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Twenty-nine sets of sandstone and stratigraphically adjacent, fine-grained, organic-rich samples representing diverse depositiona! environments (fluvial, coastal, paludal, and marine) have been obtained from cores of the Mesaverde Group within the Piceance Basin, Colorado. These units have been buried to a sufficient depth that organic maturation has progressed to an advanced state (vitrinite-reflectance values range from 1.0 to 2.2 in the study area). The authigenic-mineral paragenesis and organic data are integrated with burial and thermal-history modeling to place diagenetic events and hydrocarbon generation into a temporal framework. Three phases of diagenesis (early, late, and post-hydrocarbon) are characterized on the basis of petrography, XRD, and geochemistry of authigenic phases.

Clay-mineral distributions of sandstone/mudstone pairs indicate that mixed-layer illite/smectite dominates mudstone mineralogy, although subordinate amounts of chlorite are observed in the marine interval. Sandstone mineralogy also includes mixed-layer clays and chlorite, with the addition of kaolinite. Additional aspects of sandstone diagenesis examined include feldspar albitization and dissolution, and carbonate mineralogy.

Organic analyses indicate the presence of a type III kerogen component in all samples. The paludal interval contains the most oxygen-rich organic material and exhibits the highest sandstone porosities. Water-soluble organic compounds released from organic material as burial progresses have been invoked as agents affecting the course of mineral diagenesis in clastic sediments and sedimentary rocks. Our results indicate the potential for the field evaluation of the effects of thermal maturation of organic matter on the diagenesis of closely adjacent sandstones.

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