Geological controls on shale gas accumulation in a synclinal reservoir and implications for natural gas migration

Abstract

Shale gas exploration in the marine systems of southern China has predominantly targeted anticlines, while synclines have been regarded as high-risk areas due to potential gas loss along shale layers governed by structural morphology. Recently, commercial shale gas production has been identified in the Upper Ordovician Wufeng and Lower Silurian Longmaxi shales within the Baima Syncline, Southeast Sichuan Basin, sparking significant interest in exploring the geological controls on shale gas accumulation and migration in synclinal settings. Here, we characterize the shale microstructure, free and adsorbed gas capacities, and the chemical and isotopic composition of shale gas in relation to the tectonic evolution of the syncline, based on a sample set from five wells in the structurally isolated Baima Syncline. Total gas contents decrease from the center of the syncline (5.2 m³/t) through the limb (4.9 and 4.8 m³/t) towards the margin (3.8 and 2.4 m³/t), primarily due to a reduction in free gas content. Seismic profiles and microstructural analysis of organic matter (OM)-hosted pores reveal increasing structural deformation from the syncline core to its margins. This strong deformation reduces the free gas content through two mechanisms: (1) porosity loss due to compression of OM-hosted pores caused by structural deformation, and (2) lower pressure coefficients and reduced gas saturation at the syncline margins, where bedding-parallel microfractures formed by strong tectonic stress and enhance permeability and facilitate gas escape. Variations in δ¹³C₁ values and the wetness of shale gas indicate minimal gas migration along shale layers within the syncline, typically less than 2 km, even over a geological timespan of approximately 90 million years. These findings suggest that the core and limb areas of synclines with weak structural deformation and adequate distance from faults could serve as promising targets for shale gas exploration, even in tectonically complex regions. Moreover, the limited gas flow along shale layers provides valuable insights into the CO₂ storage potential of shale reservoirs.