Gas Hydrate Recoveries in the Gulf of Mexico: What Is the Shallow Water Depth Limit for Hydrate Occurrence?

ABSTRACT Gas hydrates are known to occur in the upper 6 meters of the seafloor in the Gulf of Mexico. This statement is supported by a greater number of hydrate sample recoveries here than in any other seafloor region. Both the direct evidence of sample recovery and other indirect evidence, such as extensive core sample expansion upon recovery, are indicators of gas hydrates in many piston core samples from the region. Some of the evidence occurs for samples recovered from water depths which are less than the frequently stated shallow water depth limit of 400 or 500 m. Some of the samples recovered have been analyzed for gas type and concentration of the gas hydrate. The resulting gas concentrations have been used to calculate stability conditions for possible existence of hydrates of the measured gas concentrations and the results indicate that, for typical Gulf of Mexico water column temperature profiles, thermogenic gas hydrates could exist in water depths as shallow as 220 m. INTRODUCTION Free gas occurrence is widespread in the Gulf of Mexico and other ocean regions (Anderson and Bryant, 1990; Hovland and Judd, 1988). As reviewed by Sloan (1990), natural gas hydrates are believed to occur Widely within the seafloor of the continental margins of the world's oceans. The evidence for such widespread occurrence is primarily indirect inclUding: 1) patterns in seismic profiles (primarily bottom simulating reflectors or BSR's but also "wipeout zones"); 2) downhole log data indicating zones of increased sonic velocity and/or decreased material bulk density; 3) anomalous patterns of seafloor velocities in remote sensing (acoustic/seismic) seafloor sections; 4) seafloor slumps and slides; 5) anomalously large quantities of gas produced by samples of seafloor sediment upon retrieval to the sea surface; and 6) the occurrence of seafloor gas within a seafloor zone of combined temperature and pressure conditions which would allow the stable existence of solid gas hydrates. Because of the wide-area coverage of seismic profiles, the occurrence of a BSR in such profiles is the most commonly applied of these types of indirect evidence for seafloor hydrates. An early example of such application is that of Shipley et al. (1979). The direct evidence of seafloor hydrates is discussed below in the section on hydrate recoveries. Although actual observable pieces of gas hydrate have only been present in cores taken in water depths of 439 m or greater, indirect indications of in situ hydrates are present in some seafloor cores recovered from shallower water depths. Some of the recovered hydrate samples have been analyzed for gas content (Brooks, et al., 1986). The temperature and pressure conditions for the solid gas hydrate/free gas boundary of a phase diagram has been calculated for the measured hydrate gas mixtures in sea water; phase stability calculation methods are described in Sloan, 1990. The resulting hydrate stability conditions are described below and compared with the water column temperature profile of the northern Gulf of Mexico.