Gas Hydrate and Sediment Gas Composition, Hole 892A

Abstract

Hole 892A is located above a prominent bottom-simulating reflector (BSR) and a steeply dipping thrust fault. Gas hydrates and high sediment gas concentrations were therefore expected to be encountered in the hole. Gas hydrates were found and sam- pled, but only within the near-surface sediments, down to about 19 meters below sea floor (mbsf). Also as expected, there were relatively high sediment hydrocarbon gas concentrations in some zones, but not at the BSR at 73.9 mbsf. Finally, there were a few other unexpected organic geochemical results: (1) Very high concentrations of hydrogen sulfide (H2S) and methane (CH4) were found in the near-surface sediments. (2) At 68 mbsf there was an abrupt transition from a domination of bacterial gases to gases of thermal origin. (3) The visible gas hydrates contained a high proportion of H2S. Even though anomalously low temperatures were measured in some cores, and discrete layers of depleted chlorinity were found, indirectly suggesting intercalated layers of disseminated gas hydrates, down to a depth of 68 mbsf, no visible gas hydrates were found deeper than 19 mbsf. The 68 mbsf boundary is termed the "geochemical BSR". Theoretical hydrate stabil- ity calculations based on the geochemical and temperature observations in Hole 892A, provide somewhat surprising results: hydrates are found to be very stable down to 15.2 mbsf, where the stability decreases dramatically down to 21.7 mbsf, and then they become unstable at 45 mbsf, and only conditionally stable below that depth. The stability conditions below 45 mbsf are governed mainly by variations in the propane content of the gas mixture and by local temperature deviations from the general geothermal gradient. The results from this study therefore, do not support the widely held notion that the geophysical BSR directly corresponds with the base of gas hydrate stability zone.