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

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

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.

Item Type:Article
Full text:Publisher-imposed embargo
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Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
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Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
Download PDF
(1620Kb)
Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1130/G46280.1
Publisher statement:© 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.
Date accepted:03 May 2019
Date deposited:03 May 2019
Date of first online publication:22 May 2019
Date first made open access:29 May 2019

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