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.
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Patterns of Sedimentation, Diagenesis, and Hydrocarbon Accumulation in Cretaceous Rocks of the Rocky Mountains
In the Rocky Mountains from western Canada to Mexico, Cretaceous rocks are major sources and reservoirs for oil and natural gas, accounting for about 40% of the cumulative production to date. Resources estimates indicate that large amounts of hydrocarbons remain to be discovered in these rocks. The purpose of this volume is to examine the relationship of reservoir quality, resource evaluation, and exploration strategy to depositional environment, thermal maturity, and diagenetic history of Cretaceous rocks in the Rocky Mountain area. Chapters deal with the general characteristics of the Cretaceous Western Interior Basin and seaway, the application of organic geochemistry to hydrocarbon occurrence and exploration, principle aspects of diagenesis that affect reservoir quality and source-rock potential, and the five main depositional facies which can be recognized from west to east across the basin.