Estimating the amount of gas-hydrate using effective medium theory: a case study in the Blake Ridge

Estimating the amount of gas-hydrate and free-gas is difficult in deep seas even with scientific coring and downhole measurements. Well data may be incompatible between holes of a site as well as with depth in the same hole. In this paper, we demonstrate an approach to estimate saturation of gas-hydrate from seismic velocities at any site where data set is limited. The study is carried out in the outer Blake Ridge, which is one of the most intensively studied regions of natural gas-hydrate occurrences and a very distinctive example of studying geophysical signatures of gas-hydrate and free-gas in deep marine sediments. Although, downhole measurements from both vertical seismic profiles (VSPs) and sonic logs provide the most accurate and direct measurements of velocity, only VSP velocities at Ocean Drilling Program (ODP) Sites 994, 995, and 997 on the Blake Ridge are used to estimate the saturation of gas-hydrate and free-gas as sonic logs at ODP sites are not reliable. Here we derive a general trend of the background velocity with depth using the porosity and mineralogy from coring at discrete depth intervals. Saturations of gas-hydrate and free-gas are then estimated from this background velocity using the effective medium modeling. The porosity and mineralogical compositions are taken from four different depths at Site 995, as data quality is the best in this hole. Average saturations of gas-hydrate and free-gas at three holes are estimated as 10–14 and 2–3%, respectively.