A MINERALOGICAL AND GEOCHEMICAL CHARACTERIZATION OF LATE CAMBRIAN – PENNSYLVANIAN CARBONATES IN THE CENTRAL KANSAS UPLIFT, KANSAS WITH IMPLICATIONS FOR DIAGENESIS

The NW-SE trending Central Kansas Uplift is known to contain two major hydrocarbon reservoirs: the Arbuckle Group and the Lansing-Kansas City Formations. These are carbonate reservoirs containing interbedded shales and silts and were deposited in shallow to deep marine settings. Porosity is dominantly secondary, characterized by carbonate dissolution and precipitation. By using mineralogical and geochemical analyses we can discern diagenetic processes. Drill cuttings were obtained from three locations were analyzed to characterize the mineral assemblage and geochemistry of the sampled intervals.

For all three localities, our analyses reveal similar mineral assemblages, which include quartz, chlorite, illite, calcite, dolomite, feldspar, albite, and silicified material. Illite and quartz are the dominant minerals (~22 - 25 wt% and ~20 - 25 wt% respectively), and illite is interpreted to be associated with burial diagenesis of the shale intervals. Calcite is also abundant (~10 – 15 wt %), while dolomite is present mostly in small amounts (~3 wt%). However, there are zones of elevated dolomite content (<10 wt%) which represent periods of increased diagenesis of shallow carbonates due to post-burial interaction with seawater.

Major elements identified in the intervals includes calcium, magnesium, aluminum, silicon, phosphorous, potassium, and iron. Calcium is the most abundant element with an average of ~60 wt% (up to 90 wt% in some areas). Silicon is the second most abundant element (averaging ~25 wt% up to ~75 wt%) and is interpreted to be correlated with the clastic-rich portions of the sample areas. Magnesium averages ~3 wt% (with maximums of ~8%) and is found in increased concentrations in areas of dolomitization. Iron comprises ~6 wt% of the samples (up to 37%) with elevated concentrations related to oil-bearing zones. This increase is interpreted to be a result of microbial biodegradation of the hydrocarbons which formed the Fe-bearing minerals.