Hydrate Composition from Seismic Data: An Inverse Procedure

Abstract

Hydrate existence conditions and the associated thermal gradients are examined using equations of state for mixes of gases. Three examples are developed to illustrate the resolution possible in determining the composition of a hydrate and the required range of thermal gradient using seismic information concerning the depth and thickness of a hydrate-bearing body. The examples use shallow (70m), medium (250m) and deep (600m) water as archetypical conditions. In general, it is shown that one cannot determine a unique composition for the hydrate due to incomplete knowledge of parameters in the hydrate existence equations of state, including the lack of knowledge concerning salinity and sedimentary lithological effects on parameters in the equations of state, and also due to such parameters being massively influenced by the types and mixing fractions of gases that compose the hydrate. What is shown is that it is possible to determine the cumulative probability that a hydrate has greater than a given fraction of methane and at the same time determine the probability that the associated thermal gradient lies within geologically required constraints. In association with this determination, one can also identify the relative contributions of each uncertain parameter to the resolution of the methane fraction and the thermal gradient, including geological parameters (such as sediment density) and “kinetic” parameters in the hydrate equation of state. In this way one can determine where to place effort in any further attempt to narrow down the uncertainty in the methane fraction for a given hydrate, and one can also identify when it is necessary to do so. These aspects are developed in the paper. There is also the point that, for deep water hydrates, the attempt to use the top of the hydrate body as the shallowest depth for hydrate existence can fail and recourse to a more appropriate strategy, but one that is not so statistically sharp, is required. This aspect, too, is developed in the paper. The main point to make is that the quantitative procedure developed here allows one to use seismic data directly to assess a hydrate body, to determine what can be deduced concerning hydrate composition, the sort of uncertainty one has to contend with, and also to determine the worth of proceeding further with more information collection to help resolve better the uncertainties.