Abstract

Slave Point carbonates form a series of broad, low relief, reef complexes which grew in a restricted, shallow basin, the Cranberry Basin. This basin was bounded by the Peace River Arch to the south and the Presqu’ile Barrier Reef to the northwest. The largest gas field in this region is the Cranberry Field. The objective of this project was to perform a petrophysical evaluation of the Slave Point carbonates in the Cranberry Field. The results were incorporated into a regional exploration effort and were used in reserve evaluations.

The Slave Point Formation is divided into three facies: platy stromatoporoid grainstones, amphipora mudstones to packestones, and bulbous stromatoporoid grainstones. These facies have a mixed limestone and dolomite lithology.

Two productive rock types have been identified in the Cranberry Field. Limestones (grainstones) with intergranular porosity and fractured dolomites with vuggy porosity. The limestones have an intergranular pore network and smaller pore throats than the vuggy and fractured dolomites. The dolomites display much more variability in pore throat size and geometry, and exhibit a large scatter in rock properties measurements.

Slave Point carbonates are at immobile Sw’s, and wells in the Cranberry Field produce virtually water-free. Dolomites associated with faults produce at up to ten times the rate of the limestones because of the contribution of fractures to flow capacity. Hydrocarbon pore volumes of limestones and dolomites are similar. Four wells, drilled into the vuggy dolomites account for two thirds of the gas, 67 Bscf, and all of the oil, 2.2 MMbbls, produced to date.