Images of the Base of Gas Hydrate Stability, Northwest Walker Ridge, Gulf of Mexico

Abstract

Abstract The gas hydrate stability zone, the zone below the seafloor where gas and water should form gas hydrate is predicted to be present in the tophole sections in deepwater areasaround the world including the deepwater Gulf of Mexico. Few, if any, bottom simulating reflectors (BSR) or other manifestations of the gas hydrate stability zone have been interpreted in the Gulf of Mexico. This paper presents convincing images of the base of the gas hydrate stability zone extracted from exploration 3-D data in Northwest Walker Ridge in the Gulf of Mexico from the upper 1,050 m of sediment. This area is on the margin of an uplifted and compressed mini-basin and in the vicinity of numerous giant gas mounds at the seafloor. Interpreted tophole stratigraphy includes laterally extensive, steeply dipping basin floor silts and sands. This paper shows images of hydrate-trapped gas at multiple reflectors at depths that are coincident with the predicted base of the hydrate stability zone if modeled with a geothermal gradient of 19.6 ± 0.5 °C/km. A BSR is seen in the seismic data in vertical section, but the more convincing images of the base of gas hydrate stability are seen in map view. We expect that the subsurface conditions here are not unique and the hydrate stability zone is similarly imaged, but perhaps has not been recognized, in similar settings in other parts of the Gulf of Mexico and elsewhere in the world. Recognition of hydrate-trapped gas can aid in better decision making for shallow-gas avoidance strategy for exploration drilling and optimal design and placement of conductor strings for deepwater development wells. The possible concentrations of gas hydrate reservoirs in the updip margins of hydrate traps should also be of interest to those that are evaluating the feasibility of commercial exploitation of gas hydrates. Local Geologic Setting Figure 1 shows the study area in the northwestern part of Walker Ridge in the Gulf of Mexico. The focus of the study is in an uplifted and compressed minibasin with giant gas mounds to the northeast and a large salt wall with shallow buried salt or salt cropping out at the seafloor to the south. Thrust faults within the mini-basin have seafloor expression (Figures1 and 2). The source of compressive stress is apparently from shallow salt movement north-to-south on the northern margin of the minibasin. A larger, deeper salt withdrawal basin (not shown) is oriented northwest-to-southeast, northeast of the giant gas mounds. Both of the salt withdrawal basins filled with Pliocene and Pleistocene sediments transported from the shelf-edge towards the central Gulf of Mexico Basin. The minibasin is filled with typical deepwater sedimentation cycles composed of debris flows, basin floor splays, channels, thin turbidites, and hemipelagic clays.