Multiscale Characterization of the Caney Shale — An Emerging Play in Oklahoma

From a hydrocarbon perspective, the Caney Shale has historically been evaluated as a sealing unit, which resulted in limited studies characterizing the rock properties of the Caney Shale and its suitability for hydraulic fracturing. The objective of our research is to help bridge the current knowledge gap through the integration of multiscale laboratory techniques and to characterize the macro- and microscale rock properties of the Caney Shale. We employed an integrated approach for the characterization of the Caney using 200 ft (61 m) of Caney core from a target well in southern Oklahoma. Core observation and petrographic analysis of thin sections were combined to characterize the general rock types and associated fabrics and textures. Mineralogical composition, pore system architecture, and rock fabric were analyzed using x-ray diffraction (XRD), scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), and focused ion beam (FIB)-SEM. In addition, rebound hardness and indentation testing were carried out to determine rock hardness (brittleness) and elasticity, respectively. With the integrated multiscale characterization, three mixed carbonate-siliciclastic rock types were identified — mudstone, calcareous siltstone, and silty carbonate — likely representing a spectrum of deposition from low to relatively high energy environments in the distal portions of a ramp system. Silty carbonate contains mostly interparticle pores. The calcareous siltstones and silty mudstones contain a combination of organic matter pores and interparticle pores. Each of the rock types shows unique mineralogical compositions based on XRD. The mudstone lithofacies has the highest clay content and the least carbonate content. Calcareous siltstones show moderate carbonate and clay content. Silty carbonate indicates the highest carbonate content with the least clay content. In an order of mudstone, calcareous siltstone, and silty carbonate, rebound hardness and Young’s modulus show an increasing trend. As a result of rock-fluid interactions, there are potential scaling reactions during completion and production that could ultimately affect permeability and production rates. Overall, the proposed multiscale integration approach is critical for the geologic characterization of most rocks. However, in shale reservoirs dominated by microporosity and microstructure where engineered fractures are expected to provide permeability at a reservoir scale, successful integration is essential. An optimized, integrated geological characterization of the Caney Shale that is well aligned with the engineering designs in drilling, completing, and producing wellbores will ultimately lead to optimal production while providing safe and environmentally responsible operations.

Examining patterns and drivers of variability in playa water status on the High Plains of western Kansas, 2016–2019

Playa wetlands are widely distributed across the High Plains of the central United States, providing a range of ecosystem services, such as groundwater recharge, surface water storage, and wetland habitat. Although playas are essential resources, few studies have examined the variability and controls on playa water storage. The purpose of this project is to determine how playa and watershed morphology, watershed land cover, and precipitation patterns affect timing and duration of water storage in playas. This project focuses on 92 playas distributed throughout a 10-county region in western Kansas. Playa and watershed morphology were calculated in a GIS environment and classified into quartiles based on playa and watershed surface area. Watershed tilled index (i.e., percent cropland versus grassland) was determined using 2016, 2017, 2018, and 2019 Cropland Data Layers available from the National Agricultural Statistics Service and classified as either cropland (more than 75% cropland), grassland (more than 75% grassland), or mixed. Monthly precipitation data for 2016–2019 were compiled from the Oakley 22S High Plains Regional Climate Center weather station. Playa water status for 2016–2019 was classified monthly as either standing water or dry (i.e., no visible standing water) by visually examining four-band satellite imagery with 3.7 m resolution available from Planet Explorer (www.planet.com). Playa water status is influenced by a combination of factors, including playa and watershed morphology, watershed land cover, and precipitation patterns. Larger playas have larger watersheds and standing water more frequently and for longer periods than smaller playas. Playas in cropland watersheds store water more frequently and for longer periods than playas in grassland watersheds, though differences are not statistically significant. Standing water within playas is positively correlated with monthly precipitation and reflects a short-term response to precipitation patterns, regardless of playa size or watershed land cover. The strongest controls on playa water status are playa area, monthly precipitation, and watershed area. Playas are critical resources for the High Plains, providing a range of ecosystem services that are dependent upon the playa’s ability to store water. Playa functions are under continued threat from cropland expansion, climate change, and playa and watershed modifications. To sustain playa functions in Kansas, efforts should focus on conserving larger grassland playas and reducing sediment inputs to playas in cropland watersheds.

Wrench faulting and trap breaching: A case study of the Kizler North Field, Lyon County, Kansas, USA

The Kizler North Field in northwest Lyon County, Kansas, is a producing field with structures associated with both uplift of the Ancestral Rockies (Pennsylvanian to early Permian) and reactivation of structures along the Proterozoic midcontinent rift system (MRS), which contributed to the current complex and poorly understood play mechanisms. The Lower Paleozoic dolomitic Simpson Group, Viola Limestone, and “Hunton Group” are the reservoir units within the field. These units have significant vuggy porosity, which is excellent for field potential; however, in places, the reservoir is inhibited by high water saturation. The seismic data show that two late-stage wrench fault events reactivated existing faults. The observed wrench faults exhibit secondary P, R’, and R Riedel shears, which likely resulted from Central Kansas uplift-MRS wrenching. The latest stage event breached reservoir caprock units during post-Mississippian to pre-Desmoinesian time and allowed for hydrocarbon migration out of the reservoirs. Future exploration models of the Kizler North and analog fields should be based on four play concepts: 1) four-way closure with wrench-fault-related traps, 2) structural highs in the Simpson Group and Viola Limestone, 3) thick “Hunton Group,” and 4) presence of a wrench fault adjacent to the well location that generates subtle closure but not directly beneath it, which causes migration out of reservoirs. In settings where complex structural styles are overprinted, particular attention should be paid to the timing of events that may cause breaches of seals in some structures but not others. Mapping the precise location and vertical throw of the reactivated wrench faults using high-resolution seismic data can help reduce the drilling risk in analog systems.

Geological characterization of the Patterson CO2 storage site from 3-D seismic data

Approximately 26 square miles of new 3-D seismic data were acquired in July 2019 over the Patterson Site (Kearny County, Kansas) to assess its potential for carbon dioxide (CO2) storage. Seismic interpretation revealed that the Patterson Site contains multiple structural closures that lie on uplifted fault blocks, bounded by two reverse faults that strike nearly perpendicular to each other. These faults offset Precambrian through Pennsylvanian sections, including several primary reservoir and seal intervals. Fault displacements are maximum at the Precambrian basement and decrease upward. Data indicated a range of structural and combination traps exists at the Patterson Site in the Cambrian-Ordovician Arbuckle through Mississippian Osagian reservoirs. The three-way closures along the NW–SE fault have structural relief of ~130 ft (40 m), and the four-way closures contain relief of ~60 ft (18 m). Erosional surfaces and multiple basement fractures also are observed on the top of the Precambrian. A Mississippian-aged incised valley system also was observed at the Patterson Site. The incised valleys formed during the Meramecian-Chesteran Stages with an incised depth up to 250 ft (76 m). The motion of the reverse faults likely captured existing meandering and linear channels, causing the current deeply incised morphology. The incised valleys observed at Patterson are similar in age, structural style, shape, incision depth, and seismic attribute properties to incised valleys observed by other workers at Pleasant Prairie South, Eubank, and Shuck oil fields (southwest Kansas). Further research should focus on estimating reactivation tendency and sealing characteristics of the reverse faults to evaluate the seal integrity of the saline reservoirs. This will reduce uncertainty concerning the risk of CO2 migration during injection and storage. Further reservoir description, modeling, and simulation are also underway to characterize the storage potential at the Patterson Site.

Diagenesis of Hunton Group Carbonates (Silurian) West Carney Field, Logan and Lincoln Counties, Oklahoma, U.S.A.

The West Carney Hunton Field (WCHF) is an important oil field in central Oklahoma. Deposited during a series of sea-level rises and falls on a shallow shelf, the Cochrane and Clarita Formations (Hunton Group) have undergone a complex series of diagenetic events. The Hunton section of the WCHF comprises dolomitized crinoidal packstones, brachiopod “reefs” and grainstones, thin intervals of fine-grained crinoidal wackestones, and infrequent mudstones that were diagenetically affected by repeated sea-level change. Widespread karst is evidenced by multiple generations of solution-enlarged fractures, vugs, and breccias, which extend through the entire thickness of the Hunton. Karst development likely occurred during sea-level lowstands. Partial to complete dolomitization of Hunton limestones is interpreted to have occurred as a result of convective circulation of normal seawater during sea-level highstands. Open-space-filling calcite cements postdate dolomitization and predate deposition of the overlying siliciclastic section, which comprises the Misener Sandstone and Woodford Shale. Petrographic evaluation and carbon and oxygen isotope values of the calcite cements suggest precipitation by Silurian seawater and mixed seawater and meteoric water. Carbon and oxygen isotopic signatures of dolomite may have been partially reset by dedolomitization that was concurrent with calcite cementation. Fluid inclusions in late diagenetic celestite crystals observed in the Clarita Formation indicate that the WCHF was invaded by saline basinal fluids and petroleum after burial, during later stages of diagenesis. The timing of late diagenetic fluid flow and petroleum generation likely was during the Ouachita orogeny, which was occurring to the south. There is no evidence that late diagenetic fluids significantly altered the dolomite reservoir that formed earlier. The WCHF provides an ancient example of early diagenetic dolomitization by seawater that remains relatively unaltered by later diagenetic events.

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.