Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs

The geological processes that create fluid storage capacity and connectivity in global fractured basement reservoirs are poorly understood compared to conventional hydrocarbon plays. Hosting potentially multi-billion barrels of oil, the upfaulted Precambrian basement of the Rona Ridge, offshore west of Shetland, gives key insights into how such reservoirs form. Oil presence is everywhere associated with sub-mm to m-thickness mineralized fracture systems cutting both basement and local pre-seal cover sequences. Mineral textures and fluid inclusion geothermometry suggest a low temperature (90-220ºC), near-surface hydrothermal system, as does the preservation of clastic sediments in the same fractures. These fills act as permanent props holding fractures open, forming long-term fissures in the basement that permit oil ingress and storage. Calcite fill U-Pb dating constrains the onset of mineralization and contemporaneous oil charge to the Late Cretaceous. The additional preservation of oil-stained injected sediment slurries and silica gels along basement faults suggests that rift-related seismogenic faulting initiated lateral oil migration from Jurassic source rocks into the adjacent upfaulted ridge. Subsidence below sea level in the latest Cretaceous sealed the ridge with shales, with buoyancy-driven migration of oil into the pre-existing propped fracture systems continuing long after the cessatio