Salt-driven evolution of a gas hydrate reservoir in Green Canyon, Gulf of Mexico

Abstract

The base of the gas hydrate stability zone (GHSZ) is a critical interface, providing a first-order estimate of gas hydrate distribution. Sensitivity to thermobaric conditions makes its prediction challenging particularly in the regions with dynamic pressure-temperature regime. In Green Canyon in the northern Gulf of Mexico (Block GC955), the seismically inferred base of the GHSZ is 450 meters (1476 ft) below the seafloor, which is 400 m (1312 ft) shallower than predicted by gas hydrate stability modeling using standard temperature and pressure gradient assumptions, and an assumption of structure I (99.9% methane gas) gas hydrate. We use 3D seismic, log data and heat flow modeling to explain the role of the salt diapir on the observed thinning of the GHSZ. We also test the alternative hypothesis that the GHSZ base is actually consistent with the theoretical depth. The heat flow model indicates a salt-induced temperature anomaly, reaching 8 °C at the reservoir level, which is sufficient to explain the position of the base of the GHSZ. Our analyses show that overpressure does develop at GC955, but only within a ~500 m (1640 ft) thick sediment section above the salt top, which does not currently affect the pressure field in the GHSZ (~1000 m (328 ft) above salt). Our study confirms that a salt diapir can produce a strong localized perturbation of the temperature and pressure regime and thus on the stability of gas hydrates. Based on our results, we propose a generalized evolution mechanism for similar reservoirs, driven by salt-controlled gas hydrate formation and dissociation elsewhere in the world.