Gas hydrate-related sedimentary pore pressure changes offshore Angola

The thickness of the free gas column below the base of the gas hydrate stability zone is a proxy for the pore pressure in the free gas bearing sediments underneath. In the study area the pore over-pressrues derived under this assumption range between 0 and 1430 kPa ± 30% which appear to be the typical maximum over-pressure that the gas hydrate sediments can withstand before hydro-fracturing occurs. A number of gas chimneys rise from the gas cloud to the seabed. Over-pressures of about 1000 kPa are found under these gas chimneys. We interpret the fact that over-pressures under chimneys are significantly below the maximum over-pressures as an indication of draining of the gas reservoirs by gas blow-out events. The upslope prolongation of gas clouds at the base of the gas hydrate stability zone away from known gas sources suggests that gas hydrate formation changes the permeability of the host sediments and steers fluid migration pathways. Both the calculated over-pressure distribution and the calculated free gas distribution indicate that gas hydrates significantly reduce permeability of marine sediments on a basin-wide scale. INTRODUCTION Continental margins are dynamic environments. Fluids are migrating through the sediments because of sediment compaction, in situ production of biogenic gases, and by thermogenic break-down of organic carbon during burial and by heating of sediments due to volcanic intrusions (Berndt, 2005). When rising gas-rich fluids enter the gas hydrate stability zone they form gas hydrates if temperatures are low and pressures are high (Sloan Jr., 1998). The way in which gas hydrate formation influences the fluid flow systems in continental margins is still poorly understood. In particular, it is debated whether gas hydrates can significantly change the permeability of sediments which would allow pore pressure build-up. It is important to understand the effects of gas hydrates on fluid flow systems, because the presence of gas hydrates has important consequences for their role in slope stability in a time of deep-water exploration, for their mobility in response to climate change and for the risks associated with their exploration as an energy resource after all 500 to 2500 Gt of carbon are stored in marine gas hydrates and the free gas reservoirs associated with them (Archer and Buffett, 2005; Milkov, 2004). This is up to half of all the carbon in the Earth’s carbon cycle. In this paper we evaluate a large, high-resolution 3D seismic data set from the Angolan Margin mapping the thickness of the free gas cloud beneath the bottom simulating reflector (BSR) as a proxy