Most hydrocarbons in nonmarine Cretaceous rocks of the Western Interior occur in fluvial-channel sandstones that were deposited on the western side of the seaway by meandering and anastomosing streams. Channel sandstones surrounded by fine-grained organic-rich rocks provide stratigraphic traps for gas and oil. Reservoirs in the Cut Bank Sandstone Member of the Kootenai Formation are examples of oil production from fluvial rocks. The upper Mannville Group of Alberta and Saskatchewan contains heavy oil and tar, the degraded remnants of oil that migrated updip from the basin to the west. Reservoir properties in these sandstones are excellent because of their quartzose composition and low-intensity thermal history. In most cases, gas reservoirs in channel sandstones, such as those of the Green River Basin, have had a more intense thermal history and have low permeability. Low permeability results from porosity loss and from increases in internal surface areas of the reservoirs due to clay formation. Because authigenic clays in sandstones are different from the detrital clays of shales, the effects of clays in sandstones cannot be extrapolated from shales. Porosity reduction in tight gas reservoirs results mainly from ductile grain deformation in lithic sands and from quartz cementation in more quart-rich sandstones. Secondary porosity is ubiquitous, but volumetrically small and clay-filled. In low-permeability reservoirs, measured permeability is greatly reduced by water saturation and confining pressure.