Thomas D. Lorenson¹, Keith A. Kvenvolden²

(1) U. S. Geological Survey, Menlo Park, CA
(2) U.S. Geological Survey, Menlo Park

ABSTRACT: A Worldwide Assessment of Coincidental Gas Hydrate and Petroleum Gas Occurrences

There are 79 localities worldwide where there is evidence for gas hydrate; at 21 of these locations, gas-hydrate samples have been recovered. A comparison of the global occurrence of gas hydrate and the global occurrence of oil and gas deposits shows that 24 gas-hydrate localities lie over or nearby (within 100 km) producing oil and gas fields. Most methane in sampled gas hydrate is microbial in origin; however, dilution by thermally produced methane has been observed, particularly in gas hydrate from permafrost regions. Gas hydrate composed of mainly thermal methane has been sampled in the Gulf of Mexico and the Caspian Sea.

The production of microbial methane for gas hydrate, occurring in shallow sediment, should be decoupled from the thermal generation of methane in most oil and gas deposits, occurring in sediment more than one kilometer deep. Nevertheless, migration of gas from the deep sediments into the shallow sediments is always possible, given suitable conduits. Methane from sampled gas hydrate at five localities show the presence of various amounts of thermally-sourced methane. Thus, at the 24 gas-hydrate localities at or near to producing oil and gas fields, any recovered gas hydrate may contain microbial methane diluted with thermal methane if the oil and gas deposits have leaked gas toward the surface. From an exploration prospective, then, the finding of gas hydrate with a thermal component at a location not related to current oil and gas deposits, should encourage a further search for the source of the thermal gas, leading perhaps to a new discovery of an oil and gas deposit.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado