An Unconventional Facies Classification Scheme for from Unconventional Oil Reservoir: Interpreting Tidal Paleoenvironments in the Heterolithic Bluesky Fm., Peace River, Alberta

Duncan A. Mackay and Robert W. Dalrymple
Department of Geological Sciences, Queen's University, Kingston, ON, Canada

Modern tidal systems have remained poorly understood until very recently, forcing workers in ancient deposits to assign low confidence to interpretations of ancient tidal environments. The Lower Cretaceous Bluesky Fm. is used here as one such example of a tidal deposit where difficulties in the correct identification of ancient tidal environments cascade down into extremely low-confidence subsurface correlations that impact the robustness of subsurface geological models.

Lithologies in the Bluesky Fm. are typical of tidal deposits and include: heterolithic mudstones, heterolithic sandstones and monolithic sandstones. They span continua of sandstone-to-mudstone ratios and of the thickness of sandstone - and mudstone -beds. As such, an unmanageably large number of discrete lithofacies could be identified using traditional classification schemes. Instead, a reductionist approach is used, focusing on the following characteristics: (1) sandstone-to-mudstone ratio; (2) the thickness and type of mudstone beds; (3) sandstone grain size and sedimentary structures; and (4) the degree and type of bioturbation. Each of these criteria, used in isolation, produces its own set of interpreted architectural elements and/or proximal-distal positions within tidal deltaic or estuarine systems. Comparison of the four independent interpretations allows one to reconcile combinations that are mutually exclusive, thereby promoting combinations that support one another.

Based on this new classification scheme, the Bluesky Fm. is subdivided into tide-dominated deltaic and tide-dominated estuarine deposits that include: tidal-fluvial channel-bar deposits, mixed sand-mud tidal flats, compound dunes formed at distributary mouths, delta-front sub-tidal flats, prodelta, outer estuary compound dunes, sandy tidal flats and sub-tidal channel bars.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas