CO2 Geological Storage Potential of Longmaxi Shale: Insights from Geochemistry, Modelling, and Imaging

Carbon geological sequestration is central to China’s energy transition through carbon capture, utilization, and storage (CCUS). While saline aquifers are commonly regarded as primary targets for large-scale CO₂ storage, the storage potential of gas shales remains poorly constrained due to their complex and heterogeneous pore systems. The Upper Silurian Longmaxi Formation in the Sichuan Basin represents one of China’s most significant shale targets. This study integrates pore-scale characterization and basin-scale modelling to explicitly link intrinsic shale properties with regional CO₂ storage estimates, addressing a key limitation in current CCUS assessments that treat shale as a reservoir unit for CO₂ storage.

The Pengye-1 well is located in the Pengshui block on the southeastern margin of the Sichuan Basin, a transitional zone between the stable platform and the Wuling fold belt. It targets organic-rich shales of the Lower Silurian Longmaxi Formation deposited in a deep-water shelf environment and, unlike overpressured reservoirs in the basin interior, is currently preserved under normal-pressure conditions due to tectonic uplift and denudation during the Yanshanian and Himalayan orogenies. To bridge the gap between regional tectonic evolution and microscopic storage capacity, a multi-scale characterization approach was adopted. At the pore scale, core samples from the Pengye-1 well were analysed using Brunauer–Emmett–Teller (BET) adsorption, scanning electron microscopy (SEM), and X-ray computed tomography (CT).  Specific surface area (SSA) ranges from approximately 19 to 60 m² /g over a depth interval of 2095–2140 m. Porosity does not show the expected reduction with increasing depth, SSA and pore volume do not exhibit a simple monotonic decrease attributable solely to burial compaction. Instead, imaging reveals a depth-dependent evolution in pore morphology from relatively open and regular pores to predominantly slit-shaped pores, with locally preserved ink-bottle geometries. The substantial variability in SSA and pore volume among samples at comparable depths highlights the importance of mineralogical heterogeneity, particularly variations in clay mineral assemblages and brittle mineral phases which in controlling pore preservation and surface development. In addition, a subset of samples shows a negative correlation between pore volume and total organic carbon (TOC), in contrast to commonly reported trends, suggesting a role of TOC in pore evolution.

At the basin scale, CO₂ storage capacity was evaluated for both saline aquifers of the Xujiahe Formation and organic-rich shales of the Upper Silurian Longmaxi Formation within the Sichuan Basin, which together form an interbedded sandstone–shale system. Storage capacity was estimated using multiple volumetric approaches and further constrained by geological modelling that explicitly represents shale intra-beds within sandstone reservoirs. Reservoir-scale simulations using the Permedia® CO₂ module were conducted for the Xujiahe Formation in the central Sichuan Basin, comparing sandstone–shale interbeds with shale-dominated scenarios. The simulations indicate near-complete storage efficiency (≈99%), reflecting restricted CO₂ migration and enhanced trapping in low-porosity, tortuous shale pore networks. While saline aquifers offer higher injectivity, shale formations contribute substantially to the total basin-scale storage resource due to their extensive areal distribution and adsorption-dominated storage mechanisms.