Multiphase Fracturing, Fluid Evolution, and Geothermal Sustainability in Mesoproterozoic Carbonates of the North China Craton

Understanding the origin and evolution of fractures is critical for evaluating the long-term sustainability of geothermal water production from carbonate reservoirs, an emerging low-carbon energy source aligned with global carbon-neutrality goals. In this study, we develop a fracture-fluid evolution model for Mesoproterozoic carbonates of the North China Craton (NCC) by integrating U–Pb dating of fracture-filling dolomites with petrological, micro-CT, and multi-isotopic (C, O, clumped and ⁸⁷Sr/⁸⁶Sr) analyses. Three principal phases were identified at ~1550 to 973 Ma, 669 to 597 Ma, and 106 to 58 Ma. Clumped isotope-derived temperatures (57.1°C–93.6°C) and calculated burial depths (mostly < 2 km, with few reaching ~2.5 km) indicate predominantly shallow diagenetic conditions. Fluids responsible for Phases I–II fracturing were primarily seawater-derived, whereas meteoric water dominated Phase III fracturing beginning in the late Mesozoic. Our results demonstrate that high reservoir connectivity—primarily driven by multiphase fracturing—exerts a first-order control on reservoir quality, while porosity and pore-throat dimensions play a secondary role. This connectivity, coupled with the absence of large karstic cavities, sustains high hydrostatic pressures and sustained geothermal yields. An estimated ~1.1 × 10²² J of geothermal energy—derived from meteoric recharge over recent geologic time—underscores the carbon-neutral potential of these fractured carbonates. The integrated methodology presented here offers a transferable framework for evaluating fractured geothermal systems worldwide.