REGIONAL STRATIGRAPHY, SEDIMENTOLOGY AND PETROLEUM GEOLOGY OF THE GRAND-RAPIDS FORMATION, MANNVILLE GROUP, NORTHEASTERN ALBERTA

In northeastern Alberta, the Grand Rapids Formation comprises the uppe r 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, M cLaren and Colony formations of the Lloydminster area. The Grand Rapid s sandstones are therefore falling and lowstand sea level deposits whi ch offlap from surfaces equivalent to updip unconformities. Six allost ratigraphic units defined on the basis of regressive surfaces of erosi on cut at the bases of shorefaces and transgressive surfaces at their tops have been defined. Each offlapping allostratigraphic unit consist s in the south of thin channel and crevasse-splay sands grading northw ard into major shoreface sands. The largest sandstones are very extens ive, 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 clino-forming and terminating northward by downlap. They thin southward with abrupt facies terminations into finer-graine d deposits. Alongshore, individual shoreface sands onlap against other sands discharged from different incised valleys or, to the west, agai nst finer-grained Upper Mannville successions. Relative sea level fluc tuations caused valley incision and amalgamation of shoreface sandston es. As a result of the lack of seals, the largest reserves of hydrocar bons (both bitumen and gas) have generally not accumulated in the thic kest sandstones but in the thinner uppermost sandstone which is effect ively blanketed by the overlying Joli Fou Shale. Hydrocarbons may be t rapped in the lower sandstones in situations where the intervening sha les have been preserved to form an effective seal. The bitumen general ly is trapped to the north of the major gas fields. Many of the larges t gas reserves occur in small drape structures that occur because of p ostsedimentary dissolution of Devonian salt. Smaller pools occur in st ratigraphic traps where permeable shoreface sandstones terminate updip because of facies changes.