Gas Venting and Gas Hydrate Stability in the Northwestern Gulf of Mexico Slope: Significance to Sediment Deformation

Abstract

Abstract Gas hydrate is abundant on the Gulf of Mexico continental slope because hydrocarbon gases from the subsurface petroleum system vent prolifically to the sea floor at low temperatures and high pressures within the gas hydrate stability zone (GHSZ). Compositions of gases venting from hydrate-bearing sediments show that gas hydrate decomposition is not significant at this point in geologic time. Structure II gas hydrate is commonly encountered, andcontains C1-C5 hydrocarbons that crystallize from relatively unaltered gases that have migrated from deep in the section. Most structure II gas hydrate sites studied from the ~540-1930 m water depth range are stable, and gas hydrate isaccumulating to considerable depth in sediment because of constant gas venting. Bacterial methane hydrate is also abundant in the Gulf of Mexico. Chemosynthetic communities are preferentially associated with gas hydrate. Massive accumulation of gas hydrate in the Gulf of Mexico deforms shallow sediments. Gas hydrate may be a geologichazard because exploration and exploitation activities on the Gulf slope may cause localized decomposition to form freegas, and modify the physical properties of hydrate-bearing sediments. Introduction Gas hydrate impacts the energy industry because it is abundant at the sea floor in the vicinity of major oil and gas fields as well as along potential pipeline routes in the Gulf of Mexico continental slope. Gas hydrate is thought to represent ageologic hazard in the Gulf slope (Milkov et al., 2000). Basic knowledge of gas hydrate geology and geochemistry is important to continued safe exploration and exploitation on the Gulf slope, as well as in other deep water oil and gas provinces globally. Recent research submersible operations and piston coring have given us the opportunity to obtain intact samples of gas hydrate and associated sediment from the Gulf of Mexico continental slope, and to perform detailed molecular and isotopic analyses of the samples. The primary objective of the present paper is to characterize the occurrence of gas hydrate from shallow to ultra-deep water within the gas hydrate stability zone (GHSZ) of the Gulf slope. The second objective is to address the basic geologic question of whether there is net accumulation or decomposition of gas hydrate in the Gulf slope. Thirdly, we wish to better constrain the role of gas hydrate in deformation of sediments as a potentialgeologic hazard. Geologic setting The main structural features of the northern Gulf of Mexico are salt basins including the large Gulf of Mexico Salt Basin and a series of smaller interior salt basins that extend fromsouth Texas to Alabama. The basins formed during Late Triassic rifting of the Pangean supercontinent, and were floored by salt (Louann/Werner formations) during MiddleJurassic marine incursions (Salvador, 1987). The structural style of these basins is profoundly influenced by the effects of salt movement driven by the rapid deposition of siliciclastic sediment.