HR: 0830h
AN: OS51B-0863    [PDF]
TI: Describing the Spatial Continuity of Gas Hydrate Deposits Using Indicator Variography
AU: * Strack, E
EM: eastrack@olemiss.edu
AF: University of Mississippi, Dept of Geol & Geol Engineering, University, MS 38677 United States
AU: Kuszmaul, J S
EM: kuszmaul@olemiss.edu
AF: University of Mississippi, Dept of Geol & Geol Engineering, University, MS 38677 United States
AB: Gas hydrates observed on the Gulf of Mexico and Nigerian continental margins provide a basis for characterizing the spatial continuity of gas hydrate environments as a function of distance and direction from neighboring sampled locations. A total of 1039 sites for the Gulf of Mexico margin and 484 sites for the Nigerian margin were compiled from published data sets. Each data set included sites where gas hydrates were observed or where drilling occurred without gas hydrate contact. The spatial distribution of the deposits within the study areas were characterized by preparing multiple experimental indicator variograms with values assigned as one for hydrate presence and zero for hydrate absence. The nugget, sill and correlation length of both isotropic and anisotropic variograms were adjusted to determine the best-fit models. An isotropic variogram with an active lag of 200 km and a uniform interval of 5 km produced the clearest structure for the Gulf of Mexico study area. The isotropic variogram was fit with an exponential model with a correlation length of 52 km, a sill value of 0.047, and a relative nugget effect of 45.8% The maximum direction of spatial continuity trends approximately $43\deg$ with the minimum axis at approximately $133\deg$. Sampled sites are correlated at a distance of 85 km in the direction of maximum continuity and 55 km in the direction of minimum continuity. The sill value for an anisotropic spherical model was 0.047 with a relative nugget effect of 38.3%. An isotropic variogram with an active lag of 180 km and a uniform interval of 5 km produced the clearest structure for the Nigerian continental margin study area. The isotropic variogram was fit with a spherical model with a range of 161 km, a sill value of 0.043, and a relative nugget effect of 4.7%. No significant spatial anisotropy was evident in the Nigerian continental margin for the data set as a whole. The nugget effect observed in the variogram models of both study areas may in part be the consequence of the sampling procedures used to establish each data set. The maximum direction of spatial continuity derived from the Gulf of Mexico data set corresponds with the trend of gas hydrate locations collected along the edge of the Salt Deformation Province. Indicator variograms suggest a spatial relationship does exist between gas hydrate locations as a function of both distance and direction.
DE: 4203 Analytical modeling
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting