Controls of Reservoir and Cap Rock on Gas Hydrate Formation

Review of previous gas hydrate studies indicates that the formation and occurrence of gas hydrate is controlled by the factors of sea bottom temperature, geothermal gradient, availability of gas and water, gas chemistry, pore-water salinity, gas and water migration pathways and the presence of reservoir rocks and seals and so on. This paper proposes that what controlled the formation of gas hydrate in the gas hydrate stability zone (GHSZ) are two core factors: gas concentration (Cg) and gas molecular migration velocity (Vg). Only when Cg is big enough and Vg is small enough, can gas hydrate take shape. I illustrate this with a C-V function: y=f(Cg, Vg), when y is in some particular value, gas hydrate will form. On the other hand, Cg is mainly controlled by the reservoir and Vg is mainly controlled by the cap rock. Moreover, the seal ability of the cap rock can be expressed as a function: y=f(H, Φ, K, Z). H, Φ, K, and Z stand for the thickness, porosity, permeability and grain size of the cap rock, respectively. Based on this theory, this paper illustrates the relationship between sediment and Cg, Vg. Especially it will discuss the influences of the four factors controlling the seal abilities of cap rock for Vg. Thereby, we conclude that the different thicknesses, porosities, permeabilities and grain sizes of cap rock and different combinations of reservoir and cap rock, which mainly contributes to the formation and accumulation of gas hydrate. On the basis of the above research, as there are different combinations of reservoirs and cap rocks in different systems tracts of sequence stratigraphy, this paper wants to study which systems tract and which sedimentary facies are the best for gas hydrate formation and accumulation. This would be very helpful to gas hydrate exploration.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California