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Hydrocarbon prospectivity in the Greater Barents Sea remains enigmatic as gas discoveries have dominated over oil in the past three decades. Numerous hydrocarbon-related fluid flow anomalies in the area indicate leakage and redistribution of petroleum in the subsurface. Many questions remain unanswered regarding the geological driving factors for leakage from the reservoirs and the response of deep petroleum reservoirs to the Cenozoic exhumation and the Pliocene-Pleistocene glaciations. Based on 2D and 3D seismic data interpretation, we constructed a basin-scale regional 3D petroleum systems model for the Hammerfest Basin (1 km × 1 km grid spacing). A higher resolution model (200 m × 200 m grid spacing) for the Snøhvit and Albatross fields was then nested in the regional model to further our understanding of the subsurface development over geological time. We tested the sensitivity of the modeled petroleum leakage by including and varying fault properties as a function of burial and erosion, namely fault capillary entry pressures and permeability during glacial cycles. In this study, we find that the greatest mass lost from the Jurassic reservoirs occurs during ice unloading, which accounts for a 60-80% reduction of initial accumulated mass in the reservoirs. Subsequent leakage events show a stepwise decrease of 7-25% of the remaining mass from the reservoirs. The latest episode of hydrocarbon leakage occurred following the Last Glacial Maximum (LGM) when differential loading of Quaternary strata resulted in reservoir tilt and spill. The first modeled hydrocarbon leakage event coincides with a major fluid venting episode at the time of a major Upper Regional angular Unconformity (URU,~0.8 Ma), evidenced by an abundance of pockmarks at this stratigraphic interval. Our modelling results show that leakage along the faults bounding the reservoir is the dominant mechanism for hydrocarbon leakage and is in agreement with observed shallow gas leakage indicators of gas chimneys, pockmarks and fluid escape pipes. We propose a conceptual model where leaked thermogenic gases from the reservoir were also locked in gas hydrate deposits beneath the base of the glacier during glaciations of the Hammerfest Basin and decomposed rapidly during subsequent deglaciation, forming pockmarks and fluid escape pipes. This is the first study to our knowledge to integrate petroleum systems modelling with seismic mapping of hydrocarbon leakage indicators for a holistic numerical model of the subsurface geology, thus closing the gap between the seismic mapping of fluid flow events and the geological history of the area.

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<p>Oceanic Transform faults are one of manifestations of the three major plate boundaries and a key tectonic feature of oceanic crust. They are broadly considered to accommodate strike-slip displacement along simple vertical faults and to be largely without magmatic addition. We present the first observations from broadband 3D seismic of buried, Cretaceous-aged transform faults in the Gulf of Guinea with complex internal architectures including crustal scale detachments and rotated packages of volcanics within oceanic crust. In the study area, several Oceanic Fracture Zones (OFZ) are described from Top Crust to Moho. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho, necking the adjacent crust in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness within the OFZ. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. These insights demonstrate complexity previously only predicted in numerical simulations. We propose a model with inside corner extension at a high angle to the spreading orientation along a low angle shear zone that acts as a conduit for decompression related melt and volcanism. Late-stage lavas indicate a second stage of magmatic accretion as dykes propagate through the transform to adjacent crust, as inferred from bathymetric studies. These observations come from a unique 3D seismic dataset and are placed within a kinematic model which combines insights from numerical models. We conclude that these oceanic transforms were non-conservative and not simple strike slip fault zones, contradicting the conventional view.</p>

<p>Oceanic Transform faults are one of the three major tectonic plate boundaries and yet their evolution and deformational mechanism is not well understood. They are broadly considered to be dominated by strike-slip displacement along simple planar vertical faults and to be conservative in nature with no magmatic addition. Observations from Pre-Stack depth-migrated (PSDM) 3D seismic of Cretaceous-aged transforms in the eastern Gulf of Guinea allow complex internal architectures to be described, including crustal scale detachments and rotated packages of volcanics.</p><p>These insights demonstrate additional complexity previously only predicted in numerical simulations of spreading ridge-transform interaction, namely intra-transform extension at a high angle to the spreading orientation, and the addition of significant extrusive volcanic material. In the study area of São Tomé and Príncipe, several Oceanic Fracture Zones (OFZ) are identified, consisting of a broad deformational zones that can be described from top to base crust. OFZ scarps are observed to connect at depth with zones of low angle reflectivity which dip into the OFZ and perpendicular to the spreading orientation. At depth they detach onto the Moho below, necking the adjacent crust along the length of the OFZ in the manner of extensional shear zones. Thickly stacked and tilted reflectors, interpreted as extrusive lava flows, are common above the shear zones and infill up to 75% of the crustal thickness. The entire OFZ stratigraphy is overlain and sealed by late-stage lavas that are continuous from the abyssal hills of the trailing spreading ridge. This constrains a process of oblique extension at a high angle to the spreading orientation along a low angle shear zone which also acts as a conduit for decompression related melt.</p><p>We demonstrate that transforms in São Tomé and Príncipe were both non-conservative and not a simple strike slip fault zone, contradicting the current understanding of modern systems. This style of deformation has similarities with anomalously deep and smooth nodal basins which form at slow spreading inside-corner crust. Our model adds strong observational constraints to complement recent numerical models that predict oblique extension within transform zones.</p>

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