Carbonate reservoirs are characterized by extremely heterogeneous porosity and permeability-These heterogeneities are caused by the wide spectrum of environments in which carbonates are deposited and by subsequent diagenetic alteration of the original rock fabric. Pore systems range from thick, vuggy reservoirs in the coarse grained skeletal-rich facies of the reef margin or platform margin to highly stratified, often discontinuous reservoirs in the reef interior, platform interior and nearshore facies.
Eight pools from the Western Canada Basin have been selected to illustrate the variety of reservoir configurations which can be found. Boundary Lake and Steelman are thinly bedded sequences of cyclic nearshore carbonates which are best suited to pattern waterflood. Quirk Creek is a thick, overthrusted open platform gas pool in which fractures associated with the leading edge of the structure provide high deliverabllity in an otherwise low-permeability reservoir. Clarke Lake is a fractured, dolomitized platform margin gas reservoir which is highly susceptible to water channelling. Golden Spike, Judy Creek, Norman Wells and Redwater are reef bioherms which display the classic reservoir configuration of highly permeable reef margin facies enclosing strongly stratified reef interior sediments. With increasing reef size, the relative volume occupied by oil becomes smaller and the aquifer becomes larger and more effective.
Golden Spike was entirely filled with oil when discovered and early production was characterized by rapidly declining pressure. An initial gas flood depletion plan was followed by a gravity-controlled, gas-driven miscible flood. This was very effective until the miscible bank was broken up and dispersed by several thin impermeable beds in the reef interior which proved to be barriers to vertical flow.
Judy Creek had a small ineffective original aquifer and was produced initially by a downdip peripheral waterflood- This ultimately proved to be ineffective in repressuring the discontinuous porosity in the reef interior and was supplemented with a pattern waterflood.
Depletion plans for these pools require detailed, integrated geological-engineering studies to develop accurate reservoir models. These models are then used in computer simulation studies to forecast reservoir behavior under various depletion methods and to assist in on-going reservoir surveillance studies.