No 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.

The Devonian Orcadian Basin in northern Scotland belongs to a regionally linked system of post-Caledonian continental basins extending northwards to western Norway and eastern Greenland. Extensional fault systems that cut the Orcadian Basin sequences are commonly assumed to be Devonian, with some limited inversion and reactivation proposed during the Carboniferous and later times. We present a detailed structural study of the regionally recognized fault systems exposed in the Dounreay area of Caithness, which host significant amounts of authigenic mineralization (carbonate, base metal sulphides, bitumen). Structural and microstructural analyses combined with Re–Os geochronology have been used to date syndeformational fault infills (pyrite) suggesting that faulting, brecciation and fluid flow events are likely to have occurred during the Permian (267.5 ± 3.4 [3.5] Ma). Stress inversion of fault slickenline data associated with mineralization suggest NW–SE regional rifting, an episode also recognized farther west in Sutherland. Thus a dominant set of Permian age brittle faults is now recognized along the entire north coast of Scotland, forming part of the regional-scale North Coast Transfer Zone located on the southern margin of the offshore West Orkney Basin. Supplementary material: Onshore and offshore fault and fracture lineament data are available at https://doi.org/10.6084/m9.figshare.c.2182433 .

The lattice preferred orientation (LPO) of both muscovite and biotite were measured by electron backscatter diffraction (EBSD) and these data, together with the LPOs of the other main constituent minerals, were used to produce models of the seismic velocity anisotropy of the Alpine Fault Zone. Numerical experiments examine the effects of varying modal percentages of mica within the fault rocks. These models suggest that when the mica modal proportions approach 20% in quartzofeldspathic mylonites the intrinsic seismic anisotropy of the studied fault zone is dominated by mica, with the direction of the fastest P and S wave velocities strongly dependent on the mica LPOs. The LPOs show that micas produce three distinct patterns within mylonitic fault zones: C-fabric, S-fabric and a composite S–C fabric. The asymmetry of the LPOs can be used as kinematic indicators for the deformation within mylonites. Kinematic data from the micas matches the kinematic interpretation of quartz LPOs and field data. The modelling of velocities and velocity anisotropies from sample LPOs is consistent with geophysical data from the crust under the Southern Alps. The Alpine Fault mylonites and parallel Alpine schists have intrinsic P-wave velocity anisotropies of 12% and S-wave anisotropies of 10%.

The Labrador Sea is a small (~900 km wide) ocean basin separating southwest Greenland from Labrador, Canada. It opened following a series of rifting events that began as early as the Late Triassic or Jurassic, culminating in a brief period of seafloor spreading commencing by polarity chron 27 (C27; Danian) and ending by C13 (Eocene-Oligocene boundary). Rift-related magmatism has been documented on both conjugate margins of the Labrador Sea. In southwest Greenland this magmatism formed a major coast-parallel dike swarm as well as other smaller dikes and intrusions. Evidence for rift-related magmatism on the conjugate Labrador margin is limited to igneous lithologies found in deep offshore exploration wells, mostly belonging to the Alexis Formation, along with a postulated Early Cretaceous nephelinite dike swarm (ca. 142 Ma) that crops out onshore, near Makkovik, Labrador. Our field observations of this Early Cretaceous nephelinite suite lead us to conclude that the early rift-related magmatism exposed around Makkovik is volumetrically and spatially limited compared to the contemporaneous magmatism on the conjugate southwest Greenland margin. This asymmetry in the spatial extent of the exposed onshore magmatism is consistent with other observations of asymmetry between the conjugate margins of the Labrador Sea, including the total sediment thickness in offshore basins, the crustal structure, and the bathymetric profile of the shelf width. We propose that the magmatic and structural asymmetry observed between these two conjugate margins is consistent with an early rifting phase dominated by simple shear rather than pure shear deformation. In such a setting Labrador would be the lower plate margin to the southwest Greenland upper plate.