2. Motivations for the Geophysical Investigation of Gas Hydrates

Abstract

Recent years have witnessed an array of international field expeditions designed to investigate the nature of Arctic and marine gas-hydrate geologic systems and the potential for gas-hydrate accumulations to be tapped as a future supply of natural gas. At the same time, numerical models designed to assess gas-hydrate reservoir response to production-related or natural perturbations have continued to mature. With regard to resource potential, it now appears that the most promising reservoirs will be those accumulations that are housed at high saturations in sand- and sandstone-dominated lithologies at or near the base of gas-hydrate stability. Essential to assessing and realizing this potential will be technologies to effectively survey deepwater shallow sediments to remotely detect and characterize gas-hydrate occurrence. The refined focus of attention on sand and sandstone reservoirs has significant implications for this effort. We now have the opportunity to move gas-hydrate exploration beyond the primary reliance on geochemical and geophysical indicators taken from the margins of the gas-hydrate stability zone. Future gas-hydrate exploration will instead incorporate that information into a fuller approach centered on the improved imaging and characterization of discrete prospects. This potential for delineation of specific targets is particularly true for those accumulations that are of sufficient gas-hydrate saturation and thickness to be attractive exploration prospects. Further mitigation of the uncertainties inherent in exploration will be provided through geologic, geophysical, and geochemical data that support the presence of the various elements of the gas-hydrate petroleum system, such as gas and water charge, suitable migration pathways, and porous and permeable reservoir facies. In addition, these emerging geophysical technologies and approaches will contribute critical insight to the assessment of the role of gas hydrates in dynamic natural processes such as carbon cycling and global climate change by more accurately capturing the natural variability in gas-hydrate distribution and concentration, the geologic nature of the enclosing media, and other factors that affect gas-hydrate stability and gas and water mobility.