Abstract: Petrophysical Characteristics of Carbonate Resources and Their Influences on Withdrawal Efficiency

Lillian Hess-Tanguay

Within the dolostones (U2, U4, U6, U8) of Cabin Creek Field, Cedar Creek Anticline, Williston basin, Montana, of the Ordovician Red River Formation, there is a positive relationship between porosity and withdrawal efficiency during mercury porosimetry. The maximum withdrawal efficiency is correlated to a minimum range in porosity and throat-size distribution (1 to 2 microns). The greatest apparent porosity occurs in dolostones in which the total pore system is mercury saturated by 500 psia. Dolostones with average pore-throat radii from 0.2 to 1.0 microns have the highest porosity. Porosity between 10% and 22% generates the greatest extrusion. Within the U4 dolostone, as porosity increases greater than 15%, withdrawal efficiency declines.

The maximum-threshold extrusion radius indicates the capillary pressure that must be maintained for maximum withdrawal. As the maximum-threshold extrusion radius increases greater than 2 microns, the withdrawal efficiency decreases.

Well-sorted pore systems with low aspect ratios have the highest withdrawal efficiencies, whereas poorly sorted pore systems generally have no withdrawal.

The area between the intrusion curve and the extrusion curve defines reservoir grade. Samples with optimum reservoir grades are characterized by >90% dolomite, maximum-threshold extrusion radius less than or equal to 2 microns, median pore-throat radius volume between 0.5 and 3.0 microns, unimodal or bimodal distribution of pore throats, average pore radius between 0.2 and 2 microns, and maximum incremental volume between 0.005 and 0.015 cc per gram.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994