Temperature/Pressure Changes Impact Hydrates from Coast to Slope, Northern Gulf of Mexico

Abstract

The northern Gulf of Mexico's passive margin is interrelated and synergistic with both regional and local inputs. Regional inputs may be thought of as deterministic and may include eustatic sea-level oscillations, sediment deposition from erosion of uplifted areas, climate above and below sea level, compression and dewatering of sediments once deposited, and related tectonics. The synergistic sum of these deterministic processes impacts the occurrence of natural gas and their hydrate phases. Along the foot of the continental slope, compression is transmitted to the sediment beneath and down-dip. Hydrates occur when suitable cold temperatures and high pressures are present. In water depths of ∼4 km, the hydrate stability zone (HSZ) may be up to 1 km thick. A prograding slope imparts regional pressures. Local pressures may be sufficient to catalyze the gas-to-hydrate phase change. Further compression may cause the zone to heat and/or to generate fractures, promoting hydrate disassociation and/or escape. The impact of a salt/shale lateral wedge or diapir (or both) entering the zone can be observed in several ways. Increased salinity raises the gaseous phase, thereby reducing hydrate existence. Increased heat also augments gaseousness. Note that shales dissipate heat and that the top of a salt diapir can be cool, even though overall salt is a heat conductor relative to sediments. Migrating salt/shale can promote fracturing, where the size and rate of migration determine the dimensions and frequency of fractures. As gases enterfractions, gas may expand and possibly block further gas entrance. Local sediment shattering and thermal anomalies could obviate the gas expansion effect. As the Louann Salt, which is located over the slope-sediment wedge, advances basinward, the amount of compression rises. Thus, temperatures and pressures change and the pattern of gases/hydrates are dynamic. Specific local predictions are subtle and may be regarded as probabilistic in model development. Equal subtlety is experienced along the ocean/land contact. Hydrate existence is determined by gas chemistry, whether methane or ethane is present. As the sea level oscillates, temperatures and pressures also oscillate. Landward, the HSZ limit during sea-level highs is disassociated, and a new, more basinward HSZ is established. Specific landward HSZ limit is determined by the gas chemistry.