Diagenesis includes all physical and chemical changes undergone by sediments from the time of deposition until that of metamorphic transformation. Hydrocarbon source-bed potential and limestone and sandstone reservoir quality are controlled by diagenesis. During the Cretaceous, organic matter exceeded oxygen supply and sediments served as a sink for organic carbon in the Western Interior. In marine sediments with an excess of organic matter, early diagenesis is a depth-zoned process with three principal metabolic zones: (1) aerobic oxidation, (2) sulfate reduction, and (3) methane generation. Products of these zones may be used to reconstruct the early burial history of the sediment. Shales in the Western Interior have a composition similar to Tertiary mudstones of the Gulf Coast, with a clay suite dominated by mixed-layer illite-smectite (I/S). These I/S clays have undergone temperature-related burial diagenesis. In the principal transformation, smectite was converted to illite through a series of discrete steps, with the addition of K+ and the loss of Si4+. Reactions have occurred within the organic matter of the Cretaceous shales simultaneously with the clay transformations. Waters expelled from the shales during early diagenesis and during thermal transformations of organic matter and clays controlled water chemistry in the basin. Most reservoir rocks are sandstones made up of derived from orogenic highlands. Diagenesis in these reservoir sandstones was controlled by the starting composition and by fluids introduced from adjacent shales. As a group, the Cretaceous sandstones have low porosity and permeability. Most porosity loss resulted from compaction of ductile grains and from quartz
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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.