Joint analysis of P-wave velocity and resistivity for morphology identification and quantification of gas hydrate

A knowledge of hydrate morphology is essential for assessing its resource potential, understanding its formation, and determining optimum strategies for exploitation. Previous studies have suggested two general morphologies of gas hydrates distributions in sediment settings: pore- and fracture-filling. However, because of a lack of effective morphology identification methods, it is currently difficult to quantitatively model gas hydrates. Here we propose a joint analysis of P-wave velocity and resistivity to identify hydrate morphology and estimate hydrate saturation in a continuous depth profile. First, we perform numerical modeling to investigate the effects of hydrate morphology on the P-wave velocity and resistivity properties of gas hydrate-bearing sediments (GHBS). The results demonstrate that, in the case of identical hydrate concentration, fracture-filling GHBS typically exhibit higher resistivity but lower P-wave velocity than those of pore-filling GHBS. Consequently, the cross plot between these two properties are strikingly different for the two types of GHBS. By comparing the cross plot of field measurements to the theoretical cross plot, we hypothesize that hydrate morphology can be identified and hydrate saturations estimated. We test and verify the hypothesis using the velocity and resistivity log data at different sites in China's second gas hydrate expedition in 2013 in the South China Sea, where the two morphologies were confirmed via core samples. Cross plots of field measurements agree closely with the theoretical results, and gas hydrate morphologies are successfully identified. The estimated hydrate saturations are comparable to measurements of pore-water freshening in the majority of cases.