Abstract

In northeastern Alberta, the Grand Rapids Formation comprises the upper part of the regressive Upper Mannville Group and consists dominantly of thick sandstones. Regional correlations to the south show that the sands were fed from a number of incised valleys cutting the Waseca, McLaren and Colony formations of the Lloydminster area. The Grand Rapids sandstones are therefore falling and lowstand sea level deposits which offlap from surfaces equivalent to updip unconformities. Six allostratigraphic units defined on the basis of regressive surfaces of erosion cut at the bases of shorefaces and transgressive surfaces at their tops have been defined. Each offlapping allostratigraphic unit consists in the south of thin channel and crevasse-splay sands grading northward into major shoreface sands. The largest sandstones are very extensive, reaching dimensions of 100 km by 200 km by 50 m thickness because they are composite, consisting of several individual shoreface sands as well as incised valley fills. Individual sands within these larger composite bodies extend seaward from a few kilometres to a few tens of kilometres before clinoforming and terminating northward by downlap. They thin southward with abrupt facies terminations into finer-grained deposits. Alongshore, individual shoreface sands onlap against other sand discharged from different incised valleys or, to the west, against finer-grained Upper Mannville successions. Relative sea level fluctuations caused valley incision and amalgamation of shoreface sandstones. As a result of the lack of seals, the largest reserves of hydrocarbons (both bitumen and gas) have generally not accumulated in the thickest sandstones but in the thinner uppermost sandstone which is effectively blanketed by the overlying Joli Fou Shale. Hydrocarbons may be trapped in the lower sandstones in situations where the intervening shales have been preserved to form an effective seal. The bitumen generally is trapped to the north of the major gas fields. Many of the largest gas reserves occur in small drape structures that occur because of postsedimentary dissolution of Devonian salt. Smaller pools occur in stratigraphic traps where permeable shoreface sandstones terminate updip because of facies changes.

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