuan Basin, this study investigates the gas source, charging processes and enrichment patterns of gas reservoirs based on reservoir characterization, natural gas geochemical analysis, reservoir testing, well logging-seismic data interpretation, as well as basin modeling and dynamic analysis. The results are obtained in three aspects. First, four sets of highly efficient source rocks are developed beneath the salt of the Triassic Jialingjiang Formation, dominated by the Cambrian source rocks. The reservoirs exhibit strong heterogeneity, with six sets of effective reservoirs being isolated from each other yet dynamically connected. Multi-stage strike-slip fault-related fault-fracture-cavity-unconformity systems constitute the hydrocarbon migration network. Second, overpressure generated by hydrocarbon generation in the Cambrian source rocks drove bidirectional hydrocarbon expulsion from the source kitchen. Multiple sources, including cracked gas from paleo-oil reservoirs and residual hydrocarbons within source rocks, contributed to the hydrocarbon supply. The Sinian–Permian system underwent multiple dynamic hydrocarbon accumulation processes, resulting in the formation of extensive “sweet spots” within multi-layered heterogeneous reservoirs, which were subsequently modified by late-stage gas adjustments to their current form. Third, a three-dimensional accumulation model for deep marine natural gas is established, with multi-source hydrocarbon supply, three-dimensional migration, multi-stage accumulation, dynamic adjustment and lithology-controlled distribution. Large-scale reservoirs within positive structural settings, late-stage structurally stable areas, and slope structures are identified as favorable plays for gas exploration.