Natural hydraulic opening-mode fractures are widespread in low-permeability organic-rich shales and could serve as preferred conduits for fluid migration during hydrocarbon generation. However, in response to fluid overpressure under a certain stress regime, the fracturing of differentially oriented sets and how they cooperated to affect hydrocarbon migration are still uncertain. Here we present an illustrative example by studying formation of a near-orthogonal fracture network—comprising both bedding-parallel and bedding-normal, natural hydraulic opening-mode fractures in organic-rich Wufeng-Longmaxi shales in northeastern Sichuan foreland basin, China. Through analyses of fracture petrography, cementation, and fluid inclusions combined with basin modeling and hydrocarbon maturation, we provide evidence for a broadly simultaneous formation of the two near-orthogonal fracture sets driven by the combined effect of compressive tectonic stress and super-lithostatic pore fluid overpressure. Such extreme overpressure was recorded by monophase CH4 inclusions, which were trapped throughout the synkinematic, broadly simultaneous blocky cementation in both sets during the gas generation stage. The required local, coeval horizontal compressive conditions that enabled the broadly simultaneous opening of the near-orthogonal sets are compatible with the regional tectonic regime in the northeastern basin at the time. This regime was characterized by near-orthogonal compression resulting from the southwestward expansion of the Dabashan fold-thrust belt and coeval northwestward propagation of the Xuefengshan fold-thrust belt. Thus, during cementation with super-lithostatic overpressure, the permeable space kept widening within both fractures and new hydro-fracturing proceeded at the tips. Such hydraulic opening-mode fracturing in a tectonic compressive regime would promote enhanced permeability and connectivity, and propagate the flow conduit system in all directions for accelerating gas expulsion from shales.
Paper Information
Wang, M., Chen, Y., Lin, T., Li, Y., Zhou, Y., Ren, Q., Zhou, Z., Steele-MacInnis, M. (2025) Near-orthogonal, natural hydraulic opening-mode fractures in a compressive setting as conduits for gas expulsion from shales. GSA Bulletin, online. https://doi.org/10.1130/B38346.1
