Quantitative characterization of fracture networks in the Permian organic-rich shales of the Whitehill formation in the Karoo Basin, South Africa and implication for hydrocarbon exploration, CO2 sequestration, and other fractured geofluid systems

Abstract

Mudstone formations (shales) are increasingly recognized as targets of hydrocarbon exploration and production, sites for CO₂ storage, and repositories for nuclear and other waste. Due to their low matrix permeability, considerable amount of fluid flow in shales may occur within fractures, and consequently significant effort is placed on the characterization of fracture networks. We report results of a multidisciplinary field and borehole study of a fracture network in Permian organic-rich shales of the Whitehill Formation in the Karoo Basin, South Africa, developed in proximity to Lower Jurassic dolerite intrusive sills. The goal was to systematically document the different fracture types, differentiate the underlying fracturing mechanisms, and quantify their contribution to fracture networks in intrusion-related organic-rich shales. Our observations comprise high-resolution digital outcrop models constructed from ground- and drone-based photogrammetry, and petrographic and geochemical data from fracture cements and fluid inclusions contained in them.

Our results indicate that the fracture network in the studied shale developed from a suite of processes related to intrusion of the sills. These resulted in the formation of five distinctive fracture types, with varying orientation and fracture-filling material: (i) vertical solid bitumen veins; (ii) sub-vertical to horizontal solid bitumen-calcite veins; (iii) closely-spaced subvertical calcite veins; (iv) horizontal bitumen and calcite veins; and (v) sub-vertical joints. Based on the spatial distribution of these fracture types, three fracture domains are identified: upper, central, and lower domains, where the upper and lower domains are near the intrusions and the central domain at a greater distance.

The upper and lower domains are dominated by solid bitumen and solid bitumen-calcite veins that exhibit a strong vertical orientation with a N-S strike. In contrast, the central domain comprises nearly horizontal solid bitumen-calcite veins. The vertical tensile fractures in the upper and lower domains likely developed in response to increased pore fluid pressure associated with the mobilization hydrocarbon and other hydrothermal fluids caused by intrusive heating. The horizontal tensile fractures in the central domain may be related to the thermal maturation and poroelastic deformation of the organic-rich shales. The development of the N-S tensile fractures is consistent with the normal fault regime associated with the E-W Early Jurassic extensive tectonics that led to the breakup of Gondwana.

The relative contributions of the vertical and horizontal tensile fracture types in the Karoo shale to spatial variation of fracture orientation, intensity, and connectivity were evaluated using digital fracture network quantification. All observed fracture types contributed to hydrocarbon and fluid migration and storage. Our observations demonstrate that outcrops containing natural fracture sets can be used to characterize crucial fracture attributes such as fracture length, spatial arrangement, and connectivity and contribution of different fracturing mechanisms to fluid flow and storage in organic-rich shales impacted by igneous intrusions.