METHANE HYDRATE STABILITY AND BREAKDOWN

Methane hydrate is a substance in which the molecules of gas and water chemically interact to form an ice-like crystalline structure. This structure is stable within a certain pressure-temperature regime. The phase diagram in Figure 1 shows clearly that this pressure-temperature regime requires either low pressure/low temperature or high pressure/high temperature. Associated with the base of the hydrate stability field are bottom simulating reflectors (BSRs) that parallel the seafloor at a subbottom depth of several hundred meters. Seismic reflection profiles often show a discontinuous or fading behavior of these BSRs (Figure 2), which might be explained by perturbations in the pressure-temperature profile in the shallow marine sediments. Changes in the pressure and temperature will ultimately affect the position and characteristic of the BSR since it will result in different hydrate stability conditions.

 

phase
Figure 1 Phase diagram showing the boundary between methane gas and methane hydrate. Redrawn after Kvenvolden (1993).

Several possible scenarios that can cause such pressure/temperature perturbations can be imagined, such as changes in sea level, land slides, faults and diapirs, etc.. Figure 3 illustrates the cases of sea level fall and mass sliding. In both events, pressure reduction and temperature will affect the hydrate stability field, placing the BSR into a zone of hydrate instability. This will result in the breakdown of the hydrate and, consequently, in a change of the BSR. While the BSR will be affected on a global scale in case of sea level fall, mass slides will have a more local effect on the BSRs. In order to understand and successfully characterize seismic sections showing discontinuous, fading BSRs, we must have knowledge of the behavior of the BSRs under hydrate breakdown. Moreover, several crucial questions related to this breakdown must be answered if we are to characterize the hydrate structures. How will this breakdown affect the BSR? Will the BSR disappear and rebuild in the new hydrate stability zone? If so, how long would it take to be rebuilt? Is there still hydrate, even though there is no longer a BSR? What will happen to the gas that will be released from the dissociating hydrate? Will it migrate upwards undisturbed, or will it form hydrate again? Fluid-flow simulation of a disturbed hydrate zone should be a good way to get a first-order insight into these questions. It should enable me to anticipate the changes in hydrate structure and BSR.

 

near-ann
Figure 2 Near-offset stack section of data from the Blake Outer Ridge, offshore Florida and Georgia, showing a discontinuous BSR.

 

break
Figure 3 Illustration of hydrate breakdown. The upper part illustrates the change in the hydrate stability due to a drop in sea level thus affecting the BSR on a global scale. The lower part displays the stability behavior for mass slides which affects BSRs on a local scale.