Characterizing Stress Orientations in Southern Kansas

ABSTRACT Induced seismicity predominantly occurs along faults that are optimally oriented to the local principal compressive stress direction, and the characterization of these stress orientations is an important component of understanding seismic hazards. The seismicity rate in southern Kansas rapidly increased in 2013 primarily due to the disposal of large volumes of wastewater into the Arbuckle Group. Previously, local stress orientations in this area were poorly constrained, which limited our understanding of the complex faulting and diverse earthquake mechanisms in this region. We use shear-wave splitting and focal mechanism inversion techniques to create multiple, independent estimates of maximum horizontal stress directions (SHmax) and their spatial variation across the study area. We then create an integrated model of stress orientations for southern Kansas and northern Oklahoma using our local results in conjunction with previous, regional stress orientation estimates. We find that SHmax in both southern Kansas and central Oklahoma exhibits an east-northeast (∼N78° E) orientation, and these regions bound a northeast (∼N59° E) rotation within a ∼20 km area in northern Oklahoma near the Nemaha ridge.

Attribute-assisted characterization of basement faulting and the associated sedimentary sequence deformation in north-central Oklahoma

Patterns of recent seismogenic fault reactivation in the granitic basement of north-central Oklahoma necessitate an understanding of the structural characteristics of the inherited basement-rooted faults. Here, we focus on the Nemaha Uplift & Fault Zone (NFZ) and the surrounding areas, within which we analyze the top-basement and intrabasement structures in eight poststack time-migrated 3D seismic reflection data sets. Overall, our results reveal 115 fault traces at the top of the Precambrian basement with sub-vertical dips, and dominant trends of west-northwest–east-southeast, northeast–southwest, and north–south. We observe that proximal to the NFZ, faults dominantly strike north–south, are fewer (<10), and have the lowest areal density and intensity, while displaying the largest maximum vertical separation. However, farther away (>30 km) from the NFZ, faults exhibit predominantly northeast–southwest trends, fault areal density and intensity increases, and maximum vertical separation decreases steadily. Of the analyzed faults, approximately 49% are confined to the basement (intrabasement), ~28% terminate within the Arbuckle Group, and approximately 23% transect units above the Arbuckle Group. These observations suggest that (1) proximal to the NFZ, deformation is dominantly accommodated along a few but longer fault segments, most of the mapped faults cut into the sedimentary rocks, and most of the through-going faults propagate farther up-section above the Arbuckle Group; and (2) with distance away from the NFZ, deformation is diffuse and distributed across relatively shorter fault segments, and most basement faults do not extend into the sedimentary cover. The existence of through-going faults suggests the potential for spatially pervasive fluid movement along faults. Further, observations reveal pervasive, subhorizontal intrabasement reflectors (igneous sills) that terminate at the basement-sediment interface. Results have direct implications for wastewater injection and seismicity in north-central Oklahoma and southern Kansas. Additionally, they provide insight into the characteristics of basement-rooted structures around the NFZ region and suggest a means by which to characterize basement structures where seismic data are available.

STRUCTURAL ANALYSIS OF THE WICHITA UPLIFT AND STRUCTURES IN THE ANADARKO BASIN, SOUTHERN OKLAHOMA

We have constructed regional structural transects across the Wichita Uplift and adjacent Anadarko Basin to show the relationship between thick-skinned basement-involved structures and thin-skinned detached fold-thrust structures. Slip from the basement-involved structures in the Wichita Uplift is transferred along two major detachments into the Anadarko Basin. Our interpretation is that along the northwestern margin, the Wichita Uplift is marked by a zone of frontal imbricates forming a triangular wedge with most of the slip dissipated along the Wichita front. Paleozoic units show tight folding with overturned beds in the frontal zone. The uplift is episodic as indicated by the truncation of major faults along unconformities and their subsequent reactivation. In contrast, along the southeast margin, a significant part of the slip is transferred into structures in the Anadarko Basin. These structures are tight faulted-detachment folds that formed above a major detachment within the Springer Shale, cored by broader structures detaching at the base of the Arbuckle Group. Examples include the Carter-Knox, Cement-Chickasha, and Cruce structures. Oblique faults with normal and strike-slip components cut some of these structures, resulting in more complex geometries. We propose that pre-existing normal faults of Precambrian-Cambrian age were either reactivated along the Wichita Uplift, or controlled the location of the Pennsylvanian age structures in the Anadarko Basin. Progressive rotation of regional stresses from northeast-southwest to a more east-northeast-west-southwest direction during the Pennsylvanian impacted the tectonic history of the area. We used 2D and 3D seismic, well log data, and surface geology were used to evaluate the structural styles and tectonic evolution of the Wichita Uplift and the Anadarko Basin.

Diminishing Depth to Water in Cambrian-Ordovician Arbuckle Group Disposal Wells in Kansas

Industrial and municipal wastewater and oilfield brines have been disposed of into the Cambrian-Ordovician Arbuckle Group for decades in Kansas and nearby states in the midcontinent United States. The industrial and municipal wastewater disposal wells (designated Class I disposal wells) are regulated by the Kansas Department of Health and Environment. The oilfield brines are disposed of in Class II disposal wells, which are regulated by the Kansas Corporation Commission. Annual testing of formation pressure and static fluid levels in Class I wells compose a body of data that is useful in monitoring movement of water and fill-up of Arbuckle disposal zones. In western Kansas, the depth to water in wells penetrating the Arbuckle can be several hundred to more than a thousand feet (305 m) below ground surface, but in parts of southern and southeastern Kansas, the depth to water locally can be less than 100 ft (31 m). Furthermore, most Class I wells indicate Arbuckle fluid levels in central and south-central Kansas are rising ~10 ft (~3 m) annually, suggesting that at current disposal rates, the Arbuckle may lose its capacity to accept wastewater under gravity flow in parts of the state in the next few decades, principally south-central and southeastern Kansas along the Oklahoma state line. At present in parts of six Kansas counties along the Oklahoma state line, low-density (~1.0 g/cc or slightly greater density) wastewater in a wellbore does not have a sufficient hydrostatic head by gravity alone to force its way into the more dense resident Arbuckle formation water. In general, Arbuckle formation water flows west to east in Kansas. Arbuckle disposal wells in Kansas collectively dispose of ~800,000,000 barrels (~127,000,000 m3) of wastewater per year, although some of this is recycled from Arbuckle oil production. Declines in oil price since mid-2014 have resulted in less oilfield disposal in the Arbuckle since 2015. The number of Class I wells recording annual fluid rises have also declined since 2015, as has the median of their annual change in static fluid level, but overall, more Class I wells are still recording fluid rises. There is a poor correlation between changes in fluid levels in Class I wells and the volume of fluid disposed in them annually, thereby indicating that more regional characteristics may control water movement in the Arbuckle. More monitoring wells are needed to better understand the movement of water in the deep subsurface and to anticipate any potential problems that may occur with reduced disposal capacity and possible migration of fluids through unplugged or improperly plugged older wells.

Characterization of Arbuckle-basement wastewater disposal system, Payne County, Northern Oklahoma

Over the past eight years, north-central Oklahoma has experienced a significant increase in seismicity. Although the disposal of large volumes of wastewater into the Arbuckle Group basement system has been statistically correlated to this increased seismicity, our understanding of the actual mechanisms involved is somewhat superficial. To address this shortcoming, we initiated an integrated study to characterize and model the Arbuckle-basement system to increase our understanding of the subsurface dynamics during the wastewater-disposal process. We constructed a 3D geologic model that integrates 3D seismic data, well logs, core measurements, and injection data. Poststack-data conditioning and seismic attributes provided images of faults and the rugose top of the basement, whereas a modified-Hall analysis provided insights into the injection behavior of the wells. Using a Pareto-based history-matching technique, we calibrated the 3D models using the injection rate and pressure data. The history-matching process showed the dominant parameters to be formation-water properties, permeability, porosity, and horizontal anisotropy of the Arbuckle Group. Based on the pressure buildup responses from the calibrated models, we identified sealing and conductive characteristics of the key faults. Our analysis indicates the average porosity and permeability of Arbuckle Group to be approximately 7% and 10 mD, respectively. The simulation models also showed pockets of nonuniform and large pressure buildups in these formations, indicating that faults play an important role in fluid movement within the Arbuckle Group basement system. As one of the first integrated investigations conducted to understand the potential hydraulic coupling between the Arbuckle Group and the underlying basement, we evaluate the need for improved data recording and additional data collection. In particular, we recommend that operators wishing to pursue this type of analysis record their injection data on a daily rather than on an averaged basis. A more quantitative estimation of reservoir properties requires the acquisition of P-wave and dipole sonic logs in addition to the commonly acquired triple-combo logs. Finally, to better quantify flow units with the disposal reservoir, we recommend that operators acquire sufficient core to characterize the reservoir heterogeneity.