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New Insights Into Falling-Stage Delta Complexes Using Virtual Outcrop Analysis: Three-Dimensional Data From the Turonian Ferron Sandstone of Southern Utah

Abstract

Subsurface reservoir models contain many uncertainties due to sparse subsurface datasets that extrapolate surfaces from limited data. Outcrop analogues are often used as a source of detailed, spatially-resolved data to populate three-dimensional geocellular models. Collecting relevant outcrop data can prove challenging when attempting to account for lithofacies complexities and three-dimensional geometries. Our study overcomes this issue by using two LiDAR (light detection and ranging) derived virtual outcrop models (VOMs) to serve as the foundation for an analogue reservoir model. One of the two VOMs displays a 1.5 km cliff face trending north-south, the second VOM is positioned 0.5 km down depositional dip and exhibits truly three-dimensional exposure along strike and dip. The fluvial-deltaic Turonian Ferron Sandstone Member of Mancos Shale in south-central Utah, USA is an ideal target for analogue research due to the spectacular exposure. The Ferron comprises an offlapping sequence set composed principally of forced regressive deposits in which lithofacies representing delta plain, distributary channel, mouth bar, and proximal-medial-distal-delta front settings are preserved in a series of top-truncated stratal cycles. New findings from this research document complete cycles of sea level change over a few meters of vertical section. Three facies variants overlie a sequence boundary; incised fluvial channel, delta plain heterolithic strata, and marine-influenced distributary channel. Updip, the proximal expression entails three stacked channel stories (6 m thick, 150-200 m wide) of medium-grained sandstone separated by thin, recessive bioturbated partings. Thin, fine-grained sandstone bodies (0.7-1.5 m thick) that are laterally extensive are the more distal expression of the same, marine-influenced terminal distributary channel downdip. The cycle is capped by a transgressive surface overlain by a thoroughly bioturbated fine-grained, shoreface sandstone. Using industry standard reservoir modeling software as the visualization platform, true bed thickness, stratal geometries, and lithographic relationships are correlated between the two VOMs. An improved, truly three-dimensional model of the Ferron-Notom Delta Complex provides additional insight into modeling systems with variability and heterogeneous strata for geocellular reservoir models and lithofacies prediction of fluvial-deltaic petroleum reservoirs.