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Facies Associations of Humid Terminal Splays in the Distal DFS Model and Implications for Reservoir Connectivity: A Case Study of the Raton Formation

Abstract

The high accommodation fluvial strata of the Cretaceous-Paleogene Raton Formation in Colorado are characterized by fine grained floodbasin dominant deposits, with discrete channel bodies, and poorly understood laterally extensive thin sandstone beds. Few recently published studies have focused on the facies associations of these strata, and previous publications have been vague in addressing the geometric relationships and internal characteristics of thin sandstone beds found within the Raton Fm. Through the use of outcrop and core data analysis, we propose that the facies of the Raton Formation are consistent with that of the distal deposits of a distributive fluvial system. Channel bodies and valley fill complexes of the Raton Fm. are discrete, and composed of fine to medium grained sandstones. Paleosols are predominantly poorly drained and weakly developed, defined by medium to dark gray clay rich mudstones, with Fe concretions, well preserved organics, slickenlines, and rhizoliths. Coals are bituminous to subanthracitic, interpreted as forming in low lying floodplain swamps. Thin sandstone beds are fine to medium grained, extensive, and occur between poorly drained paleosols or coals, exhibiting vegetation induced sedimentary structures (VISS). These are interpreted as humid terminal splay deposits that occurred on a forested floodplain, with VISS representing the burial of trees during deposition. The presence of poorly drained paleosols, coals, discrete channel bodies, and humid terminal splay deposits are all consistent with the facies expected to be found in the distal portion of a DFS. High accommodation fluvial reservoirs are commonly overlooked as petroleum reservoirs due to their presumed low connectivity and low net to gross; however, terminal splay sandstones have potential to increase the production potential of these reservoirs. In outcrop terminal splays are commonly associated with organic rich and coaly deposits, juxtaposing potential reservoir with source rock. Additionally, valley fill complexes and channel sandstones incise into terminal splays creating permeability pathways, increasing reservoir capacity between otherwise discrete sandy reservoir elements. Understanding the facies characteristics and geometric relationships of the terminal splay within a distal DFS context could thus prove key in enhancing production from high accommodation fluvial reservoirs, and serve as the backbone for the thin-bed fluvial play.