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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

SKGS-AAPG

Fifth International Williston Basin Symposium, June 14, 1987 (SP9)

Pages 212 - 216

USING CAPILLARY PRESSURE TECHNIQUES TO MAP AND IDENTIFY AN ECONOMIC FAIRWAY IN THE ORDOVICIAN RED RIVER ON THE WESTERN FLANK OF THE WILLISTON BASIN

JEFFREY B. JENNINGS, Consulting Geologist, 2156 South Field Street, Lakewood, Colorado

ABSTRACT

Capillary pressure curves have been confined mainly to the realm of petroleum engineering but have some excellent applications in petroleum geology. Capillary pressure, for any rock type possessing porosity and permeability, can be viewed as the force required to push a fluid through a given pore size. The raw data are collected by injecting mercury, at increasing pressures, into plugs obtained from core or side wall samples. These measurements, when plotted, produce a curve of injection pressure versus fluid saturation from which a number of parameters can be extracted for the purposes of interpretation and mapping. Pore throat sorting, reservoir grade and oil columns required for economic oil saturations are some of the more important of these parameters.

Pore throat sorting is an empirical number which reflects the pore geometry. It is highly influenced by the diagenetic history and grain- or crystal-sorting, thus making it useful as a stratigraphic indicator.

Reservoir grade is a number which defines the rock's ability to accept fluid and permits qualitative comparisons between reservoirs.

Oil column heights can be calculated to define the vertical trap closure (structural or stratigraphic) required to achieve economic oil accumulations. A 50 per cent oil saturation approximates the conditions for which marginal production can be expected while a 75 per cent oil saturation should result in good economic oil production.

Pore throat sorting, reservoir grade and oil column heights may be compiled and contoured to produce exploratory maps. The maps are useful for delineating "Oil Reservoir Fairway" trends and highlighting areas in which to concentrate further investigative efforts.

A capillary pressure study was conducted on the third porosity of the Ordovician Red River Formation on the western flank of the Williston Basin in Montana and North Dakota. Sixteen wells with core data were sampled and mercury injection capillary pressures were obtained. Values for pore throat sorting, reservoir grade and oil column requirements for 50 per cent and 75 per cent oil saturation were then plotted and contoured to produce four regional maps. Each of the four maps identified a north-south trending fairway along the Montana-North Dakota border containing the best development of reservoir rock. The maps provide a good example of how capillary pressure techniques can be applied to assist with exploration programs.

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