Caledonian detachment deformation and deposition in the Fuling gas field of the southeastern Sichuan Basin in China: Implications for the Lower Silurian Longmaxi Shale gas exploration and production

Abstract

The Fuling gas field in the southeastern Sichuan Basin is the first and the largest shale gas play in China that has been producing primarily from the organic-rich shale in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. Newly processed 3D seismic data along with well-completion data in the gas field reveal important structural, depositional, and reservoir details in the Lower Paleozoic sedimentary section. Lateral (along-stratal) variations in time structure and bed curvature demonstrate the diversity in faults that can be classified based on their orientation (regional and cross-regional), scale (small, intermediate, and large), and mode (contractional, extensional, and wrench). Vertical (cross-stratal) variations in time structure and bed curvature demonstrate that the deformational intensity increases from the Lower Cambrian to the Upper Ordovician, then decreases from the Upper Ordovician to the Silurian. Seismic isochron and facies analyses indicate that the structural deformation influenced the syntectonic deposition of turbidite sand in a channel complex above the reservoir. The pore pressure, porosity, and gas productivity of the reservoir are the highest in the central portion of the field, where small-scale faults dominate, but drop significantly at the edges of the field, where large-scale lineaments dominate. The relationships suggest that faults and fractures could either reduce or enhance pore pressure, porosity, and gas productivity, depending on their scale. Large-scale faults have the most negative impact on gas enrichment and pressure build-up, leading to reduced pressure, porosity, and productivity; whereas, small-scale ones have the least negative or even positive impact on gas enrichment and pressure build-up, leading to increased pressure, porosity, and productivity. These observations and interpretations offer new insight into the dynamic interplay among tectonic deformation, syn-tectonic sedimentation, and reservoir integrity during the Caledonian (Late Ordovician to Silurian) in the southeastern Sichuan Basin (China).