THE INHERITED PORE STRUCTURE AND STRATIFICATION OF THE ARBUCKLE GROUP, OKLAHOMA - SOUTHERN KANSAS

The Arbuckle Group is the primary target for fluid injection in Oklahoma and southern Kansas, which numerous studies have shown is associated with the ongoing sporadic seismicity of the region. However, there remains puzzling complexities in the distribution of the seismicity, highlighting a need to better understand the variations in structure, lithology, and stratigraphic packaging within the injection zones. Our study is focused on characterizing the lithology, diagenesis, porosity & permeability, and stratigraphic packaging that create heterogeneity for fluid flow. We perform a multiscale characterization of the Arbuckle Group across the region, using core samples (southern Kansas & northern Oklahoma), well logs (southern Kansas), and outcrops (southern Oklahoma).

Our analyses show that the entire Arbuckle in Kansas is a succession of subtidal to peritidal carbonate cycles that vary in thickness from meter scale up to ~100 m, defining three distinct scales of stratigraphic packaging. In general, the cycles consist of porous lithofacies (11 - 30% porosity, 10 - 1500 md permeability), which include mixed packstone-grainstone, ooid packstone-grainstone, wackestone-mudstone; and non-porous facies (0 - 8% porosity, 0.0001 - 0.1 md permeability), including microbialites (thrombolites, stromatolites), peloidal packstone-grainstone, and intra-Arbuckle shale. Important porosity types include matrix, vuggy, karst, & fracture porosity. We observe similar facies and pore types in the Oklahoma sub-region of these carbonate sequences.

The alternating packaging of porous and non-porous lithologies at different scales, and the different porosity types result in variable vertical and lateral heterogeneity that could control fluid flow patterns. Matrix porosity is important throughout the Arbuckle and where dominant, horizontal fluid flow is preferentially favored due to interbedding with non-porous facies that form barriers and baffles to vertical fluid flow. The presence of karst or fracture porosity provides connectivity for vertical fluid flow between porous and non-porous horizons.

We demonstrate that integrated sedimentologic, stratigraphic, diagenetic, and petrophysical studies can aid in understanding and predicting dispersal patterns of injected fluids and their relations to induced seismicity.