AVO Numerical Simulation of Gas Hydrate Reflectors Beneath Seafloor

Abstract

The AVO numerical method is used to 6 models of layered gas hydrate systems beneath seafloor, composed of methane hydrate bearing sediment, free gas bearing sediment and saturated sediment, to simulate the formation mechanism of BSR, double BSR and the vertical distribution of the gas hydrate system to make theoretical explanation of some practical problems. The main conclusions are as follows: (1) The presence of evident ocean bottom simulating reflector (BSR) is closely related to the existence of free gas and the seismic profile “BSR” maybe is not uniquely linked with methane hydrate but only shows the existence of gas. Inversely, methane hydrate may exists at the place without obvious BSR. (2) Free gas is possible to exist on the top of methane hydrate and forms a positive BSR above the normal BSR (BSR1). (3) There are two possible elastic mechanisms for double BSR0 under normal BSR1. If the methane gas meets some obstruct layer on the path of upward migration or different gas components naturally are stratified, the vertical gradient distribution of methane gas would be formed with upper part saturation higher than lower part resulting in a negative reflector. On the other hand, if some residual hydrate exists in the free gas layer under methane hydrate, some kind of BSR with certain amplitude would occur. But for this case the polarity of BSR0 maybe is difficult to be discriminated, depending on the gas saturation above or beneath the residual hydrate and the thickness of residual hydrate.