Ocean Drilling Program, United States

A pressurized core with CH (sub 4) hydrate or dissolved CH (sub 4) should evolve gas volumes in a predictable manner as pressure is released over time at isothermal conditions. Incremental gas volumes were collected as pressure was released over time from 29 pressure core sampler (PCS) cores from Sites 994, 995, 996, and 997 on the Blake Ridge. Most of these cores were kept at or near 0 degrees C with an ice bath, and many of these cores yielded substantial quantities of CH (sub 4) . Volume-pressure plots were constructed for 20 of these cores. Only five plots conform to expected volume and pressure changes for sediment cores with CH (sub 4) hydrate under initial pressure and temperature conditions. However, other evidence suggests that sediment in these five and at least five other PCS cores contained CH (sub 4) hydrate before core recovery and gas release. Detection of CH (sub 4) hydrate in a pressurized sediment core through volume-pressure relationships is complicated by two factors. First, significant quantities of CH (sub 4) -poor borehole water fill the PCS and come into contact with the core. This leads to dilution of CH (sub 4) concentration in interstitial water and, in many cases, decomposition of CH (sub 4) hydrate before a degassing experiment begins. Second, degassing experiments were conducted after the PCS had equilibrated in an ice-water bath (0 degrees C). This temperature is significantly lower than in situ values in the sediment formation before core recovery. Our results and interpretations for PCS cores collected on Leg 164 imply that pressurized containers formerly used by the Deep Sea Drilling Project (DSDP) and currently used by ODP are not appropriately designed for direct detection of gas hydrate in sediment at in situ conditions through volume-pressure relationships.