Relationship Between Sources of Hydrate-Forming Gases and Gas Hydrate Concentrations in Sediments

Alexei V. Milkov
BP America, Houston, TX

A comprehensive study of the origins and sources of hydrate-bound gases collected from 26 offshore and onshore areas worldwide suggests a strong relationship between sources of hydrate-forming gases and gas hydrate concentrations. Hydrate-bound gases come from autochthonous (located predominantly within the gas hydrate stability zone (GHSZ)) and allochthonous (located in deep sediments) sources. Autochthonous and diffuse allochthonous gases (mostly microbial) occur in stratigraphically and structurally simple geological settings characterized by low gas flux at the seafloor. Relatively low gas hydrate concentrations (average ~2% of pores) occur in such settings because little microbial gas is produced in cold sediments within the GHSZ. In contrast, relatively high gas hydrate concentrations (average 5-15% of pores and locally up to 100% of volume) occur where significant amounts of allochthonous gases (microbial and/or thermogenic) are focused into the shallow GHSZ along faults, within mud volcanoes, permeable carrier beds and other geological features from underlying petroleum systems. This finding implies that, similarly to conventional petroleum systems, the initial generative potential (microbial or thermogenic) of underlying sediments, their burial and thermal history, fetch area, and the presence and efficiency of focusing mechanisms are the most important factors that ultimately control gas hydrate concentration within the GHSZ. Theoretical estimates that largely depend on the amount of organic carbon in the GHSZ and disregard the depth and rate of microbial processes and the flux of gases from greater depths tend to grossly exaggerate the global gas hydrate inventory.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005