Occurrence of gas hydrate in Oligocene Frio sand: Alaminos Canyon Block 818: Northern Gulf of Mexico

Occurrence of Gas Hydrate in Oligocene Frio Sand: Alaminos Canyon Block 818: Northern Gulf of Mexico Ray Boswell a,*, Dianna Shelander b, Myung Lee c, Tom Latham d, Tim Collett c, Gilles Guerin e, George Moridis f, Matthew Reagan f , Dave Goldberg e a U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, WV 26507, USA b Schlumberger Reservoir Seismic Services, Houston, TX, USA c U.S. Geological Survey, Denver, CO, USA d Chevron North America Exploration and Production Company, Houston, TX, USA e Lamont-Doherty Earth Observatory, Columbia University, New York, NY, USA f Lawrence Berkeley National Laboratory, Berkeley, CA, USA Keywords: gas hydrate, Gulf of Mexico, Alaminos Canyon, Frio sand Abstract A unique set of high-quality downhole shallow subsurface well log data combined with industry standard 3D seismic data from the Alaminos Canyon area has enabled the first detailed description of a concentrated gas hydrate accumulation within sand in the Gulf of Mexico. The gas hydrate occurs within very fine grained, immature volcaniclastic sands of the Oligocene Frio sand. Analysis of well data acquired from the Alaminos Canyon Block 818 #1 (‘‘Tigershark’’) well shows a total gas hydrate occurrence 13 m thick, with inferred gas hydrate saturation as high as 80% of sediment pore space. Average porosity in the reservoir is estimated from log data at approximately 42%. Permeability in the absence of gas hydrates, as revealed from the analysis of core samples retrieved from the well, ranges from 600 to 1500 millidarcies. The 3-D seismic data reveals a strong reflector consistent with significant increase in acoustic velocities that correlates with the top of the gas-hydrate-bearing sand. This reflector extends across an area of approximately 0.8 km and delineates the minimal probable extent of the gas hydrate accumulation. The base of the inferred gas-hydrate zone also correlates well with a very strong seismic reflector that indicates transition into units of significantly reduced acoustic velocity. Seismic inversion analyses indicate uniformly high gas-hydrate saturations throughout the region where the Frio sand exists within the gas hydrate stability zone. Numerical modeling of the potential production of natural gas from the interpreted accumulation indicates serious challenges for depressurization-based production in settings with strong potential pressure support from extensive underlying aquifers. 1. Introduction The term ‘‘gas hydrate’’ is commonly used to describe clathrate compounds of water and various appropriately-sized gas molecules, most commonly methane. Gas hydrates form naturally in sediments given suitable temperature and pressure and sufficient supplies of both water and gases. The macroscopic form in which gas hydrate occurs varies considerably, including large massive mounds at the seafloor, as nodules, veins and fracture fillings; and as disseminated pore-filling grains in more deeply buried sediments. Gas hydrate occurrences are generally restricted to the upper several hundred meters of marine sediments under deep continental shelves and slopes and to areas in the Arctic, where the temperature regime is conditioned by the presence of permafrost. The recognition of widespread and large-scale occurrences of gas hydrate in marine environments has raised many fundamental scientific and public interest concerns. In particular, marine gas hydrate-bearing sediments have potential as a future energy source, are a suspected hazard to oil and gas drilling operations, and have potentially significant implications for global environmental processes such as climate change and the carbon cycling (Kvenvolden, 2000). In comparison to accumulations encased in unconsolidated and impermeable shallow marine muds, gas hydrate reservoirs in porous and permeable sands are the most promising targets for potential natural gas production (Moridis and Reagan, 2007a,b; Moridis and Sloan, 2007) they provide: 1) the means to transport pressure or temperature perturbations into a gas-hydrate-bearing formation

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