Petrographic and geochemical data from cores in the Wyoming Overthrust belt are used to integrate thermal maturation of the Phosphoria with timing of cementation and porosity development in the Tensleep and Madison. Vitrinite reflectance studies from the Phosphoria indicate that it contains both bitumen and kerogen and that it has already generated some hydrocarbons and retains the potential to generate more. These hydrocarbons migrated into the Tensleep during an intermediate stage of silica cementation and into the Madison prior to the last phase of dolomitization.

The observed diagenetic sequences for the underlying Tensleep and Madison, in conjunction with depositional interpretations, demonstrate that porosity generation was critically affected by both depositional environment and early diagenetic history. The Tensleep and Madison underwent dramatically different histories of cementation and porosity development. This is reflected in the present maximum of 5% porosity in the Tensleep contrasted with 20% porosity in parts of the Madison. If porosity in the Tensleep was not destroyed by early evaporitic cements, then continuous destruction of porosity by silica and dolomite cementation occurred throughout its burial history. In contrast, preliminary isotopic studies indicate porosity in the Madison was developed at temperatures of less than 35& 176; C. These regionally correlatable porous zones in the Madison which have persisted from shallow burial to present depths of 15,000 ft (4,572 m) emphasize the potential of the Madison as a hydrocarbon reservoir both now and in the geologic past.

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