Abstract

Mississippian and Pennsylvanian carbonate reservoirs in Kansas have produced over 2 billion barrels of oil and presently represent 33% of Kansas oil production. Mississippian reservoirs, deposited on a gentle sloping ramp, comprise principally mudstone to bioclastic moldic grainstone limestones, dolomites, and cherts. Postdepositional regional uplift, subaerial exposure and differential erosion of the ramp strata at the pre-Pennsylvanian unconformity resulted in paleotopographic highs and formation of reservoirs subcropping the unconformity. In contrast, oolitic packstones and grainstones are the most prolific reservoir lithofacies for the Pennsylvanian Lansing-Kansas City groups. The broad Kansas shallow shelf and oscillating sea level resulted in lateral migration of oolite shoal conducive environments and successive creation of stacked oolite cyclothems and wide geographic distribution.

Though the influence of depositional environment and facies on reservoir properties has been well studied for Mississippian and Pennsylvanian reservoir strata in the Midcontinent, there are patterns to field location and productivity that are not completely explained by the distribution of facies. Ongoing studies, of which this is a part, indicate that regional- and local-scale structures (e.g. faults, fractures, lineaments) segmented the broad shelf and shelf-margin areas in Kansas, and were re-activated throughout the Paleozoic, acting as important influences on depositional patterns, paleotopography, weathering intensity, and movement of fluids. The correlation between structural features and reservoir properties argues that the contribution of re-activated (and syndepositional) structural elements to reservoir character may have been critical to reservoir development.

Mississippian and Pennsylvanian fields provide examples to examine the influence of faulting and fault reactivation on deposition, diagenesis, and hydrocarbon accumulation for different times and at different locations on the shelf-shelf margin. Examples include the Mississippian shelf margin (Spivey-Grabs and Glick fields), Pennsylvanian mid-shelf (Dickman field), and Pennsylvanian lower shelf (Victory field). Given the importance of structure to reservoir properties, basement configuration and structural elements might serve as a template for understanding and predicting areas of segmentation and oil accumulation.