Pore Size and Pore Throat Types in a Heterogeneous Dolostone Reservoir, Devonian Grosmont Formation, Western Canada Sedimentary Basin

Abstract

The Devonian Grosmont Formation in northeastern Alberta, Canada, is a giant heavy-oil reservoir. The main reservoir rocks are dolomitized and karstified platform and ramp carbonates, and the best reservoir facies occur in the upper Grosmont (UGM) units 3 and 2. In these units, reservoir properties are highly heterogeneous. Hand specimen, thin section, UV, and SEM petrography, as well as grading scales, mercury capillary pressure curve analysis, and statistics, have been used to characterize reservoir heterogeneity. Our investigation led to a new pore size classification for carbonate reservoirs; this new classification has four pore sizes: microporosity (pore diameters 256 mm). A combination of microscopic observations and capillary pressure curve characteristics led to the recognition of four pore throat texture types on the microporosity scale, and to five types on the mesoporosity scale. Microporosity pore types include (1) intracrystal dissolution porosity, (2) pervasive intercrystal and intracrystal dissolution porosity, (3) intergranular and/or intercrystal porosity in grainstones, and (4) primary or solution microporosity in mud matrix (only in limestones). Mesoporosity pore types include (1) intercrystal porosity, (2) solution-enhanced intercrystal porosity, (3) oversized porosity, (4) intragranular solution porosity, and (5) intergranular solution porosity. Some of these types are homogeneous (e.g., non-fabric selective dissolution porosity and inte crystal primary porosity), whereas others are heterogeneous. Generally, hydrocarbon recovery efficiency is good in the homogeneous pore throat types, but poor in the heterogeneous types. Reservoir heterogeneity has further been characterized by comparison and statistical analysis of plug and full-diameter core (FDC) data that indicate (1) plug porosities, on average, are higher than FDC porosities; (2) plug permeabilities, on average, are higher than FDC permeabilities; (3) plug porosity and permeability show relatively good linear relationships in certain core intervals, but FDC data do not; and (4) generally, the more homogeneous reservoir rocks have relatively high matrix porosity. Heterogeneity is common in Grosmont dolostone reservoir rocks, and affects overall reservoir quality. At the megascopic scale, the reservoir properties can be classified as homogeneous, dual-porosity, and multilayered. The dual-porosity model has three subcategories: fractured, channeled, and pressure solution derived.