ABSTRACT

The Glauconitic Sandstone reservoir in the Countess "D" Pool (Twp. 18, Rge 15, W4M), is currently in the later stages of secondary recovery by waterflood. As part of a detailed reservoir analysis to determine the "D" Pool's enhanced recovery potential, the geology of the Countess Field has been evaluated.

Throughout the life of the reservoir, relatively simple geological models have been used to describe the geometry of the reservoir sands in the "D" Pool area. However, owing to small-scale perturbations in production/injection history during 25 years, it became evident that a more complex geological model was needed to describe the pool.

Results of a detailed core study indicate that the productive interval comprises two major lithofacies. Lithofacies I is composed of light grey to brown, fine to medium grained, well-sorted quartzarenites with a slightly fining-upward grain size distribution. The bases of the sand bodies within this lithofacies are erosional, while the top grades into coal. Tidal couplets, mud drapes reactivation surfaces, flaser beds and trace fossils are observed. Lithofacies I is interpeted as a meso- to macrotidal estuarine deposit.

Lithofacies II comprises dark grey to light brown, fine to coarse grained, poorly-sorted litharenites. The bases of sand bodies within this lithofacies are erosional, with abundant rip-up clasts. Near the top of the lithofacies surface, the sandstone grades upward into floodplain sediments. High angle trough crossbeds with carbonaceous laminae on the foresets, lag deposits, rip-up clasts, and a fining-upward grain size occur commonly. Absent are tidal couplets, reactivation surfaces and bioturbation. Lithofacies II is interpreted as a fluvial meandering channel deposit.

Stratigraphic correlation of wireline logs supports the interpretation that the Glauconitic estuarine facies has eroded pre-existing Upper Mannville sediments, sometimes to the Ostracod Beds, and was later eroded by the fluvially-dominated facies. Both lithofacies are of good reservoir quality. However, the presence of erosional boundaries between the lithostratigraphic units may obstruct flow patterns, resulting in heterogeneous production behaviour. Through proper delineation of the two lithofacies and adequate modelling of barrier effects at erosional contacts between the units, a detailed geological model, necessary for EOR purposes in the "D" Pool, has been achieved.

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