| In Pennsylvanian oomoldic reservoirs (Raytown Limestone equivalent, central Kansas), late diagenetic processes control porosity and permeability more than early processes associated with subaerial exposure. Cement stratigraphic studies show that early diagenesis did not enhance reservoir character significantly. Most oomoldic porosity formed penecontemporaneously with chemical and physical compaction. Late cementation reduced most porosity. Fluid inclusion and stable isotope data indicate that late cements precipitated during burial conditions by Permian refluxing brines, and by later hydrothermal fluids. After initial burial, greater permeability existed downdip, where the smaller amount of early meteoric cement allowed for compaction. Then, preferred reflux cementation downdip degraded downdip reservoirs, but later, more reflux cementation updip resulted in relatively uniform reservoir porosity. Later hydrothermal events occurred during or after Laramide fracturing, and are the most important in affecting distribution of the highest quality reservoir. Highest porosity is preserved in paleotopographically low wells where hydrothermal cements were not as prevalent. |
