Current states of well-logging evaluation of deep-sea gas hydrate-bearing sediments by the international scientific ocean drilling (DSDP/ODP/IODP) programs

Abstract

Since deep-sea gas hydrate-bearing sediments were drilled for the first time in the Blake Ridge in 1970, gas hydrates have been discovered at 53 drill sites in the continental margins of global oceans with international scientific ocean drilling (DSDP/ODP/IODP Programs). As a result, massive amounts of geophysical well-logging data have been accumulated, which provide critical information for understanding the in-situ properties of gas hydrates and their host sediments. Gas hydrates have such physical and chemical properties as non-conductivity, low density, high acoustic velocity, and high hydrogen content, which form the basis of identifying gas hydrate reservoirs and predicting their distribution by well-logging data. A series of well-logging evaluation methods have been proposed to estimate gas hydrate saturation of sediments, including Archie equation, combined methods of density and nuclear magnetic resonance well logging, various forms of three-phase acoustic wave equations, and elastic wave velocity simulations based on different rock physical models. The distribution of gas hydrates is highly heterogeneous, which is mainly manifested in the selectivity of hydrate occurrence to the lithology of host sediments and to the nucleation sites within a host sediment of the same lithology. The scientific-ocean-drilling well logging data have also been preliminarily used for evaluating the heterogeneity of gas hydrate distribution and inferring the growth habit of gas hydrates in host sediments. Nevertheless, there still exist some problems. The formation models used in logging evaluation are in general oversimplified, in which only two or three stratal components are involved. The application of high-resolution logging while-drilling (LWD) data remains limited. Log interpretation is not closely integrated with core geology. Therefore, joint inversion of lithologic components, porosity and gas hydrate saturation based on more complex formation models, together with the applications of high-resolution LWD logging data and core calibration, may represent an important direction in future well-logging evaluation of gas hydrate reservoirs.