Geological Controls on Reservoir Properties: Midale Carbonates, Southeastern Saskatchewan, Canada

John G. Kaldi

The Midale field produces oil from the lower, middle, and upper zone carbonates of the Mississippian Midale beds. Like many older fields with declining production, the Midale reservoir is a prime candidate for enhanced recovery operations. Therefore, recent geological investigations in this field have been directed toward detailed characterization of the rocks in terms of their potential response to the movement of various fluids. The reservoir was thus divided into flow units. Dividing was done by first detailing the depositional and diagenetic lithofacies. Next, quantitative data, including porosity and permeability measurements, pore geometry, and mercury injection capillary pressure curves, were generated for each lithofacies. From these analyses, comparing aspect ratios (pore to pore-throat relationships), coordination numbers (throats per pore), and recovery efficiencies (S_max - S_or on drainage and imbibition MICP curves) is possible, thereby determining flow units. The lower zone constitutes flow unit 1, made up of lime packstones to wackestones, locally having a vuggy texture where late-stage replacive anhydrites have been dissolved. Middle zone carbonates comprise 4 additional flow units. Oolitic and/or crinoidal grainstone shoals (flow unit 2) are commonly cemented by blocky calcite in the oolite and syntaxial ov rgrowths in the encrinite. Intershoal deposits (flow unit 3) consist of argillaceous lime mudstones deposited in paleotopographic lows. The broad flanks of the grainstone shoals are made of packstones to wackestones having a dolomitized matrix with calcite allochems (flow unit 4), or a dolomitized matrix with moldic pores (flow unit 5). Upper zone carbonates (flow unit 6) are uniformly microcrystalline dolomites with a pervasive nonfabric selective microfracture system. Reservoir properties in the Midale reservoir are defined by the behavior of individual flow units; these, in turn, are controlled mainly by the rocks' sedimentary and diagenetic history.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.