Sedimentology of the Sparky Formation, Lower Cretacious (Albian), Wainwright heavy oil pool, east-central Alberta

Abstract

Much of the 2.5 billion barrels of recoverable reserves in the LLoydminster heavy oil area lie within the Sparky Formation. A study of the Sparky Formation in the Wainwright heavy oil pool was conducted to provide detailed sedimentologic information _to enhance further development of this economically important reservoir. The 20-30 m thick Sparky study sequence is divided into two informal members: the Lower and Upper Sparky. The Lower Sparky member has a "layer-cake" stratigraphy and consists of three subdivisions (in ascending order): Basal Sparky, Sparky 'D', and Sparky 'C'. The Sparky 'D' and Sparky 'C' constitute two distinct, 4-7 m thick, coarsening-upward sequences comprising shale-to-siltstone and shale-to-very fine sandstone, respectively. In each sequence, biogenic structures are succeeded up-section by inorganic sedimentary structures. Ichnofacies recognized in these two sequences, include Zoophycus(?), cruziana, Skolithos, and Psilonichnus. Physical sedimentary structures include graded-laminated beds, convex ripples, low-angle inclined stratification (hummocky cross stratification?), and wave ripples. In contrast, the 10-15 m thick Upper Sparky member consists of a highly variable three-dimensional distribution of mudstones, siltstones, sandstones and coals. Two major Upper Sparky, 20-30 m thick, sandstone-dominated, channel deposits locally truncate the Lower Sparky member. The Sparky sequence is capped by the regionally-extensive, 2-3 m thick Sparky Coal. The two Lower Sparky coarsening-upward sequences are interpreted as thin, progradational, brackish-water, delta lobe deposits whose sediments were cleaned and sorted by storm processes. Each sequence is truncated by a flooding surface and overlain by a thin, transgressive , brackish-water shale unit deposited within a bay following lobe abandonment. The Upper Sparky is interpreted to have formed in a mosaic of lower delta plain bay, marsh and swamp environments. Upper Sparky channels are interpreted as deltaic distributaries. In the Wainwright pool, the Sparky succession therefore comprises three stacked delta lobe cycles interpreted to have formed via progradation of a shallow-water deltaic complex into the Cretaceous epicontinental Boreal Sea. Each lobe prograded further than its predecessor across a slowly subsiding low-gradient shelf. Following abandonment of the entire deltaic complex, peat-forming environments were established resulting in the development of the Sparky Coal.