Depositional Environment and Diagenesis of the Teapot Sandstone, Southern Powder River Basin, Wyoming

The Teapot Sandstone is interpreted as a regressive, wave/fluvial dominated deltaic sequence, which prograded eastward into the Cretaceous Seaway. Marine lithofacies coarsen upward from bioturbated offshore siltstone to nearshore sandstone with large, pellet-lined Ophiomorpha. The overlying well-sorted, horizontally-laminated, foreshore sandstone exhibits ridge and runnel topography. Marine foreshore sandstone is overlain by complexly interbedded sandstone and carbonaceous shale in stacked, lining-upward sequences of the delta plain. Rootlets and contorted beds are common. Fining-upward units are interpreted as abandoned channels. Coarsening-upward sequences are interpreted as interdistributary bay or lagoonal deposits. Capping the sequence is a thick, crossbedded, fluvial section consisting of levee, point bar, and channel sand deposits. Slumped beds, intraformational basal conglomerates, and minor eolian ripple laminations are present in fluvial sandstone. The Teapot Sandstone has a complex diagenetic history. Siderite and framboidal pyrite formed early in the diagenetic sequence at shallow depths of burial under anaerobic conditions. Pore-filling kaolinite, chlorite, and quartz overgrowths formed coevally following dissolution of relatively unstable framework grains. Poikilotopic calcite is locally abundant and extensively replaces framework grains. Depositional facies exert strong control of diagenetic patterns. Kaolinite occurs predominantly in fluvial sandstone. Chlorite is restricted to marine facies and calcite is further restricted to well-sorted foreshore marine sandstone. Quartz overgrowths occur only in relatively well-sorted sandstone, whereas pyrite and siderite are common in shaly sandstone and siltstone. Nearshore marine and fluvial sandstones are potential hydrocarbon reservoirs, although authigenic clays have significantly reduced permeability. Reservoir potential of we/I-sorted foreshore marine sandstone was destroyed by pore-filling calcite cement. Delta plain deposits are too thin, discontinuous, and poorly-sorted to form significant reservoirs.