FACIES ARCHITECTURE AND CONTROLS ON RESERVOIR BEHAVIOR IN THE TURONIAN WALL CREEK MEMBER OF THE FRONTIER FORMATION, POWDER RIVER BASIN, WYOMING

Inter-well heterogeneities influencing fluid migration in deltaic reservoirs are controlled by lateral lithofacies changes and vertical complexities such as low permeability thin-beds. Subsurface tools often cannot predict the spatial and stratigraphic organization of these architectural elements, nor their influence on effective reservoir properties and connectivity. This study integrates sedimentological, stratigraphic, and fluid simulation data to 1) document the facies architecture and depositional evolution of the Turonian Wall Creek Member (WCM) of the Frontier Formation, and 2) quantify the role of multi-scale stratigraphic heterogeneity on reservoir behavior. Upscaled permeability properties derived from fluid simulation of nested, small-scale facies models condition the observed architecture within a 500m x 715m geocellular model. Key surfaces recognized across the study area separate the WCM into three depositional sequences, each of which contain multiple parasequences that form the geomodel framework. Sequence 1 consists of a top-truncated package of river-dominated delta lobes, interpreted as highstand deposits (HST1); sequence 2 is made of wave-dominated delta sandstones deposited during subsequent highstand (HST2); sequence 3 consists of heterolithic tidal bar deposits of a tidally-influenced delta (LST). Detailed mapping of the HST1/HST2 show the spatial distribution of intra-parasequence lithofacies is largely controlled by their proximity to high energy conditions above wave-base and near distributary channels. Modelling results show that permeability of the fine-grained component within heterolithic deposits is the most critical parameter in reservoir behavior. In wave-dominated environments, relatively simple bed geometries of thin-beds induce low vertical permeability. Conversely, more architecturally complex tidal deposits maintain better vertical connectivity but limited horizontal permeability. Flow compartmentalization on any scale happens only when thin-beds are assumed to be impermeable barriers; mud drapes with lower clay content act only as flow baffles. Fine-scale heterogeneities carry through as controlling factors in geomodel (500m x715m) reservoir simulations. In the wave-dominated setting, continuous horizons of low vertical permeability facies delineate parasequence-scale flow units. Within individual parasequences, the lithofacies distribution plays an important role on effective permeability pathways and total volume in place. Results from this outcrop-to-geomodel study can be applied to WCM reservoirs in the subsurface and used as guidance to build more accurate geomodels in other basins