Recent observations of extensive methane release into the oceans and atmosphere have raised concern as to whether rising temperatures across the Arctic could drive rapid destabilization of gas hydrate reservoirs. Here, we report modelling results from hydrate-modulated methane seepage from Vestnesa Ridge, offshore western Svalbard, suggesting that continuous leakage has occurred from the seafloor since the early Pleistocene up until today. Sustained by modelled deep subsurface thermogenic sources of Miocene age, large scale hydrocarbon fluid migration started ~6 million years ago and reached the seafloor some 4 million years later. The modelling results indicate that widespread methane seepage offshore western Svalbard commenced in earnest during early Pleistocene, significantly older than late Holocene as previously reported. We propose that the onset of vertical hydrocarbon migration is the response of rapid burial of potential hydrocarbon sources induced by increased sediment deposition following the onset of Northern Hemisphere glaciations, ~2.7 million years ago. From the modelling results we propose that source rock intervals capable of generating hydrocarbons and hydrocarbon reservoirs buried kilometers deep have continuously fueled the gas hydrate system off western Svalbard for the past 2 million years. It is this hydrocarbon system that primarily controls the thermogenic methane fluxes and seepage variability at the seabed over geological times.

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Equation of state modelling of multicor~ponent hydrocarbon mixtures has been incorporated into a program which models the secondary migration of oil and gas. Migration is modelled by means of a ray-tracing technique which operates on backstripped depth maps of a carrier bed. Reservoir beds are represented as rectangular grids (arrays) in the computer, ensuring the high resolution required for proper secondary migration modelling. The resulting software, named SEMI, can be calibrated against the hydrocarbon composition in drilled traps and then used to compute the composition in undrilled prospects. This is especially useful in moderately I explored basins where some discoveries have already been made. A model for the loss of Hydrocarbons during secondary migration is ulilized and, by modelling secondary migration within a synthetic basin, it is shown that compositional differences in traps may result from differences in loss of the vapour and liquid phases. Using a simple model for primary migration, it is shown that the primary and secondary migration losses influence the hydrocarbon composition in traps differently. Thus it is hoped that the proposed methodology can be used to quantify some of the unknown effects of migration and that a decrease in the quantified uncertainty in exploration for oil and gas may result.

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Publication informationMarine and Petroleum Geology, 26 (6): 764-774 Publisher ElsevierLink to online version http://dx.doi.org/10.1016/j.marpetgeo.2008.05.004Item record/more information http://hdl.handle.net/10197/3028 Publisher's statement þÿ T h i s i s t h e a u t h o r s v e r s i o n o f a w o r k t h a t w a s a c c e p t e d f o r p u b l i c a t i o n i n M a r i n e a n dPetroleum Geology. Changes resulting from the publishing process, such as peer review,