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

R.E Holdsworth,K.J.W Mccaffrey,A Conway,K Hardman,A Morton,N.M.W Roberts,M Feely,E Dempsey,J Hunt,A Robertson

doi:10.1130/g46280.1

Abstract

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.

Highlights

Fractured basement hydrocarbon reservoirs are recognized worldwide, but they are relatively poorly understood and underexploited (Trice, 2014)
We show that seismicity related to rifting may have initiated migration of oil from nearby organicrich source rocks into the ridge
We suggest that the emergent Rona Ridge was affected by the contemporaneous and long-term development of near-surface (>0.5 km depth) open fissure systems that hosted hydrothermal mineralization from below and sediment ingress from above (Fig. 4A)

Summary

Introduction

Fractured basement hydrocarbon reservoirs are recognized worldwide, but they are relatively poorly understood and underexploited (Trice, 2014). U-Pb calcite geochronology, and fluid inclusion geothermometry to explore the nature, age, trapping temperatures, and significance of widespread fracture fills (minerals, clastic sediment) associated with oil observed in basement cores along the ~200-km-long Rona Ridge, west of Shetland, UK. Regional Setting Significant oil discoveries have been made in basement rocks of the Rona Ridge, west of Shetland (Fig. 1A).

Results

Conclusion