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ABSTRACT: Basin Scale Anisotropy of Permeability as a Function of Depositional Environment

J. -L. Rudkiewicz, E. Brosse, G. Allen

The cells of numerical basin models commonly reach kilometers in length and hundreds of meters in height. In order to model compaction and fluid flow, Kx and Kz absolute permeability must be estimated in these cells. In a complex delta, like the Mahakam, these permeabilities represent the fluid flow behavior of a multilayered system of stacked sandstone bodies, the thicknesses of which are far smaller than the height of a basin model cell.

Therefore, the distribution of sandstone or shale bodies inside the big cells has to be estimated. The various depositional environments in the delta differ in the size and orientation of the sand bodies and their net-to-gross

ratio. These data are known from sedimentological studies on the actual delta and from underground data.

HERESIM software, designed to characterize heterogeneous reservoirs, has been fed with this data and applied to selected basin-scale cells. HERESIM uses stochastic methods to build heterogeneous reservoirs in a geologically consistent way. It then scales up the permeability of this complex media.

For a given mean size of the sand bodies, the permeability ratio jumps abruptly: 1-10 to 104-105 and back to 1-10 at two threshold values of sand/shale ratio. Kx/Kz is linked to the connection between sand bodies or shaly layers.

The percolation theory predicts this result but only for simple (geologically unrealistic) shapes of sand bodies and remains qualitative. Our approach uses geologically quantified geometries and shows how thresholds depend on the depositional environment. The provided Kx and Kz were used in the THEMIS modeling of the Mahakam delta.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990