Deep and ultra-deep basin brittle deformation with focus on China

This volume describes progress in understanding brittle structures in deep and ultra-deep (>4 km to > 7 km) sedimentary basins. Under deep conditions in sandstone, carbonate rocks, shale, and other rocks, fluid charge and resource recovery are sensitive to faults and opening-mode fractures. In China, work is in progress on deep, deformed, and tectonically active basins including drilling of wells expected to exceed 10 km in depth. Papers describing fractures in horizontal wells indicate locally highly clustered spatial arrangements. Orientation patterns record protracted superposed deformation. Despite deep settings, open fractures are abundant, and wide (>1 mm) fractures with varying amounts of sealing calcite are common. Differences in cement abundances are due to the diagenetic history of fractures, not their origins (e.g., tectonic loading or elevated pore fluid pressure). In carbonate rocks, solution enhanced strike-slip faults and fractures with cavernous porosity are present, and in sandstone enigmatic enhanced host-rock porosity halos a few mm wide locally surround sealed fractures. Owing to differences in thermal exposure due to recent (>6 Ma to present) rapid deep burial (in some cases >2000 m) and locally low geothermal gradients, some fractures at great depths are less diagenetically altered than those at shallower depths in the North American Cordillera. Contrasts in diagenesis may affect fracture size, spatial arrangement, and connectivity. •Deep (>4 km) and ultra deep (>7 km) basins have open quartz lined and calcite sealed fractures, solution enhanced strike-slip faults, enhanced porosity zones.•Protracted thermal exposure rather than depth governs quartz accumulation.•In horizontal wells in China, clusters of fractures can be found at depths greater than 4900 m.•Ultra-deep faults and fractures in China have marked similarities and differences compared to deep basins elsewhere.•A potential area for future research is investigation of interconnected structural and diagenetic processes.