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

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.

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


Authors

E. Dempsey

N.M.W. Roberts

K. Hardman

A. Morton

M. Feely

J. Hunt

A. Conway

A. Robertson



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.

Citation

Holdsworth, R., McCaffrey, K., Dempsey, E., Roberts, N., Hardman, K., Morton, A., …Robertson, A. (2019). Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs. Geology, 47(8), 700-704. https://doi.org/10.1130/g46280.1

Journal Article Type Article
Acceptance Date May 3, 2019
Online Publication Date May 22, 2019
Publication Date Aug 1, 2019
Deposit Date May 3, 2019
Publicly Available Date Mar 29, 2024
Journal Geology
Print ISSN 0091-7613
Publisher Geological Society of America
Peer Reviewed Peer Reviewed
Volume 47
Issue 8
Pages 700-704
DOI https://doi.org/10.1130/g46280.1

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