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Durham University

Department of Earth Sciences

Profile

Publication details for Prof Ken McCaffrey

Holdsworth, R.E., McCaffrey, K.J.W., Dempsey, E., Roberts, N.M.W., Hardman, K. , Morton, A. Feely, M. Hunt, J. Conway, A. & Robertson, A. (2019). Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs. Geology 47(8): 700-704.

Author(s) from Durham

Abstract

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 multibillion barrels of oil, the upfaulted Precambrian
basement of the Rona Ridge, offshore west of Shetland, UK, gives key insights into how
such reservoirs form. Oil presence is everywhere associated with sub-millimeter- to meterthickness mineralized fracture systems cutting both basement and local preseal cover sequences. Mineral textures and fluid inclusion geothermometry suggest a low-temperature
(90–220 °C), 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, and buoyancy-driven migration of oil into the preexisting propped fracture systems continued long after the cessation of
rifting. These new observations provide an explanation for the viability of sub-unconformity
fractured basement reservoirs worldwide, and have wider implications for subsurface fluid
migration processes generally.