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

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 meter-thickness 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.

Modeling petroleum expulsion in sedimentary basins: The importance of igneous intrusion timing and basement composition

“…Our model cannot account for the very recent charge (since 20 Ma) seen in many fields within the FSB, based on the modeled biodegradation rate of oils and reservoir temperature histories. On this basis, re-migration of oil from basin sands or long-lived open fracture systems in the basement (Holdsworth et al, 2019) could provide the source of oil required for recent replenishment of oil fields and discoveries in the basin margins. Igneous intrusions may act as migration barriers within the heavily intruded sections of the basin, providing a complex migra-tion route to accumulations on the basin margins (Rateau et al, 2013).…”

“…The three main factors that should be considered when undertaking basin modeling in such circumstances are 1. The addition of intrusive igneous material at the correct time in order to correctly model the subsidence history of the petroleum Re-Os (Finlay et al, 2011) U-Pb (Holdsworth et al, 2019)…”

“…Organic geochemical interpretations suggest that the primary source rock of most oils within the FSB is the locally oil-prone, lacustrinefluvio-deltaic-marginal marine shales of the Upper Jurassic to Lower Cretaceous KCF (Scotchman et al, 2018). Isotopic dating of oils and fluid inclusions indicates a range of implied petroleum generation ages, from 68 ± 13 Ma based on Re-Os isotopes (Finlay et al, 2011), to 69-93 Ma from U-Pb dating of calcite mineralization (Holdsworth et al, 2019), to ca. 83 Ma from Ar-Ar dating of feldspars surrounding oilfilled inclusions (Mark et al, 2005).…”

Fractured basement play development on the UK and Norwegian rifted margins

“…The Rona play is located on the Atlantic margin, West of Shetland, and comprises a narrow (,15 km wide) upfaulted elongated ridge c. 200 km long of naturally fractured Neoarchean granitic-tonalitic orthogneisses, which are likely to be related to the Lewisian Gneiss Complex in NW Scotland (Ritchie et al 2011;Holdsworth et al 2018Holdsworth et al , 2019. The basement ridge is overlain by a diverse Paleozoic-Cenozoic sedimentary succession (Brooks et al 2001, Ritchie et al 2011, as summarized in Figure 4a.…”

“…The Rona Ridge basement play is demonstrably affected by Mesozoic extensional faults and fissures, combined with near-surface hydrothermal and weathering processes (Holdsworth et al 2019). At Lancaster and Lincoln, for example, mineralized sediment-and breccia-filled fissures (e.g.…”