Methane Hydrates

A methane hydrate is an ice-like, crystalline lattice of water molecules in which gas molecules are trapped physically without the aid of direct chemical bonds. They are only stable under certain pressure and temperature conditions, which are illustrated in Figure . These conditions for hydrate stability limit the occurrence of methane hydrates to two regions: polar and deep oceanic. In polar regions, hydrate structures are normally associated with permafrost both onshore in continental sediment and offshore in sediment of the continental shelves. In deep oceanic regions, hydrates are found in outer continental margins in the sediment of slopes and rises where cold bottom water is present.

 

phase
Figure 1 Phase diagram of the stability of methane hydrate. Redrawn after Kvenvolden (1993).

Surface seismic data can image hydrate formations through the presence of bottom simulating reflectors (BSR), which are associated with the base of the hydrate stability field. BSRs parallel the seafloor at a subbottom depth of several hundred meters and are characterized by strong negative reflection coefficients and increasing subbottom depth with increasing water depth, a characteristic which is governed by the phase diagram of hydrate stability (Figure ). They are therefore not structural reflectors, but occur at the phase transition of frozen methane hydrate to gas or water.

Two models have been proposed to account for the formation of methane hydrates and the development of BSRs: (1) the BSR is caused by hydrate overlying gas-saturated sediment and (2) the BSR is caused by hydrate overlying brine-saturated sediment. The first model assumes the local generation of methane from organic material at the depth of the hydrate. Gradual thickening and thus deepening of the hydrated zone causes it eventually to subside into a temperature region where hydrate is unstable. Consequently, free gas can be present in this region Kvenvolden and Barnard (1983a). The BSR is then caused by the impedance contrast at the base of the hydrated zone and the top of the gas layer. The second model, in contrast, supports the formation of methane hydrates through the removal of methane from rising pore fluids being expelled from deeper in the sediment section Hyndman and Davis (1992). Most of the methane is generated microbially at a depth below the hydrate stability zone, but not at depths sufficient for the formation of thermogenic methane. Therefore, free gas does not have to be present beneath the BSR. In this case, the BSR can be the consequence of the impedance contrast between overlying sediments containing substantial amounts of hydrate and underlying brine-saturated sediments.

In addition to seismic methods, several attempts have been made to recover hydrates from drilling. However, since the risk of heating and destabilizing the initial hydrate conditions during the process of drilling is considerably high, only limited information is available. Thus the core samples and well-logs do not necessarily reflect the correct in-situ hydrate characteristics and properties. Consequently, much information is inferred remotely from seismic data of the bottom simulating reflector and should be tied with the results available from drilling.

Although numerous investigations have been performed to determine the hydrate and BSR characteristics from surface seismic, they were restricted to determining only the P-wave velocity behavior around the BSR while assuming possible hydrate Poisson's ratios based on laboratory results. Little work has been done in estimating both the P- and S-wave velocity behavior at the transition from in-situ hydrates to the underlying sediments, and subsequently in relating them to physical rock properties. The ultimate goal of interpreting seismic reflection data is to estimate the amount of hydrate present in the sediment, its distribution in the pore space, permeability and potential recoverability. This goal requires an integrated seismic analysis - rock physics approach.