ABSTRACT: Secondary Recovery from a Stromatoporoid Buildup: Devonian Duperow Formation, Ridgelawn Field, Montana

Laird D. Little

Ridgelawn field is located in Richland County, Montana, in the western part of the Williston basin. It is a multiple-pay field, with production from Ordovician, Devonian, and Mississippian carbonates. Discovered in 1980, the field was recently unitized in the Devonian Duperow Formation for purposes of secondary recovery by waterflood.

In this part of the Williston, the Duperow consists of a repetitive succession of shoaling-upward carbonate cycles, each deposited under increasingly restricted conditions on a shallow marine shelf. Production at Ridgelawn occurs from dolomites within one of these cycles, cycle IIIa. Three separate, laterally continuous porosity zones (here termed a, b, and c, from lowest to highest) are recognized and mapped individually in the field. The reservoir has a lensoidal geometry; porous dolomite thins and grades laterally into tight carbonate. The Duperow pool at Ridgelawn is a solution gas drive reservoir.

Computer log analysis of the Duperow pay interval indicates an average true porosity of 11.8% and an average initial water saturation of 17.7%. Net pay, defined as greater than 5% crossplot porosity, averages 16.6 ft across the field. Petrographic analysis and log calibration suggests that different facies in each of the three porosity zones were preferentially dolomitized to create reservoir-quality rock; each is now a sucrosic dolomite with intercrystalline porosity. Porosity can be occluded (most often in the upper two zones b and c) by both calcite and anhydrite cements. The lowermost zone, a, is related to a stromatoporoid/coralline bank, and has excellent but highly variable porosity and permeability. The two upper zones, b and c, are more finely crystalline dolomite and represe t shallower water depositional facies. Maps for each zone, including porosity, porosity-feet, net pay, and water saturation were constructed and used for equity determination in the unit. Knowledge of Duperow reservoir facies can be useful in both exploratory and development projects. At Ridgelawn, the reservoir description was input into a Cray-based reservoir model along with production data to predict future reservoir performance. This model indicates secondary recovery of about 4.5 MMBO, or about 30% of original oil in place.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990