Finding the Source for Hydrocarbon Leakage on the Vestnesa Ridge, NW Svalbard

Abstract

Abstract The Vestnesa Ridge, a gas and gas-hydrate-charged sediment drift on oceanic crust in eastern Fram Strait, is the result of the tectonic rifting processes at the North American-Eurasian plate boundary and the initial water mass exchanges between the Nordic Seas and the Arctic Ocean. A revised chronostratigraphic framework with age control based on correlation to ODP Leg 151 holes, constrain the onset of the drift to at least 11 Ma. The drift deposits consist of fine-grained sediments and large quantities of methane stored as marine gas hydrates. The predominant source for the methane is, however, still debated. Potential gas sources may be a mix of biogenic, abiogenic, and thermogenic gas. For the latter, organic-rich Miocene deposits underlying the drift deposits, and leakage of gaseous hydrocarbons from deep-seated reservoirs are debated. The main objective of this study is to carry out a quantitative study on the controlling mechanisms of hydrocarbon migration from potential kitchen areas in the Fram Strait to the leakage points on the Vestnesa Ridge. The study includes the set-up and application of migration modeling techniques by applying the software package Migri to understand potential sources of hydrocarbons, timing of expulsion, and migration pathways towards the seabed. As a baseline for the study, depth converted seismic lines, fault interpretations, borehole data, and other available data are compiled along a 2D line from the central Fram Strait towards the Vestnesa Ridge at a lateral cell resolution of 100 m. Further input to this basin model comprises a siliciclastic lithology set-up at a high vertical resolution that is based on the log-data from ODP Hole 909C. The included Miocene source rock model accounts for lateral and vertical variations of the organic matter quality derived from ODP Hole 909. The results of this modelling study will be a set of gas migration and leakage scenarios that explain the present day gas leakage on the Vestnesa Ridge for either or likely combinations of the three potential gas sources debated and include the Cenozoic basin's history. Besides a best case basin model scenario, most likely (upper 10%) and least likely (lower 10%) estimates on model solution spectrum are derived. These estimates provide information on the sensitivity of the best case solution as they give insight on the range and span of feasible input parameters permitted to explain the present day gas leakage patterns in the study area.