Geophysical Study on the Landward Limit of Marine Gas Hydrate Stability Zone

Abstract

Study on gas hydrate has gained attention all over the world. The gas hydrate stability zones (GHSZs) in continental margins are wedge zones, controlled mainly by pressure and temperature conditions. The landward limit of gas hydrate stability zone (LLGHSZ) is indicated by the intersection location of BSR with the seabed in seismic data, representing the shallowest water depth of GHSZ. In this research, the location of LLGHSZ offshore Mauritania is recognized, which is parallel to the seabed contours. A coral reef belt is observed at ~ 4 km landwards from the LLGHSZ, also parallel to the seabed contours. We hypothesize that the location of the coral reef belt presents the previous LLGHSZ, which has been proved by modeling the bottom simulating reflection (BSR).

According to the phase diagram of gas hydrate stability, the seabed-BSR intersection (LLGHSZ) depth, is determined by the intersection of hydrate stability curve and the seawater temperature profile, which is irrelevant to geothermal gradient. However it is difficult to get the precise seawater temperature profile for a depth bigger than 500 m. But the seabed temperature can be obtained from the World Ocean Database. In this research, we assume the hydrate is 100% methane hydrate, and use Makogon’s hydrate stability equation to model the location of BSR. The seabed pressure is calculated by P_sb= ρ_sw×g×H_sb. Results show that the modeled LLGHSZ fits well with the observed one, and the location of LLGHSZ is irrelevant with the geothermal gradient.

Palaeo-geomorphology reconstruction is simply conducted, with the bottom of the coral reef representing the palaeo-seabed. Sedimentation rate is investigated, as well as the sea level change and related palaeo-seabed temperature. Based on the paleo-geomorphology reconstruction result, the model is run again to calculate the previous location of the LLGHSZ. Result shows the previous LLGHSZ fits well with the coral reef belt. The LLGHSZ is sensitive to seabed temperature change or other events which may change the stability conditions of GHSZ like sedimentation or erosion. We propose that when the LLGHSZ resets seawards, the gas hydrate between the previous and present LLGHSZ dissociates, releasing free gas at the seabed which probably feed the deep water coral reef. Furthermore, it is possible that the coral reef only consumed a small part of the released gas, most may have entered the seawater or even the atmosphere.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018