THE EFFECTS OF HIGH-FREQUENCY CYCLICITY ON RESERVOIR CHARACTERIZATION OF THE MISSISSIPPIAN LIMESTONE, ANADARKO BASIN, KINGFISHER COUNTY, OKLAHOMA

Mississippian-aged limestones along the northern edge of the Anadarko basin in north-central Oklahoma and southern Kansas contain considerable amounts of hydrocarbons and have been exploited through vertical drilling for more than 50 years. Recent technological advancements have caused a shift in exploitation from vertical to horizontal drilling and completion techniques. Despite the increased level of drilling activity, subpar well performance persists due to a lack of understanding of the reservoir heterogeneities associated with the vertical and lateral distribution of depositional facies using current subsurface mapping techniques.

The “Mississippian limestone” is characterized by a hierarchical stratigraphy of sequences (100s of meters thick), high-frequency sequences (10s of meters thick) and high-frequency cycles (few meters thick) caused by fluctuations in eustatic and relative sea level due to Milankovitch-band cyclicity. Detailed facies analysis using cored intervals of the “Mississippian limestone” suggests deposition on a distally steepened carbonate ramp. The vertical stacking patterns of depositional facies defines high-frequency sequences and cycles (probable 4^th and 5^th-order) within a shoaling-upward succession. From base to top within an ideal sequence, the shoaling-upward succession of facies consists of argillaceous to weakly calcareous and slightly burrowed mudstones and wackestones followed by progressively higher-energy environments of deposition indicated by traction-laminated and/or more heavily bioturbated wackestones, packstones and grainstones.

Deviations from this ideal shoaling-upward succession are hypothesized to be caused by high-frequency, Milankovitch-band cyclicity and are the primary controlling factor in the compartmentalization of hydrocarbon reservoirs. Preliminary core analyses suggest that potential hydrocarbon reservoirs most often occur in the higher-energy packstone and grainstone facies and are vertically compartmentalized by the impermeable, lower-energy mudstone and wackestone facies. Cyclostratigraphic subsurface correlations based on core-measured porosity, permeability and gamma ray data will yield production-scale predictability of hydrocarbon reservoirs in the “Mississippian limestone”.