A REFINED LITHOSTRATIGRAPHY FOR THE RATON FORMATION: IMPLICATIONS FOR ALLUVIAL HETEROGENEITY, COAL-FORMING ENVIRONMENTS AND COAL BED DISTRIBUTION – RATON BASIN

Coal-bearing late Cretaceous and Paleocene sedimentary strata within the Raton Basin are amongst the most prolific coalbed methane (CBM) reservoirs in the Rocky Mountains. The established coal bearing stratigraphy is divided into two productive formations, the Vermejo Formation (Kv) and the overlying Raton Formation (Tr). Despite extensive exploration for, and development of the Raton Basins coal resource, depositional patterns and resulting coalbed distribution remain poorly constrained outside of proprietary industry datasets. This paper details outcrop and subsurface data in order to refine our understanding of the Raton Formation, and the dynamics of humid climate fluvial depositional systems.

The Raton Formation is subdivided allostratigraphically to form four generic, basin-scale depositional sequences identifiable due to fluvial heterogeneity, coal development and gross facies architecture. The Raton Conglomerate (Tr-C) comprises a 1-30m thick non coal-bearing incised valley-fill succession, composed of coarse-grained extraformational clasts. The unconformable base to this deposit constitutes a regional sequence boundary, while the topmost beds grade into the coal-bearing Lower Raton facies (Tr-L). Although coals at this level are well developed the limited thickness of Tr-L (<20m) precludes significant net coal thicknesses. The Middle Raton facies (Tr-M) is characterized by an 40-80m thick succession of fluviatile strata, transported via large truck river channel-forms, within a well-drained floodbasin setting. These conditions limited both swamp development and peat preservation; consequently Tr-M has restricted coal assemblages. By contrast, the Upper Raton facies (Tr-U) show good coal bed development grouped into five parasequences (Tr-U1 to Tr-U5), over a 200-300m thick interval. Floodbasin coal groups are interbedded with sand bodies deposited by meandering and braided fluvial systems. These strata are ordered to form disconformity-bounded alluvial-cycles that reflect variation in hinterland uplift and basin subsidence (sediment supply and accommodation space). By refining the existing Raton Formation stratigraphy, a new allostratigraphic model has been developed which allows the evaluation of in-situ coal resource and a predictive means for CBM development.