GINSBURG, GABRIEL, and VALERY SOLOVIEV, Institute for Geology and Mineral Resources of the Ocean, St. Petersburg, Russia

ABSTRACT: Shallow Gas Hydrate Within the Areas of Subaquatic Seepage

Sedimentary framework of worldwide hydrate-bearing areas and structures of sediments containing hydrates suggest that fluid filtration is the major process responsible for its generation. A study of seepage-associated hydrate accumulations that are accessible without drilling is useful for gaining an understanding of subaquatic gas hydrate formation in general. Localities of shallow hydrate are indicative of oil and gas content. They also may be hazardous for oil and gas field development.

The paper presents the results of exploration of gas hydrate accumulations that associate with diapirs, mud volcanoes, faults, subaquatic canyons, and pockmarks in the Caspian, Black, Okhotsk, and Barents seas. Data acquisition included echo sounding, seismic survey, ground sampling geothermic measurements, chemical and isotopic analyses of gas and water, definition of water content, and measurement of equilibrium pressures and temperatures.

Hydrate content in sediments of discovered accumulations was up to 30-40% by volume. Somewhere, hydrate rests immediately on the bottom. Hydrate accumulation requires not only gas but also water input. It may be filtering either water bringing dissolved gas or pore/sea water migrating to meet gas diffusing bottomward. The models of gas hydrate formation have been developed for both gas-saturated water and free gas.

Isotopic composition of water oxygen mainly results from exchange with carbonate inclusions rather than from the effect of hydrate fractionation.

It is possible to evaluate hydrate content in sediments from the amount and composition of water.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.