ABSTRACT: Geology and Reservoir Description of a Dolomitized Upper Devonian Reef, Westerose Field, Alberta

L. B. McNamara, N. C. Wardlaw, M. McKellar

The Westerose field produces hydrocarbons from a dolomitized Upper Devonian Leduc reef. The reef is 1.6 km wide, 5.5 km long, and up to 175 m thick. Core description, statistical analysis of whole core data, well logs, and pressure buildup data were integrated in the development of a reservoir model.

In contrast to well-known limestone reservoirs in western Canada, the organic reef margin facies and reef interior facies are not strongly differentiated. The reef interior does not contain laterally extensive barriers to vertical fluid flow. The secondary pore system dominates porosity. The large solution vugs and fossil molds are well connected through the dense matrix by a vertical fracture system. Porosity and permeability increase from the reef flank to the interior and are optimum within the lower reef interior. The reservoir model, interpreted from pore system distribution and supported by reservoir pressure response, is a disordered arrangement of permeability blocks.

Cumulative probability plots indicate that the horizontal permeability data set is top truncated, with up to 35% of the highest permeability values not represented in the data set. Examination of a Leduc equivalent outcrop yielded data on pore-size frequency and porosity at scales larger as well as smaller than core diameter. The sample size necessary to make representative measurements of reservoir properties can be estimated from the outcrop analysis.

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