ABSTRACT

It is generally agreed that there is no preferred relationship between porosity and permeability unless additional parameters, such as grain size, and shape, or pore size distribution, are used as a basis of initial selection. Most mechanically deposited sediments obviously retain directional properties such as preferred orientation of elongated grains, imbrication, etc., unless substantially modified by diagenesis. This anisotropy is also reflected in their respective permeability properties. Somewhat less obvious is the occurrence of permeability anisotropy in fine-grained carbonate deposits referred to an intertidal and supratidal origin. This is surprisingly true of the uniform microsucrosic dolomites.

Silurian dolomites from the Montana subsurface demonstrate a vertically consistent dielectric and permeability anisotropy. This is based on the simultaneous solution of three equations of the form y-h = a sin (kx-b), where three permeability plugs of 120° apart are analyzed for each foot of core. Use of the sine function, calculated in the expansion identity form

sin (kx-b) = sin kx cos b - cos kx sin b,

assumes that there is one maximum and one minimum value lying within 180° horizontal rotation, since permeability is a two-directional feature. The resulting calculations yield an ellipse whose major and minor axes provide a ratio equivalent to the permeability contrast based on azimuthal orientation.

Assuming that the concepts of shoreward distribution of common matrix carbonate particles in a tidal-flat complex, and local source dolomitization are valid, both primary deposition and secondary dolomitization habits would provide a condition of final permeability anisotropy. Based on remnant magnetic orientation of the described core samples, the anisotropy is shown to be concordant with presumed directions of regressive facies progression and normal to regional structural axes and small scale fracturing.

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