Abstract

Hydrocarbons have been produced from the Grand Rapids oil sands for decades; however, large volumes still remain in place, mostly due to the extreme complexity of the reservoirs. Detailed examination of wireline logs and cores from 1127 wells within the 400 km² Manatokan Field (Twp 62–63; Rge 4–5 W4M) are used to characterize the deposits, define the stratigraphic framework, establish the depositional architecture, and infer the mechanisms and parameters controlling the deposition of the formation. Twenty-one facies are identified based on detailed core analysis (sedimentology, ichnology, geochemistry), and are organized into five recurring facies associations: 1) wave-dominated shoreface deposits; 2) delta-influenced shoreface deposits; 3) interdistributary bay deposits; 4) marine-influenced fluvial deposits; and 5) coastal plain deposits. Shoreface deposits, which form correlatable 3–30 m-thick coarsening-up cycles, are classified as wave-dominated fluvial-influenced to fluvial-dominated wave-influenced, potentially tide-affected (Wft to Fwt). Marine-influenced fluvial deposits are mostly emplaced as point bars within 100–2000 m wide and 3–30 m deep fluvial bodies distributed within the strata.

The established stratigraphic framework subdivides the formation into 11 parasequences, 7 parasequence sets, 7 stratigraphic levels containing fluvial bodies and 2 depositional sequences capped by sequence boundaries. Those observations contrast with previous interpretations that recognized seven sequences boundaries capping each of the parasequence sets. However, stratigraphy of the Grand Rapids Formation is primarily controlled by eustacy: sequences and parasequence sets are primarily influenced by long-period moderate-amplitude (1.2–2.4 m.y.; 15–30 m) and short-period low-amplitude (0.4 m.y.; <10 m) glacio-eustatic sea-level cyclicity, respectively. Individual parasequences are likely controlled by short-period allocyclic (100 to 20 kyr climatic variations) and/or autocyclic parameters (delta lobe switching every 5 to 2 kyr) that affect regional and local sediment flux. These parameters and different timescales explain the peculiar distribution of reservoirs in the field with small and isolated geobodies.