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Abstract

The Barents Sea is a frontier for hydrocarbon exploration where activity has been renewed after recent oil discoveries. However, previously this province has been dominated by gas finds, with the largest discoveries being Snøhvit, Albatross and Askeladd gas fields, located in the Hammerfest Basin. Cenozoic erosion and high latitude Quaternary glaciations are thought to have driven the hydrocarbons out of the traps and contribute thus to the lack of significant oil discoveries. Hydrocarbon leakage is a widespread phenomenon and has significant impact on climate, marine ecosystem, geotechnical installations and petroleum exploration. In this study, we aim to elucidate the impact of Cenozoic erosion and Pliocene-Pleistocene glaciations on the dynamics of hydrocarbon leakage from the thermogenic reservoirs. We use high resolution and vintage 3D seismic reflection datasets to analyse hydrocarbon plumbing system above the Snøhvit and Albatross gas fields to investigate the geo-morphological manifestation and the dynamics of leakage from the reservoir. We then use 3D Petroleum Systems Modelling (PSM) to simulate the basin history in terms of generation, migration and leakage of hydrocarbons through time in response to erosion, glacial loading and deglaciations. Based on this integrated approach, we then are able to compare numerical modelling results with seismically observed leakage indicators. Numerous EW trending reactivated faults are present in the study area which link the Jurassic hydrocarbon reservoirs of the Snøhvit and Albatross field with the shallow Paleocene strata. Reactivation of polygonal fault networks has formed an interconnected network of Paleocene faults, which served as migration avenues for thermogenic fluids in the vicinity of deep reactivated tectonic faults. Numerous pockmarks and mega pockmarks on the seabed and buried pockmarks on the base Quaternary Upper Regional Unconformity (URU) provide evidence of migration pathways as they are connected to seismic blow out pipes, Paleocene fault networks and deep reactivated tectonic faults. A gas cloud anomaly has been interpreted as a Bottom Simulating Reflector (BSR), whose depth coincides with the estimated base of the hydrate stability field for a thermogenically-derived gas hydrate with around 90 mol % methane. At least two fluid venting episodes have been inferred based on seabed and URU pockmark distributions, following the Last Glacial Maximum ~17-16 ka and prior to the Late Weichselian, older than ~0.7 Ma. Results of the 3D PSM modelling show that hydrocarbon leakage from the Jurassic reservoirs takes place through faults during each deglaciation, with most of accumulated mass lost (60-80 %) during the first instance of fault dilation. Subsequent leakage during deglaciations results in a sequential loss of remaining accumulated mass in the Snøhvit reservoir. The first modeled leakage event (0.8-0.78 Ma) coincides with a major fluid escape event at the time of a major regional unconformity (URU older than ~0.7Ma), and is in agreement with shallow subsurface hydrocarbon leakage indicators such as pockmarks, shallow gas clouds and blow out pipes observed in the seismic data analysis.