Abstract The aim of this report is to describe a Middle Devonian dolostone reservoir in western Canada that has produced more than 57 million barrels of oil and water from moldic-pore reservoir facies with low permeability. The Slave Point Formation consists of six carbonate depositional facies, the relative proportions of which change in response to location on the basement paleotopographic surface. The most significant porosity in Slave Point dolostones is moldic porosity that formed by leaching of fossil fragments; not all Slave Point facies contain fossils. The distribution of fossiliferous carbonate facies, and moldic pores, is ultimately controlled by basement paleotopography. The main conclusion is that there is not a good correlation between permeability and porosity in these rocks. Permeable zones are restricted to dolostones that have touching moldic pores—a constraint that has limited oil production to fossiliferous-carbonate belts that fringe Precambrian granite-basement highs. In contrast, expanses between basement knolls accumulated mostly carbonate mudstone. Mudstone lacks fossils and consequently does not contain moldic pores. The simplest description of this reservoir is that it contains pods of permeability that are surrounded by less-permeable rock, a description suited to many carbonate reservoirs. The desired impact of this work is to draw attention to methods for estimating the probability of finding more-permeable or less-permeable facies types at different locations within these types of reservoirs. This approach would lead to design of more efficient exploration and production programs in complex moldic-pore carbonate reservoirs. Both oil and water are produced from these dolostones. Understanding the effects of high fluid flow in high-permeability zones can enhance oil recovery and reduce water production from similar vuggy-dolostone reservoirs.

Abstract The Middle Devonian carbonates of the Slave Point Formation, Hamburg field, northwestern Alberta, are composed mainly of stromatoporoid and Amphipora floatstones and rudstones, with interbedded mudstone and grainstone facies characteristic of deposition in open to slightly restricted marine platform environments. These carbonates have undergone a complex diagenetic history, from shallow to deep burial, as represented by fracturing, calcite cementation, silicification, and dolomitization. Petrographically, four different types of dolomite have been identified (from early to late): (1) fine-crystalline matrix dolomite; (2) pseudomorphic dolomite; (3) medium-crystalline pervasive dolomite; and (4) saddle dolomite. Fine-crystalline dolomite (5–50 (μm) replaces the mud matrix and slightly penetrates the edges of allochems. It occurred in mud-supported facies and was precipitated by marine fluids. Oxygen isotope values range from −11.62 to −9.34‰ (Peedee belemnite), lower than postulated values for Devonian carbonates. The enriched 87 Sr/ 86 Sr isotope value from this phase (0.71002) suggests that later diagenetic fluids may have recrystallized this dolomite. Pseudomorphic dolomite (50–100 μm) replaces crinoids and occurs as single, large dolomite crystals. Its oxygen and carbon isotopic values range from −10.58 to −9.65 and +4.24 to +4.49‰, respectively. Medium-crystalline pervasive dolomite (10–100 μm) occurs along dissolution seams and obliterates all previous fabrics. It is proposed that this medium-crystalline dolomite formed during shallow to intermediate burial because of its association with dissolution seams and high iron content. The range of oxygen isotope values for this dolomite (−11.74 to −9.5‰) suggests precipitation from a warm fluid, possibly in a burial environment, and/or later recrystallization by hydrothermal fluids. The relatively wide range of carbon isotope values (+1.19 to +4.49‰) and enriched strontium isotope ratio (0.710020) suggests recrystallization. Saddle dolomite (250–2000 μm) partially to completely occludes void spaces (both fractures and vugs) and also occurs as a minor replacement mineral. The oxygen isotope values for saddle dolomite (−?13.95 to −?11.97‰), as well as the nonradiogenic to enriched strontium isotope ratios for saddle dolomite (0.70494 to 0.710351), and the fluid-inclusion data (homogenization temperature, T h , range between 125 and 161°C and estimated salinity, between 22.2 and 24.7 wt.% NaCl equivalent) indicate precipitation from hot, highly saline, hydrothermal fluids, which were probably expelled tectonically during the Late Devonian-Mississippian Antler thrust belt development.