Short Migration of Methane into a Gas Hydrate-Bearing Sand Layer at Walker Ridge, Gulf of Mexico

Cook, Ann; Malinverno, Alberto

The occurrence and distribution of natural gas hydrates within continental margin sediments is not well understood. In this research, we focus on an interesting distribution of gas hydrate observed in Hole Walker Ridge 313-H drilled during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg 2, at a site approximately 280 kilometers south of Louisiana. The base of gas hydrate stability in this area is estimated at 900 mbsf (meters below seafloor).

In Hole Walker Ridge 313-H, a 2.5 m-thick sand layer containing gas hydrate (average 40% hydrate saturation) occurs at about 293 mbsf, within a thick fine-grained mud interval (from 165-315 mbsf) containing gas hydrate in fractures. The gas hydrate-bearing sand is surrounded by two distinct hydrate-free zones (10 m-thick above and 3 m-thick below) and is composed of marine mud. There are two possible migration mechanisms that supplied methane to the sand layer: 1) gas could have been transported by fluid flow over several kilometers from below the gas hydrate stability zone along the sand layer, or 2) microbial methane may have been generated in situ within the hydrate-free zones and traveled only a few meters from the adjacent mud into the sand. We explore if the latter option, which we call short migration, is feasible.

To be a realistic mechanism, short migration must be able to produce a large enough gas hydrate accumulation and cannot require too much time. To estimate how much gas was likely generated in the hydrate-free zones, we quantify the gas hydrate content of the fractured mud interval by modeling the response of the propagation resistivity logs. We find that the amount of methane produced in the hydrate-free zones is enough to explain the gas hydrate content of the sand layer. Additionally, we use biostratigraphic information from a neighboring hole in Walker Ridge to estimate the age of the fractured mud interval. We find there is enough time during sedimentation for dissolved methane to diffuse from the hydrate-free zones into the sand. We conclude that methane transport over significant distances via fluid flow is not required and that microbial methane could migrate a few meters to form gas hydrate in the sand layer at Walker Ridge.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013