Late Diagenesis: the Key to Pennsylvanian Regressive Oomoldic Reservoirs (Central Kansas)

Poteet, Jessica E.¹, Evan K. Franseen², Robert H. Goldstein² (1) Univeristy of Kansas, Lawrence, KS (2) University of Kansas, Lawrence, KS

Analyses of regressive oolites typically have emphasized the importance of subaerial diagenesis in controlling distribution of reservoir porosity. This study, of Pennsylvanian oomoldic reservoirs in Kansas (Raytown Limestone equivalent), indicates that late processes control porosity more than early ones associated with subaerial exposure. The uppermost one-half meter contains evidence for subaerial exposure, including fenestral fabrics, rhizomolds, and meniscus cements. Cement stratigraphic studies show that early diagenesis associated with subaerial exposure did not enhance reservoir character significantly. Intervals of oolitic grainstone affected by subaerial diagenesis retain low porosity and permeability (3 to 10%; 0.1 to 13 md). Other oolitic grainstone preserves the highest porosity and permeability (25 to 32%; 15 to 300 md).

The majority of oomoldic porosity creation was penecontemporaneous with burial-related chemical and physical compaction. Late cementation reduced most porosity. A late event of dissolution occurred after stylolitization, and was followed by a late phase of calcite and dolomite cementation. Fluid inclusion microthermometry and stable isotope data indicate that the majority of cements were precipitated during normal burial conditions by Permian refluxing brines, and by later hydrothermal fluids. These late events are the most important in affecting distribution of the highest quality reservoir. All wells had approximately the same relatively low porosity after reflux cementation (25-30%). After later dissolution, porosity rose to 45-50%. Partial cementation with the latest cements led to reservoirs with 20%-32% porosity. Highest porosity was preserved in paleotopographically low wells where the latest cements were not as prevalent, indicating that models for reservoir character in regressive oolites can be improved by incorporating structural (including fracture, fault data) and burial history data that can help identify pathways and conduits for hydrothermal fluids.