Multiscale Anatomy of a Reservoir: Geological Characterization of Pembina-Cardium Pool, West-Central Alberta, Canada

Geological reservoir elements that control a reservoir's conduit system are mineralogy, grain size, packing, pore network, lithohydraulic units, lithologic continuity, fractures and faults, and rock-fluid interactions. All occur at different scales. We have focused on four scales typical of a developed reservoir to characterize geological reservoir heterogeneity in the Pembina-Cardium pool: (1) megascale, the field-wide view; (2) macroscale, the interwell continuity; (3) mesoscale, the neighborhood of the well; and (4) microscale, the domain of grains and pores. 1. Megascale observations. The reservoir top is characterized by a ridge-and-swale topography oriented northwest-southeast. A similar but more subdued topography is also characteristic of the unconformity between conglomerates and sandstones. Conglomerates are distributed irregularly and are generally shaly in beds thinner than 1 m. Thick conglomerates are common on the east and south flanks of underlying ridges and impart northwest-southeast fluid production trends and yield zones of reduced pressures. In thick conglomerates, high flow capacities are detrimental to hydrocarbon recovery as they result in bypassing of injection fluids. The conglomerate is a lithohydraulic unit as it is a body of strata distinct from neighboring strata by depositional, diagenetic, and tectonic imprintin s, and by a characteristic set of petrophysical properties. 2. Macroscale characteristics. Reservoir lithofacies and lithohydraulic units are laterally discontinuous and form irregular patterns that can possess an irregular northwest-southeast orientation. Fluid production trends oriented northeast-southwest in areas of thin, shaly conglomerates appear to be the product of fractures. In these areas, reorientation of injector wells to line drives results in higher oil recoveries. 3. Mesoscale properties. In cores, five lithofacies characterize the field. They are (1) dark-gray mudstone and siltstone, (2) bioturbated, thin and very thin-bedded shale, siltstone, and very fine and fine-grained sandstone, (3) thin and very thin-bedded shale, siltstone, and very fine and fine-grained sandstone, (4) medium to thick-bedded, very fine and fine-grained sandstone, and (5) conglomerate. As each lithofacies possesses contrasting, but partially overlapping petrophysical properties, they are lithohydraulic units. Lithofacies 1 acts as the reservoir seal and restricts flow within the reservoir. Lithofacies 2 has high porosities, low permeabilities, and dominates the bottom half of the reservoir. Lithofacies 2 has a high storage capacity (^phgrh), but its flow capacity (kh) i low. Lithofacies 3 reduces cross-flow because its vertical permeabilities are very low. Lithofacies 4 has modest permeabilities and high porosities, and is common to the top half of the reservoir. This lithofacies' flow capacity in T47, R7 W5M is 17 times greater than that of lithofacies 2. Lithofacies 5 has modest porosities, but very high permeabilities resulting in intervals with very high flow capacities. 4. Microscale investigations. In the sandstones, the mineral framework consists predominantly of quartz and chert whereas chert prevails in the conglomerates. These mineralogic differences lead to reduced cementation by silica on cherts and result in the preservation of pore spaces resembling original pores in chert-rich zones. Although quartz grains are modified by megaquartz overgrowths, chert particles are altered by pressure solution, fracturing, and crushing. In both sandstones and conglomerates, framework grains are cemented by siderite, calcite, kaolinite, and illite. Experiments with reservoir cores flooded with CO2-enriched brines and data from waterflooding indicate that siderite and calcite are unstable under reservoir conditions. This behavior can result in perm ability losses from release of insoluble fine materials and weakened responses during polymer floods. Mercury capillary-pressure data from 28 measurements of lithofacies 4 sandstones indicate that mercury End_Page 1233------------------------------ recovery efficiency increases with decreasing permeability and porosity. This behavior, if confirmed elsewhere in the field, may explain the performance of areas that have reduced permeabilities and porosities but are responding well to waterflooding.