ABSTRACT: Pembina Revisited: The Origin and Significance of the Reservoir Architecture in Mobil-Operated Pembina-Cardium Properties

KREISA, R. D., D. J. FYVIE, and S. D. JOINER, Mobil Oil Canada, Calgary, Alberta, Canada

Mobil Oil Canada discovered the Pembina field in 1953 and now operates mature waterflood-units in the northeastern half of the field. A detailed understanding of reservoir quality, flow-unit geometry, pore volume, and remaining reserves is necessary to optimize the performance of these properties and as a predictive tool for possible EOR. Wire-line logs from over 1000 wells and 230 cored-well descriptions comprise the data for our geologic model, but our mythology also integrates completions and production data. Pembina produces from the Cretaceous Cardium Formation, which can be divided into two principal lithofacies groups: (1) Fine-grained sandstones and shale that comprise upward-coarsening successions from 1.5 to 8 m thick; and (2) conglomerates up to 7 m thick. The upward-coarse ing units define ten interpreted fifth-order parasequences that were deposited by the west-to-east regression of a strand-plain during falling relative sea level. Parasequence boundaries are reliable stratigraphic correlation horizons, which are critical for defining reservoir flow-units. Parasequences gradually thin and become shalier west to east and comprise a series of offlapping, broadly sigmoid-shaped units; however, at the 80 ac spacing in the field, both the sandstone and the shaley intervals of the parasequences are laterally continuous. An erosional disconformity with up to 8 m of relief toplaps the shingled parasequences. The Conglomerates were deposited as a marine-ridge complex during the transgression of the disconformity and comprise lenticular reservoir-units up to 3 km w de. They are high flow-capacity but low pore-volume facies that are thief zones for flood recovery processes.

The disconformity has a complex history. Over most of the field it is recognized as a combined sequence-boundary and "transgressive surface of erosion" (TSE), but in easternmost Pembina it is interpreted as a TSE only. Erosional relief on this surface may be due partly to scouring tidal inlets but it is largely the result of post-transgression submarine erosion. Excellent modern analogs exist for this model; however, it contrasts strongly with published explanations

for the origin of the disconformity ("E5") and overlying conglomerates. Published models appeal to multiple episodes of intrabasinal tilting, but this is not necessary. As on modern shelves, the ridge geometry is partially deposition and partially erosional. The TSE (ravinement surface) underlies the gravel ridges but crops out on the margins of the ridges. The asymmetry of these features is largely inherent to ridge forming processes. The ultimate reservoir geometry was also strongly a function of the erodability at the sea floor of the various lithologies.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)