Use of Variography in Permeability Characterization of Visually Homogeneous Sandstone Reservoirs with Examples from Outcrop Studies

Abstract

Sandstones of different ages provide economically significant oil, gas, and water reservoirs. In sandstones where heterogeneities are not visually obvious, it is particularly difficult to predict the location of permeability barriers and the scale at which high and low permeability zones occur, yet this is critical in providing information on hydrocarbon reservoir performance. This study uses variogram analysis to investigate spatial variation in permeability in visually homogeneous reservoir sandstone successions. Air permeability measurements were taken using unsteady state probe permeametry following regular grid schemes with centimeter spacing. Spatial variation in permeability was characterized using omnidirectional and directional variograms. This study combines variography with geological interpretation to assess the degree of heterogeneity of permeability in visually homogeneous sandstone successions. Variography indicates spatial dependence and short-range variation at 1 cm grid spacings that is not apparent at a larger 5 cm grid spacing in the visually homogeneous sandstones studied. The range of the models fitted to the variograms provide a potentially important index of spatial variability in permeability for different depositional settings including aeolian, fluvial, shallow marine, and marine/mass- flow turbidite.