Estimation of Gas-hydrate Saturation and Heterogeneity on Cascadia Margin from Ocean Drilling Project Leg 204 Logging-while-drilling Measurements
Aleksandra Janik, David Goldberg, Gilles Guerin, Timothy Collett, 2009. "Estimation of Gas-hydrate Saturation and Heterogeneity on Cascadia Margin from Ocean Drilling Project Leg 204 Logging-while-drilling Measurements", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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In this chapter, we present an improved method of estimation of pore space saturated with gas hydrate from the inversion of resistivity and porosity data. The method is developed through an analysis of logging data collected during logging-while-drilling (LWD) measurements on Hydrate Ridge offshore Oregon, whereby the three-dimensional distribution of hydrate is explicitly considered. The LWD tools rotate and provide 56 resistivity and 16 density azimuthal measurements of the sediment properties around the inner circumference of the borehole collected with high vertical resolution (3 cm [1.1 in.]). Additionally, LWD data are acquired only minutes after the formation is drilled, limiting the extent of hydrate dissociation in the measured in-situ properties. The resulting borehole wall images of gas-hydrate saturation reveal the detail of the geometry and shape of void spaces in which gas hydrate occurs. Several visible fractures are filled with gas hydrate, which also occur with patchy distribution, in lenticular shapes, as asymmetrical fracture fill, and in truncated thin layers. The LWD data indicate considerable azimuthal heterogeneity at the Hydrate Ridge sites, with typical variations of more than four times the azimuthal average of saturation at a given depth and on a centimeter (inch) scale around the borehole. Such heterogeneous and patchy hydrate distribution is characteristic for all occurrences of hydrate within the gas-hydrate stability zone at all of the investigated Hydrate Ridge sites.