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Facies succession, stratigraphic occurrence, and paleogeographic context of conglomeratic shorelines within the Falher "C", Spirit River Formation, Deep Basin, west-central Alberta

Dept. of Geology, University of New Brunswick, P.O. Box 4400, Fredericton, NB E3B 5A3

S. George Pemberton

Ichnology Research Group, Dept. of EAS, 1-43 E.S.B., University of Alberta, Edmonton, AB T6G 2E3

Thomas F. Moslow

Midnight Oil and Gas Ltd., 1000, 140 - 4 Avenue SW, Calgary, AB T2P 3N3

The Falher "C" is a Lower Cretaceous transgressive-regressive sequence that contains mixed coarse-grained sandstone and conglomerate gas reservoirs in the Deep Basin of west-central Alberta. A total of 11 facies were identified in cored intervals of the Falher "C" and are further grouped into five facies associations representative of deposition in an offshore-marine through continental spectrum. Coarse-grained sandstone and conglomerate facies of high reservoir potential comprise facies association two (FA2), and were deposited within wave-dominated upper shoreface and foreshore settings. Finer grained non-reservoir facies (mudstone, siltstone, and fine-grained sandstone) were deposited within storm-dominated lower shoreface (FA1), tidal inlet (FA3), back-barrier (FA4), and coastal plain (FA5) environments.

Several significant stratigraphic surfaces have been utilized to further sub-divide the Falher "C" into three parasequences termed C1, C2, and C3. In ascending order of stratigraphic occurrence these surfaces are defined as i) TSE1, ii) RSE, and iii) TSE2. The lower and upper contacts of conglomerate-rich parasequence C2 correspond to a regressive surface of marine erosion (RSE), and the youngest transgressive surface of marine erosion (TSE2). From correlations made between cored intervals and calibrated well logs, it can be recognized that reservoir-quality conglomerates are stratigraphically restricted to Twp. 67.

A paleogeographic model constructed for the Falher "C" demonstrates the distribution of important environments of deposition at four consecutive depositional intervals corresponding to distinct changes in sea level and sediment supply to the Falher shoreline. Initially, the shoreline underwent transgression in response to a relative rise of sea level whereupon parasequence C1 began to prograde northward. Transgression and progradation was followed by a relative fall of sea level and increased sediment supply that brought about a forced regression and deposition of the reservoir-bearing conglomeratic parasequence C2. Progradation of parasequence C3 was established shortly after a second period of transgression across the study area. Continued normal regression in response to progradation of parasequence C3 northward of the study area maintained aggradation of coastal-plain and deposition of the capping nonmarine C4 unit. The results of this study serve to enhance our present understanding of the scale, distribution, and three-dimensional predictability of conglomerate gas reservoirs in the Deep Basin of Alberta.