Lithofacies distribution and depositional environment in the Lower Cretaceous McMurray Formation, BlackGold Lease, northern Alberta: implications for geometry and distribution of oil sand reservoirs

Within BlackGold Lease located in northern Alberta,the Lower Cretaceous McMurray Formation contains the most prolificbitumen reservoirs deposited in fluvial to tidally-influencedestuarine environments. Based on core descriptions and wire-line logevaluation, this study reveals six lithofacies: cross-stratified sandstoneand mudstone-clast breccia (Lf1), sandstone-dominated IHS(Lf2), mudstone-dominated IHS (Lf3), thinly interbedded sandstoneand mudstone (Lf4), laminated mudstone (Lf5), and clean sandstonewith interbedded mudstone (Lf6). To understand the evolution ofdepositional environments vertical and lateral associations of lithofaciesare examined using well cross-sections and lithofacies slice maps. During overall rise in relative sea level, the McMurray Formationevolved through three stages of deposition: early stage representsfluvial channels with minor tidal influence, middle stage representstidally-influenced estuary with well-developed meandering channels,and late stage represents a drowning of tidally-influenced estuary. The potential bitumen reservoirs are fluvial channel sandstones inthe early stage and lower point-bar deposits in the middle stage. The fluvial channel sandstones are well stacked and correlatablebetween wells, forming sheet-like sandstone bodies that align in aSW–NE direction parallel to the inferred orientation of major channelsystems. The lower point-bar deposits consist mainly of base-ofchanneland sandstone-dominated IHS deposits. The direction ofpoint-bar migration, which is crucial in horizontal well design forbitumen production, is inferred from lithofacies slice maps. Thelateral changes in lithofacies from base-of-channel to abandonedchannel-fills through IHS deposits, shown in lithofacies slice maps,probably indicate that the point bar once migrated toward abandonedchannel-fills. Based on this lateral lithofacies trend, the dipdirection of some point-bar deposits are approximately estimatedto be southwestward or northwestward, which is oblique or perpendicularto the major channel orientation.