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Source to Seep - A Novel Calibration Domain Concept for Petroleum Systems Models


Seafloor methane seepage is a global phenomenon. Seepage sites are often characterised by morphologies such as mounds and pockmarks and are frequently associated with gas hydrate occurrences. Their origin is intensely discussed both in the light of the gas hydrate stability which is threatened by the amplification of global warming and as a potential seabed indicator for prolific petroleum systems. A leakage and migration model has recently been introduced for the Vestnesa Ridge gas hydrate and associated seepage system offshore West-Svalbard. Vestnesa Ridge is a > 5 km thick sedimentary drift with active seepage mostly sustained by a Miocene thermogenic methane source, possibly feeding the system for more than 2 million years. We present a novel stochastic basin model setup that elaborates this modelling effort further. It links the previously used 2D-seismic line with new P-cable 3D seismic data. Using P-cable 3D seismics significantly refines the tectono-stratigraphic resolution and, by representing both palaeo- and concurrent pockmarks, the seafloor leakage history in the model. Combined with a mass balance for the gas hydrate dynamics this approach enables an extensive assessment of the hydrocarbon migration-leakage pathways from the source rocks via intermediate storage reservoirs to the seabed seep sites. Additional input parameter from gas isotopic and pore water geochemical data at seep sites are used for model verification. These empirical data provide robust calibration anchors on the timing and rates of gas escape from the seabed, the methane generation regime and the paleo-hydrate and seepage settings. By reconstructing a petroleum system that jointly honours leakage history input from high resolution seismic data, gas hydrate dynamics at geological time scales and rigorously exploited ground truth data from seepage sites, we enforce a new, temporal calibration domain on the model. Ordinary petroleum system models are referenced against static observations at present day and often lack history verification. Meaning that a good model fit in e.g. hydrocarbon volumes today cannot decipher whether they accumulated 2, 10 or 20 Ma ago. Through the first results from the Vestnesa Ridge basin model, we present a concept that reduces the width of the history validation gap in numerical petroleum systems analysis. We acknowledge the financial support from the Research Council of Norway (grant no. 255150).