Inferences from Gas Hydrate Resource Characterization Based from Pore Fluid Geochemistry from the Mount Elbert Gas Hydrate Stratigraphic Test Well
Recent drilling of gas hydrate-bearing marine sediments show that a comprehensive multi-proxy approach, including analyses of gas and pore fluids in recovered cores is critical to establish gas hydrate saturations and to determine the processes that lead to the observed distributions. In February 2007, the Mount Elbert-01 gas hydrate well was drilled and cored from depths of 1990 to 2,494 ft in the Milne Point unit on the North Slope of Alaska. Analyses of pore fluids from this well clearly document the importance of acquiring a complete pore-water data set combined with wireline logging data to generate robust estimates of gas hydrate content. Both the dissolved chloride and the isotopic composition of the water co-vary in the gas hydrate-bearing zones, consistent with gas hydrate dissociation during core recovery, and they provide independent indicators to constrain the zone of gas hydrate occurrence. Gas hydrate saturation values estimated from dissolved chloride agree with estimates based on logging data when the gas hydrate occupies more than 20% of the pore space; however, the correlation is less robust at lower saturation values. Reasons for this discrepancy are still unclear, but may reflect the effect of the host sediment on parameterization of the gas saturation estimates from logs and water chemistries. The highest gas hydrate concentrations at Mount Elbert are clearly associated with coarse-grained sedimentary sections, as expected from theoretical calculations and field observations in marine and other arctic sediment cores. Nonetheless, pore-water data indicate that gas hydrate may also occur in finer-grained lithologies.
Sediment analyses of recovered core samples reveal that the organic carbon content of the gas hydrate hosting sediment at the Mount Elbert location is too low to support significant methane generation, and gas composition analyses indicate a thermogenic source. The occurrence of gas-hydrate therefore implies transport along high permeability horizons from deep-seated sources.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009