ABSTRACT

The Jameson Reef field of the Middle Pennsylvanian (Desmoinesian) Strawn Formation is an elongate, strike trending lime mud mound that occurs within the regional Strawn Reef Trend of west-central Texas. It consists primarily of lime wackestones, mudstones and packstones with less common grainstones and Chaetetes framestones.

Microfacies analyses indicate that, the mound was deposited on an open water carbonate ramp characterized by mudstones in downdip areas and grainstones in updip locations. The shape of the buildup was controlled by marine hydrodynamics rather than by biogenic growth patterns.

The sediments were exposed to early marine, freshwater and late subsurface diagenesis. Marine diagenetic effects are limited to micritization. Leaching occurred in the freshwater environment and produced intergranular, intragranular, moldic and vuggy porosity. During burial of the mound, cementation was induced which partially infilled pore spaces. Dolomitization followed early calcite cementation after further transgression. It is interpreted to have formed in the mixing zone. Diagenetic processes continued into the subsurface environment; the most prominent features are pressure-solution effects, including styolites. The late burial pressure-solution effects diminished porosity significantly. Wackestone facies in the mound exhibit more reservoir porosity than other facies because solution enhanced porosity was not as effected by late cementation as the grainier rocks.