Low temperature and high pressure dramatically thicken the gas hydrate stability zone in rapidly formed sedimentary basins

Abstract

Rapid sedimentation reduces the temperature and raises the pore pressure in sedimentary basins. During rapid sedimentation (>0.5 mm yr−1), cold sediment is buried fast and there is insufficient heat flow to keep the sediment at its steady state conductive equilibrium temperature. In addition, rapid deposition of low permeability mud results in overpressure due to the inability of the pore fluid to drain. It dramatically expands the thickness of the zone where hydrates are stable (the gas hydrate stability zone or GHSZ). We explore this effect with one-dimensional models. We then simulate the two-dimensional evolution of temperature and pressure of the Terrebonne Basin in the Gulf of Mexico. We use seismic, well data, and salt restoration to provide the initial and boundary conditions. We show that rapid burial reduces the geothermal gradient from ∼30 °C/km, which would be expected under equilibrium pressure and temperature, to as low as ∼10 °C/km; we also show that 25 MPa of overpressure is developed. This deepens the thickness of the GHSZ from ∼600 mbsf (its equilibrium depth) at the basin margin, to as much as 2000 mbsf basin-ward, in response to the increasing sedimentation rates. The model successfully simulates the deepening of the base of GHSZ that is interpreted from a bottom simulating reflection in the seismic data. The model and the observations suggest that the thickness of the GHSZ may be much thicker than commonly presumed and as a result the volume of carbon stored may be underestimated. Furthermore, the thickness of the hydrate stability will change significantly with time as sedimentation rate waxes and wanes implying that the hydrate stability zone is a dynamic component of the carbon cycle.