Arbuckle Group Research Articles

Reduced Injection Rates and Shallower Depths Mitigated Induced Seismicity in Oklahoma

Abstract The proximity of wastewater disposal to the Precambrian basement is a critical factor influencing induced earthquake rates in the Central United States, but the impact of reducing injection depths has not been widely demonstrated. Beginning in 2015, state regulatory efforts in Oklahoma and Kansas mandated that wells injecting into the lower Arbuckle Group, a basal sedimentary unit, be backfilled with cement (i.e., “plugged back”) so that they inject into shallower formations. This plug back activity gives us a unique opportunity to investigate the relationship between injection depth and induced seismicity rate. To evaluate the impact that decreased injection rates and plug backs had on the seismicity rates, we created a suite of rate–state earthquake models. Observed seismicity rates are best fit when only lower Arbuckle volumes are considered, suggesting the lower Arbuckle injectors were primarily responsible for the seismicity and that plug backs were effective at isolating the injected volumes to shallower formations. Our models demonstrate that if these wells had not been plugged back, seismicity rates would be multiple times larger than they are today. We find that the combination of well plug backs and injection volume decreases can be an effective strategy for reducing induced seismicity rates.

Pressure Monitoring of Disposal Reservoirs in North‐Central Oklahoma: Implications for Seismicity and Geostorage

AbstractDisposal of industrial wastewater and activities such as underground sequestration depend upon pressure conditions within deep geologic reservoirs. Sometimes, injection and storage are associated with induced seismicity, suggested to result from reservoir compartmentalization, leakage into faults, or other mechanisms in the subsurface. To understand subsurface pressure conditions within a major regional disposal reservoir, the Arbuckle Group of Oklahoma, we monitored the water levels in 15 inactive injection wells. The wells were monitored at 30‐s intervals, with eight wells monitored since September 2016, and an additional seven from July 2017. All the wells were monitored until early March 2020. Since 2016, hydraulic head decreased in 13 of the 15 wells, proportional to near‐borehole fluid pressure even considering decreasing regional injection volumes during this period. The well pressures respond to three types of perturbations: (i) gravitational fluctuations (a.k.a. solid‐earth tides) (ii) distal and proximal earthquakes, and (iii) injections into nearby wells. Parameterization of tidal responses illustrates that the near wellbore environments have negative fluid flux (i.e., are leaking). Earthquakes cause differing pressure responses from well to well, with some highly sensitive to proximal events, some to distal events, and some apparently insensitive. Injections have variable impacts in some cases masking tidal and earthquake pressure signals. Collectively, there appears to be a threshold injection rate above which well pressure becomes less sensitive to the volume of injections within 15 km. Multi‐scale geological structure and temporal permeability changes are likely controlling the pressure field, indicating leakage of fluids across the system.

Characterization of seismic-scale petrofacies variability in the Arbuckle Group using supervised machine learning: Wellington Field, Kansas

The Arbuckle Group in southern Kansas has been investigated for carbon geosequestration-related studies. In this study, we evaluated the seismic-scale petrophysically defined facies variability of the Arbuckle Group at the Wellington Field, Kansas, using quantitative seismic interpretation and a supervised Random Forest classification approach. We first defined three petrophysics-based rock types (petrofacies) from core-derived porosity and permeability measurements using the flow-zone indicator approach. Then, using the artificial neural network, we classified these petrofacies in noncored intervals. We observed that petrofacies 1 corresponds to medium and coarse-grained dolomitic packstone, wackestone, and dolomitic breccia with up to 8% porosity and D-scale permeability values. Whereas petrofacies 2 and 3 correspond to argillaceous and fine-grained micritic dolomites and dolomitic mudstones with lower permeability values for a given porosity, with respect to petrofacies 1. Using the common reflection-point gathers, we performed prestack seismic inversion and calculated various amplitude-variation-with-offset (AVO) attribute volumes. We used these elastic properties and AVO attribute volumes as input for estimating supervised seismic-scale 3D petrofacies and petrofacies probability volumes using the Random Forest algorithm. Results reveal the complex distribution of the petrofacies in the candidate injection and baffle zones in the study area, where petrofacies 1 is mainly prevalent within the lower and upper portions of the Arbuckle group, whereas petrofacies 2 and 3 are mainly present in the middle Arbuckle interval. The workflow we present through this study provides the spatial variability of facies distribution that is reflective of the actual lithology and petrophysical properties of the Arbuckle group in the study area with limited well control.

Assessing CO2 geological storage in Arbuckle Group in northeast Oklahoma

This paper examines a rigorous site characterization and analysis of geological storage capacity of CO2 in Arbuckle Group in the north part of Oklahoma to accelerate Carbon Capture and Storage (CCS) and Utilization (CCUS) technology deployment. Data obtained from the core, logs, and historic wastewater injection and production data were used to build and validate a geological model. Subsurface structure, depth required to attain supercritical CO2, rock properties and required caprock criteria were applied to Arbuckle geological model to identify suitable region for geological storage of CO2. The model estimates that the western Osage County has a storage capacity of > 50 million metric tons of CO2. More specifically, two sweet spots with a higher potential for CO2 storage were identified. The presence of several anthropogenic CO2 sources in the vicinity of site, existing pipelines, and compression infrastructure are the significant elements of a techno-economic analysis of the prospect storage project(s). This study demonstrates that the carbonate Arbuckle Group could be a strategic geological unit for CO2 sequestration, thus contributing toward emissions reduction from nearby industrial complex.

Controls on Timing of Hydrothermal Fluid Flow in South-Central Kansas, North-Central Oklahoma, and the Tri-State Mineral District

Paleozoic sedimentary rocks in the southern midcontinent of the United States have been affected by multiple events of deformation and fluid flow, resulting in petroleum migration, thermal alteration, Mississippi Valley-type mineralization, and a complex diagenetic history. This record is a hidden history of how cratonal settings respond to tectonic and non-tectonic drivers. The aim of this contribution is to better understand the controls on fluid migration in Paleozoic strata to evaluate whether hydrothermal activity is forced by tectonic or non-tectonic processes. This paper summarizes and vets the distribution of published dates related to thermal events in the southern midcontinent. In addition, we present new U-Pb dates obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) on calcite cements that were formed from hydrothermal fluids. These are from three samples from the Berexco Wellington KGS 1-32 core in Sumner County, Kansas; an ore sample from the Tri-State Mineral District, Neck City, Missouri; and a core sample from the Blackbird 4-33 well in Osage County, Oklahoma. Previous studies of these calcite samples provided evidence for hydrothermal fluid flow, with one of the Wellington samples possibly recording vertical hydrothermal fluid flow out of the basement. The sample from the Tri-State Mineral District (Missouri) yields a mid-Cretaceous age of 115.6±3.1 Ma. This age falls into the timing of the Sevier Orogeny along the west coast and the development of its foreland basin in the midcontinent. Calcites from the Mississippian interval in the Wellington KGS 1-32 core yield dates of 305±10.5 Ma and 305.1±9.1 Ma. Calcite in Mississippian strata from the Blackbird 4-33 core yields a date of 308.6±2.5 Ma. These dates from Mississippian calcite cements indicate hydrothermal fluid flow in the Late Pennsylvanian that coincides with the timing of the Marathon-Ouachita Orogeny or the Ancestral Rocky Mountains Orogeny. A calcite sample from the Ordovician Arbuckle Group from the Berexco Wellington KGS 1-32 core yielded an age of 5.6±1.6 Ma, coinciding with a time after high elevation uplift of the Rocky Mountains was already far advanced. We propose that this hydrothermal fluid flow may have been associated with increased meteoric recharge and increased regional fluid pressure in a basement aquifer that activated local seismic events far into the continental interior. The distribution of ages of hydrothermal fluid flow confirms a syntectonic driver during the Ouachita Orogeny and Ancestral Rocky Mountains Orogeny deformation. Continuation of hydrothermal fluid flow well into the Permian and tailing off early in the Triassic indicates a post-tectonic driver, where uplifted areas continued to provide the recharge from gravity-driven fluid flow, until the mountains were mostly beveled by the early part of the Triassic. A dearth of Triassic and Jurassic hydrothermal events suggests Gulf of Mexico rifting and extension were less important. Rejuvenation of hydrothermal fluid flow in the Cretaceous and continuing into the Paleogene indicates that elevation and regional flexure from both the Sevier and Laramide events continued to drive hydrothermal fluid flow far from the main sites of mountainous uplift and deformation. Finally, hydrothermal fluid flow associated with more recent uplift of the Rocky Mountains may have been activated by recharge events that pressurized a regional basement aquifer and triggered seismic activity.

Geothermal anomalies on the eastern flank of the Cherokee basin, southeastern Kansas, USA

Bottomhole temperature measurements from oil and gas drilling in southeastern Kansas on the eastern flank of the Cherokee basin, in combination with a suite of about 2,200 differential temperature logs recently obtained from wireline logging in coalbed methane wells, define several higher-temperature anomalies at the top of the Mississippian Subsystem strata. Temperatures slightly in excess of 90 oF (35 oC) at depths of about 900 ft (275 m) correspond to geothermal gradients as high as about 60 oC/km.
 Sparse historical measurements of heat flow in the cratonic Cherokee basin indicate that the thermal anomalies are not likely caused by locally high heat flow. Heat flow in the Cherokee basin is probably in line with most other shallow cratonic basins. The higher-temperature thermal anomalies defined by logging temperatures do not correspond to previously mapped faults or other structural features in the Phanerozoic sedimentary section, but some anomalies are underlain by Precambrian basement lineations that are detectable with aeromagnetic and gravity measurements. Well-log determination of shale content in the Pennsylvanian sedimentary strata overlying the Mississippian limestones indicates that low thermal conductivity caused by higher shale content may cause some of the thermal anomalies. 
 Lateral (advective) movement of warmer, highly saline water from the basin axis cannot account for the anomalies because the anomalies are not characterized by exceptionally highly saline water in Mississippian strata. Similarly, recorded static fluid levels of wells disposing of oilfield saltwater into the Mississippian strata and the deeper Cambrian-Ordovician Arbuckle Group indicate that the deeper Arbuckle strata generally do not have sufficient formation pressure to force Arbuckle formation water upward into the Mississippian through either natural fractures or leaky wellbores. Small-scale changes in salinity, in combination with geologic structuring indicating faulting, make a case for vertical (convective) movement of heated, less saline water from the Arbuckle Group into overlying Mississippian limestones in isolated localities. Buoyancy of the Arbuckle formation water (due to temperature and salinity differences with the cooler and more saline Mississippian water) could also be the primary force behind the convection. Convergence of cooler freshwater moving westward off the Ozark dome and more saline basinal water moving eastward also could be a factor in defining the limits of some thermal anomalies.

Structural geometry and evolution of the Carter-Knox structure, Anadarko Basin, Oklahoma

The Carter-Knox field is located on a northwest-southeast–trending faulted anticline in the southeastern part of the Anadarko Basin. Three-dimensional seismic and well data were used to develop a model for the geometry and evolution of the structure. The Carter-Knox structure formed during contractional deformation associated with the Wichita basement uplift in the Pennsylvanian. It is characterized by different structural styles in two main structural units. The lower unit, which includes the Cambrian Arbuckle Group to Mississippian Sycamore Limestone, is folded into a broad anticline associated with one or more frontal faults and back thrusts, with a change in vergence along trend. The upper unit, which includes the Upper Mississippian Springer shale to the Morrowan Primrose and the overlying Pennsylvanian growth units, is marked by a tight faulted detachment fold with a steep front limb, associated with multiple thrust faults that detach within the Springer shale. Kinematic reconstruction shows differential shortening between the two main structural packages. The evolution of the structure was episodic, resulting in two major angular unconformities within the Pennsylvanian. A major Permian unconformity truncates the structure. The southwestern flank of the structure was rotated along an active flexural hinge in the basement because of sedimentary loading, resulting in thickening of growth units on this flank. The Carter-Knox structure is an analogue for other thin-skinned structures in the Anadarko Basin and illustrates the impact of the thick-skinned Wichita uplift on the thin-skinned fold-thrust structures in the basin.

Precambrian Crystalline Basement Properties From Pressure History Matching and Implications for Induced Seismicity in the US Midcontinent

AbstractInjection‐induced seismicity across the US midcontinent has almost exclusively occurred in the crystalline basement that underlies the Arbuckle Group aquifer and its equivalents, the primary wastewater disposal zone in this region. However, the properties of the basement are not well known. Newly compiled data, from Class I wells in Kansas, provide a unique record of pressures in the Arbuckle and an opportunity to constrain the reservoir‐scale properties of the basement such as permeability, diffusivity, and specific storage. Constraints on these parameters are critical for modeling fluid flow and pressures across the entire Arbuckle‐basement system, and are necessary for accurate evaluation and prediction of injection‐induced earthquakes. Here, we present a detailed, three‐dimensional geological and pressure history‐matched numerical model for the Arbuckle and basement, based on data from >400 wells covering a large region in south‐central Kansas, where injection‐induced seismicity has been concentrated since 2014. Simulations of dynamic data from 319 wells indicate that Arbuckle pressures have increased by 1.1 MPa in high injection rate areas and an overpressure of <0.1 MPa may be the cause of seismicity in the basement. Pressure‐history matching also yields the likely range in porosity (0.3%–7%), permeability (0.1–0.7 mD), and diffusivity (0.004–0.07 m2/s) for the basement. The resulting estimates suggest reservoir‐scale properties of the basement are enhanced by faults and fractures. Importantly, the diffusivities determined in this study are lower than estimates derived from Kansas earthquake triggering fronts, and suggest that such seismicity‐based techniques may have limitations, particularly where space‐time patterns between injection and seismicity are complex.

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.

Evaluating the potential of carbonate sub-facies classification using NMR longitudinal over transverse relaxation time ratio

While the well log-based lithology classification has been extensively utilized in reservoir characterization, the classification of carbonate sub-facies remains challenging due to the subtle nuances in conventional well-logs. The nuclear magnetic resonance (NMR) log provides extra information of pore size and pore geometry features, improving differentiating carbonate sub-facies. Here we explore the feasibility of using the ratio between NMR longitudinal relaxation time and transverse relaxation time as a potential lithology indicator to determine carbonate sub-facies. We analyzed a series of logging data and corresponding core samples of Arbuckle Group carbonate containing mudstone, packstone, grainstone, incipient breccia, and breccia in northern Kansas for the characteristics of longitudinal relaxation times, transverse relaxation times, and longitudinal over transverse relaxation time ratios. The results show that mudstone, packstone, and grainstone exhibit high, intermediate, and low longitudinal over transverse relaxation time ratios, respectively, while incipient breccia and breccia have a wide range of longitudinal over transverse relaxation time ratios. Furthermore, we evaluated the potential of using longitudinal over transverse relaxation time ratios to classify carbonate sub-facies using multivariate analysis. By adding longitudinal over transverse relaxation time ratios to neutron porosity, total gamma-ray, and conductivity logs as inputs of automated facies classification, the prediction error decreased, especially for incipient breccia. On the contrary, when photoelectric log and computed gamma-ray are also available, adding longitudinal over transverse relaxation time ratios does not improve the accuracy of sub-facies classification. Our results suggest that longitudinal over transverse relaxation time ratio is an independent lithology indicator. However, it cannot replace other logs like gamma-ray and photoelectric logs in classifying carbonate sub-facies. Our study provided valuable evidence and credible elucidation of the importance and physicochemical mechanism of longitudinal over transverse relaxation time ratios, which is essential for deciphering NMR logging data in carbonate reservoirs. Cited as : Zhang, F., Zhang, C. Evaluating the potential of carbonate sub-facies classification using NMR longitudinal over transverse relaxation time ratio. Advances in Geo-Energy Research, 2021, 5(1): 87-103, doi: 10.46690/ager.2021.01.09

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 the 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 have determined that preexisting normal faults of Precambrian-Cambrian age were either reactivated along the Wichita Uplift or they 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 to evaluate the structural styles and tectonic evolution of the Wichita Uplift and the Anadarko Basin.

3D geomechanical modeling of the response of the Wilzetta Fault to saltwater disposal

From 2009 to 2017, parts of Central America experienced marked increase in the number of small to moderate-sized earthquakes. For example, three significant earthquakes (~<italic>M</italic>_w 5) occurred near Prague, Oklahoma, in the U. S. in 2011. On 6 Nov 2011, an <italic>M</italic>_w 5.7 earthquake occurred in Prague, central Oklahoma with a sequence of aftershocks. The seismic activity has been attributed to slip on the Wilzetta fault system. This study provides a 3D fully coupled poroelastic analysis (using FLAC3D) of the Wilzetta fault system and its response to saltwater injection in the underpressured subsurface layers, especially the Arbuckle group and the basement, to evaluate the conditions that might have led to the increased seismicity. Given the data-limited nature of the problem, we have considered multiple plausible scenarios, and use the available data to evaluate the hydromechanical response of the faults of interest in the study area. Numerical simulations show that the injection of large volumes of fluid into the Arbuckle group tends to bring the part of the Wilzetta faults in Arbuckle group and basement into near-critical conditions.

Macroscopic structural styles in the southeastern Anadarko Basin, southern Oklahoma

The Anadarko Basin in south-central Oklahoma contains a number of oil and gas fields located on anticlinal structures formed during the Pennsylvanian Orogeny. We use new 2-D and 3D seismic and well data to interpret the structural geometry and kinematic evolution of the Carter-Knox, Cruce, Chickasha, and Cement structures. These structures consist of tight faulted-detachment folds within the Pennsylvanian units related to slip along a detachment at the base of the Springer Shale, cored by deeper low amplitude fault-related folds within the pre-Pennsylvanian thick carbonate units which are detached at the base of the Arbuckle Group. Slip on these detachments was derived from the Wichita Uplift, which was a Precambrian-Cambrian failed rift that became inverted during the Pennsylvanian Orogeny. The trend and location of these structures was probably controlled by discontinuities related to pre-existing rift-related basement normal faults.Variations in the structural styles among the major structures are related primarily to the shortening and the influence of late-stage normal/strike-slip faulting. Changes in shortening are manifested in the increasing tightness and relief of the anticlines from the Chickasha-Cement to the Carter-Knox and Cruce structures. Some of the E-W structures were cut by Virgilian age strike-slip and associated normal faults which formed as the maximum compressive stress rotated from NE-SW to ENE-WSW during the Late Pennsylvanian. In summary, the geometry, trend, and evolution of the structures were influenced by the mechanical stratigraphy, pre-existing basement structures, and a rotation in stresses during the final stage of their formation.

Reply to Comment by Peterie Et Al. on “Accelerated Fill‐Up of the Arbuckle Group Aquifer and Links to U.S. Midcontinent Seismicity”

AbstractPeterie et al. question one observation in our paper: associating pressure increases to injection volumes at distances of up to 25 km from an injection well. In this reply, we show that the comment misunderstands our analysis and the evidence that led to this conclusion. We also show that gauge‐depth‐corrected pressures, used by the authors to produce statewide pressure maps, are discrepant with the static fluid level data, provided in our original compilation and analysis. The discrepancies are a result of the pressure correction method employed, which naïvely substitutes formation pressure for bottomhole pressure to calculate wellbore fluid density. Their linearly interpolated pressure maps, based on sparse data, contain interpolation and extrapolation artifacts that contradict injection trends in the state, the Theis solution, and the superposition principle. We reiterate that pressure and static fluid level increases in Class I wells existed prior to 2013, most notably in central Kansas, where recent earthquakes are cited in the comment as evidence of a pressure plume emanating from the Kansas‐Oklahoma border, 90 km away. We show that the space‐time pattern of seismicity in this area is inconsistent with a northward propagating pressure plume and, instead, seismicity appears to be centered on and near a cluster of high‐rate injection wells, two of which are among the highest rate wells in the state. These observations, along with recent M4 + earthquakes during continued decreases in wastewater injection in southern Kansas and northern Oklahoma, question the usefulness of the comment for understanding and managing societally significant earthquakes.

Comment on “Accelerated Fill‐Up of the Arbuckle Group Aquifer and Links to U.S. Midcontinent Seismicity” by Ansari et al. (2019)

AbstractSpatiotemporal analysis of Arbuckle Group formation pressures and disposal volumes provides insight into contributing factors underlying the regional trends in formation pressure identified by Ansari et al. (2019, https://doi.org/10.1029/2018JB016926) using statistical analysis. Based on trends in bivariate correlation of reported values, Ansari et al. (2019) suggested that rising formation pressures were a result of fluid injection within 25 km. Our primary concern is that the correlation trends are most likely an artifact of the method used by Ansari et al. (2019), at least in part, and the conclusion that pressure diffusion is limited to within 25 km of the injection point does not necessarily follow from the analysis. We also note that estimated formation pressure depends on the depth of the pressure gauge, which, in some cases, varies from one measurement to the next and should be accounted for to accurately assess measured changes. The spatial relationship of formation pressure changes among wells provides valuable insight into the origin of those changes. For each well, we project pressure to a baseline depth to account for changes in gauge depth and reexamine trends in formation pressure. Spatiotemporal progression suggests post‐2011 formation pressure changes originate from a spatially dense group of high‐rate saltwater disposal wells near the Kansas‐Oklahoma border well beyond 25 km (and as far as 90 km) away.

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 &amp; 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 (&lt;10), and have the lowest areal density and intensity, while displaying the largest maximum vertical separation. However, farther away (&gt;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.

Deep-Learning-Based Vuggy Facies Identification from Borehole Images

SummaryIdentification of vuggy intervals and understanding their connectivity are critical for predicting carbonate reservoir performance. Although core samples and conventional well logs have been traditionally used to classify vuggy facies, this process is labor intensive and often suffers from data inadequacies. Recently, convolutional neural network (CNN) algorithms have approached human-level performance at multiimage classification and identification tasks. In this study, CNNs were trained to identify vuggy facies from a well in the Arbuckle Group in Kansas, USA. Borehole-resistivity images were preprocessed into half-foot intervals; this complete data set was culled by removing poor-quality images to generate a cleaned data set for comparison. Core descriptions along with conventional gamma ray, neutron/density porosity, photoelectric factor (PEF), and nuclear magnetic resonance (NMR) T2 data were used to label these data sets for supervised learning. Hyperparameters defining the CNN network size (numbers of convolutional layers/filters and the numbers of fully connected layers/neurons) and minimize overfitting (dropout rates, patience, and minimum delta) were optimized. The median losses and accuracies from five Monte Carlo realizations of each hyperparameter combination were the metrics defining CNN performance. After hyperparameter optimization, median accuracy for vuggy/nonvuggy facies classification was 0.847 for the cleaned data set (0.813 for the complete data set). This study demonstrated the effectiveness of using microresistivity image logs in a CNN to classify facies as either vuggy or nonvuggy, while highlighting the importance of data quality control. This effort lays the foundation for developing CNNs to segment images to estimate vuggy porosity.

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.&#x0D; 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.

Mobility of Ba, Sr, Se and As under simulated conditions of produced water injection in dolomite

The volume of petroleum produced water (PPW) has increased dramatically over the last decade. PPWs are rich in salts and often contain high concentrations of potentially toxic trace elements such as Barium (Ba), Strontium (Sr), Selenium (Se), and Arsenic (As) which, under the right circumstances, may contaminate freshwater supplies. To avoid these issues, PPW is frequently disposed of in subsurface aquifers such as the dolomitic Arbuckle Group in the state of Oklahoma, USA. This may not be a permanent solution if the injected PPW migrates within the disposal sites to ultimately reach conductive fault zones and/or existing wells with casing and/or cementing failures. In these cases the water could pose an environmental risk to potable groundwater. In order to understand the mobility of these elements under conditions similar to produced water injection, we conducted a series of batch sorption experiments. We investigated the effect of brine salinity and temperature on the sorption of Ba, Sr, Se, and As by dolomite. The results revealed that the sorption of the tested elements on dolomite did not substantially change with increasing salinity from 18 to 90 g/L. However, the sorption for all elements did increase with increasing temperature from 22 to 150 °C. This is likely due to a combination of strong surface complexation or ion exchange reactions coupled with precipitation/co-precipitation on the dolomite mineral surfaces.

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.