| Summary: | Seabed topographic expressions such as pockmarks and domes caused by the vertical migration and accumulation of methane are found in muddy, cohesive sea beds around the world. These surface features range from 10-1000 m in diameter and 1-20 m in relief (Judd and Hovland, 2007). The mechanics of soft, cohesive sediment deformation due to overpressure from rising gas is not well understood or quantitatively defined despite the hazards they present to offshore drilling. Shallow gas pockets and overpressured sediments formed by the migration of subsurface gas to the seafloor can cause drilling blowouts, resulting in the release of large amounts of methane into the atmosphere that may contribute to climate warming. Barry et al., (2012) took initial steps to quantitatively define the sediment deformation caused by the upward pressure of rising gas against an impermeable sediment layer. They observed that the deformation to clay-bearing, cohesive sediments could be characterized by elastic thin plate mechanics up to sediment failure. Thin plate theory, also, could be used to predict the pressures responsible for known sea domes around the world. In this study, shallow gas and associated subsequent deformation to the sediments around them were identified in two study areas: the Ninilchik field along the eastern shore of the Cook Inlet in Alaska and the Thunder Horse Field in the Mississippi Canyon Protraction area of the Gulf of Mexico. Each observable upward doming feature was measured for radius, deflection, and thickness. This information is used in an elastic thin plate equation for calculating pressure (Barry et al., 2012). In the Ninilchik field, all measurements were done by interpretation of 3D seismic data in Petrel® 2011. In the Thunder Horse Oil Field, measurements of the sediment response at the seafloor were collected using a 3D three meter-binned bathymetry survey in Schlumbergers Petrel® 2011. Nine Gulf of Mexico and three Alaskan gas accumulations were identified and analyzed in this study. In addition, numerous other gas-related features were observed in the Gulf of Mexico study area including pockmarks, interpreted gas hydrates/outcrops, and subsurface gas that did not affect seafloor surface. By applying Barry et al. (2012) laboratory-derived method for predicting pressures, a range of predictive pressures for twelve gas domes was calculated using two equations based on the amount of observed deflection. Calculated pressures for individual domes ranged from 499-17,696 Pa in the Gulf of Mexico and 261-15,640 Pa in the Cook Inlet. The majority of observed domes displayed a deflection to thickness ratio (w/h) greater than 0.3 suggesting plate stretching during deformation and flexible plate behavior. Plate stretching equation best predicts the pressures for these 5 flexible domes, compensating for the added stresses created by their large deflections while still predicting comparable pressures (+/- 6%) for stiff domes (w/h<0.3) compared to non-plate stretching equation. Domes with the smallest Youngs Modulus (low-end calculations) provided pressures most similar to Barry et al. (2012) and a range of lower pressure values more likely recreated in the subsurface than high-end values. Domes in the Gulf of Mexico were better suited for the application of elastic plate theory due to their more circular plate shape and clay-rich lithology, thus producing more reliable pressure results than the Cook Inlet domes.
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