Exploration strategy for economically significant accumulations of marine gas hydrate

Abstract

Abstract There are at present no validated methods for reliably finding economically significant accumulations of natural gas hydrate in marine environments. The seismic bottom simulating reflector (BSR) has been regarded as a primary indicator of hydrate presence in marine environments, but the presence of a BSR conveys no information about the abundance of hydrate in the sediments above it. Seafloor features such as gas seeps, pockmarks or hydrate outcrops may be qualitative markers of deeper hydrate presence, but cannot be interpreted quantitatively. Another approach to exploration geophysics is required to find exploitable gas hydrate reservoirs with high reliability. It is known that in many cases gas is supplied to the gas hydrate stability zone primarily through faults or fractures. In a certain range of gas flux, these fissures should become mineralized with gas hydrate and form vertical or subvertical dykes. The dip and strike of these dykes are controlled by the principal stress directions, which can be predetermined. Thus multiple hydrate dykes are expected to be parallel. Even if the greatest volume of gas hydrate is to be found in sub-horizontal permeable beds, the steeply dipping mineralized conduits that fed gas to them may be the most reliable marker of substantial subsurface hydrate presence. Geological and geophysical survey methods sensitive to parallel arrays of vertical and subvertical hydrate dykes are presented.