Characterising the permeability and structure of fluid-escape conduits in sedimentary basins - application to geological carbon sequestration

Increased greenhouse gas emissions entering the atmosphere and hydrosphere are causing changes to global climate. Geological carbon sequestration is a proven technology, used to reduce anthropogenic emissions from the atmosphere. However, there are concerns about the unintended migration of CO2 from sub-surface storage reservoirs. Fluid-escape structures, which act as conduits for pressure-driven fluids, are observed in sedimentary basins globally. These structures can extend over 500 m across and intrude vertically through kilometres of sedimentary overburden. The quantitative assessment and nature of fluid-escape conduits are currently poorly constrained. Here we sample and characterise analogous onshore field outcrop analogues in Panoche Hills, California and Varna, Bulgaria to complement the study of active structures in the Witch Ground Basin, Central North Sea. A key aim is to quantify permeability and determine the process mechanisms of fluid flow through focused fluid conduits. Here we generate an accurate, repeatable and upscalable 3D X-ray micro-computed tomography (XCT) image-based methodology workflow to calculate porosity, effective porosity, and permeability of fluid-escape conduit samples. During fieldwork and sampling, the geometry, distribution, physical interaction with host-rocks and 3D properties are determined. Given the large scale of the structures (>150 m wide), porosity and permeability transects are performed across the intrusions and their host sediments, to characterise natural variability and identify preferential fluid flow pathways. Studies of sand intrusions in the Panoche Hills reveal permeability heterogeneity is largely controlled by silica cementation processes linked to the drainage of pore waters from silica-rich host rock sediments during intrusion formation. Sub-vertically orientated intrusions have reduced permeability due to the incorporation of clay and silt host rock sediments into the matrix of intrusions, and subsequent dewatering, grain compaction and silica Opal-A to Opal-CT transformation. Further, the investigation of fluid-escape structures in Varna and Panoche Hills highlights a three orders of magnitude reduction in permeability of unconsolidated host rock sediments caused by the transformation of methane gas by anaerobic oxidation to carbonates. The mapping of carbonate pipes revealed that focused fluid flow process mechanisms are not restricted to fine-grained host rock strata, and can occur in unconsolidated sediments. High-resolution 2D and 3D seismic reflection data, integrated with sediment core data, are used to characterise the Scanner Pockmark Complex, Central North Sea. The study has revealed that gas flows vertically upwards through chimneys, directly observed as a series of interconnected fractures. Previous studies interpret seismic chimneys as vertical conduits in hydraulic connection to a single, deeper zone of overpressured fluid. Here we observe that fluid overpressure generation, leading to chimney and pockmark genesis, represents a complex system with gas-bearing zones at multiple depth intervals. When integrated into a multi-disciplinary approach, these findings can improve our understanding of focused fluid flow within the shallow overburden, for applications to shallow geohazard assessment and carbon sequestration