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The Jurassic (Bajocian) Rock Creek Member of West-Central Alberta
Quartzarenites and coquinoid sandstones of the Jurassic (Bajocian) Rock Creek Member of the Fernie Formation form significant reservoirs for light oil and wet gas in west central Alberta within and near the Pembina Field.
The Rock Creek is unconformity bounded, with belemnite-bearing marine shales and siltstones of the Poker Chip Shale found below and younger Jurassic sediments or Lower Cretaceous clastic rocks of the Mannville Group located above. Rock Creek sequences generally contain a widespread basal coquina comprised of abraded bivalve shells within a variable quartz sandstone matrix. This is progressively overlain by complexly interbedded deposits of fine grained quartzarenites, minor muddy deposits and coquinoid units. The coarse siliciclastic beds generally fine upwards and display cross beds and current ripples with abundant muddy drapes and flasers. A marine trace fossil assemblage with characteristics of both the Cruziana and Skolithos ichnofacies is present. In general, individual rock units display large numbers of one or few ichnogenera, however, the overall Rock Creek package supported a diverse infauna.
The uppermost zone, which commonly comprises the main reservoir unit, is formed of flaser-bedded quartzarenites. These deposits are highly productive due, in part, to the presence of pervasive open fractures which emanate from the apices of pseudo-stylolites formed of muddy compacted flasers and interbeds.
The widespread extent of arenites and coquinas, the common presence of current-generated structures, the trace fossil assemblage and the lack of subaerial indicators leads to the interpretation that the Rock Creek was deposited upon a marine shelf impacted by both tidal and geostrophic flows.
The blanket nature of potential reservoir strata precludes the occurrence of stratigraphic traps except possibly near the regional subcrop to the northeast and where deep incisions filled with Lower Cretaceous non-reservoir strata occur. The presence of many separate pools in the absence of facies changes or major structural breaks is explained in part, by relating fractured units to compaction and the current stress field of the basin.
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