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Natural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth's atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid‐escape conduits, which are present within sedimentary basins globally. These conduits permit pressure‐driven fluid flow, hydraulically connecting deeper strata with the Earth's surface; however they can only be fully resolved at sub‐seismic scale. Here, a novel 'minus cement and matrix permeability' method using three‐dimensional X‐ray micro‐computed tomography imaging enables the improved petrophysical linkage of outcrop and sub‐surface data. The methodology is applied to the largest known outcrop of an inactive fluid‐escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600 to 800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17 to 27%) and permeability (≤1 to ca 500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal‐CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity−permeability ranges of ca 30 to 40% and 103 to 104 mD. These calculations are closely comparable to active sub‐surface systems in sedimentary basins. Field observations revealed that, at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore‐scale calculations and field‐scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid‐escape conduits, which has significance for hydrocarbon reservoir characterization and modelling, as well as subsurface CO2 and energy storage containment assessment. [ABSTRACT FROM AUTHOR]

We examined the potential impact on CO2 transport of zones of deformation bands in reservoir rock that transition to opening-mode fractures within overlying caprock. Sedimentological and petrophysical measurements were collected along an approximately 5 m × 5 m outcrop of the Slick Rock and Earthy Members of the Entrada Sandstone on the eastern flank of the San Rafael Swell, Utah, USA. Measured deformation band permeability (2 mD) within the reservoir facies is about three orders of magnitude lower than the host sandstone. Average permeability of the caprock facies (0.0005 mD) is about seven orders of magnitude lower than the host sandstone. Aperture-based permeability estimates of the opening-mode caprock fractures are high (3.3 × 107 mD). High-resolution CO2-H2O transport models incorporate these permeability data at the millimeter scale. We varied fault properties at the reservoir/caprock interface between open fractures and deformation bands as part of a sensitivity study. Numerical modeling results suggest that zones of deformation bands within the reservoir strongly compartmentalize reservoir pressures largely blocking lateral, cross-fault flow of supercritical CO2. Significant vertical CO2 transport into the caprock occurred in some scenarios along opening-mode fractures. The magnitude of this vertical CO2 transport depends on the small-scale geometry of the contact between the opening-mode fracture and the zone of deformation bands, as well as the degree to which fractures penetrate caprock. The presence of relatively permeable units within the caprock allows storage of significant volumes of CO2, particularly when the fracture network does not extend all the way through the caprock. [ABSTRACT FROM AUTHOR]

Abstract: The Giant Gjallar Vent (GGV), located in the Vøring Basin off mid-Norway, is one of the largest (~5×3km) vent systems in the North Atlantic. The vent represents a reactivated former hydrothermal system that formed at about 56Ma. It is fed by two pipes of 440m and 480m diameter that extend from the Lower Eocene section up to the Base Pleistocene Unconformity (BPU). Previous studies based on 3D seismic data differ in their interpretations of the present activity of the GGV, describing the system as buried and as reactivated in the Upper Pliocene. We present a new interpretation of the GGV's reactivation, using high-resolution 2D seismic and Parasound data. Despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column, the GGV appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface. The anomalies are confined to the Kai Formation beneath the BPU and the overlying Naust Formation, which are interpreted to act as a seal to upward fluid migration. The seal is breached by focused fluid migration at one location where an up to 100m wide chimney-like anomaly extends from the BPU up to the seafloor. We propose that further overpressure build-up in response to sediment loading and continued gas ascent beneath the BPU will eventually lead to large-scale seal bypass, starting a new phase of venting at the GGV. [Copyright &y& Elsevier]