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

Abstract

Estimating the amount of gas-hydrate and free-gas is difficult in deep seas even with scientific coring anddownhole measurements. Well data may be incompatiblebetween holes of a site as well as with depth in the samehole. In this paper, we demonstrate an approach to estimatesaturation of gas-hydrate from seismic velocities at any sitewhere data set is limited. The study is carried out in theouter Blake Ridge, which is one of the most intensivelystudied regions of natural gas-hydrate occurrences and avery distinctive example of studying geophysical signa-tures of gas-hydrate and free-gas in deep marine sediments.Although, downhole measurements from both verticalseismic profiles (VSPs) and sonic logs provide the mostaccurate and direct measurements of velocity, only VSPvelocities at Ocean Drilling Program (ODP) Sites 994, 995,and 997 on the Blake Ridge are used to estimate the sat-uration of gas-hydrate and free-gas as sonic logs at ODPsites are not reliable. Here we derive a general trend of thebackground velocity with depth using the porosity andmineralogy from coring at discrete depth intervals. Satu-rations of gas-hydrate and free-gas are then estimated fromthis background velocity using the effective mediummodeling. The porosity and mineralogical compositions aretaken from four different depths at Site 995, as data qualityis the best in this hole. Average saturations of gas-hydrateand free-gas at three holes are estimated as 10–14 and2–3%, respectively