Oklahoma has been continually exploring oil and gas since the turn of the last century. More than 450,000 wells have been drilled for resource development, putting several of the state’s oilfields past their primary and secondary recovery phases. The state’s oil production has declined since 1975, but the diminishing reserves are not without a silver lining. The shrinking petroleum industry is leaving room, figuratively and literally, for an economy centered on carbon capture and storage. A century of production has created new accommodation space in the subsurface as well as an extensive pipeline infrastructure on the surface. Additionally, a vast array of industrial sources ready to be retrofitted with capture technologies and the state’s industry-friendly attitude toward the energy industry uniquely position Oklahomans to benefit from existing tax incentives and the emerging carbon credit market. Here, we present an overview of Oklahoma’s CO2 storage potential.
Ongoing CCS Operations
Subsurface CO2 injection is not new to Oklahoma: It has been used for enhanced oil recovery since 1982. The state currently has three ongoing efforts: two collect CO2 from industries and one collects from a natural source. The Coffeyville, Kan. plant supports the Burbank field in the northeast; the Enid, Okla. plant supports the composite Golden Trend and Sho-Vel-Tum fields in the south and the Postle field in the western panhandle receive CO2 from Bravo Dome, N.M. Lately, the community’s CO2 focus has shifted from enhanced recovery to its permanent storage. While enhanced recovery is regulated through a Class II permit, permanent storage requires a Class VI permit with a longer bonding obligation. Permanent storage also entails more stringent subsurface scrutiny for its ability to retain the injected plume and a more rigorous plan for monitoring leakage. Fortunately, the state’s thriving petroleum and groundwater industries have endowed it with geological data necessary for planning such injections.
CO2 can be trapped in many ways: in pores by capillary and at the grain surface by either adsorption and/or chemical reactions. This is why its storage opportunities extend beyond pressure-depleted hydrocarbon fields to saline aquifers and granitic and metamorphic rocks, which, in the state, underly its entire length and width. The state has five basins containing Cambrian to Permian-age sediments that collectively span more than 75,000 square miles: Anadarko, Ardmore-Marietta, Hollis-Hardeman, Arkoma (and its shelf, the Cherokee Platform), and Ouachita (figure 1a). Except for the Ouachita basin, part of a late Paleozoic thrust belt, the other four generally formed between the late Mississippian and early Pennsylvanian ages. Although the basins are isolated today, their overlapping depositional histories have resulted in sub-sections that share thematic similarities. Identifying them is essential, as it enables us to rank the basins for their geological suitability, ultimately guiding the prioritization of CCS infrastructure development. A challenge in regional assessment is to reconcile the varying nomenclature across the basins, which has evolved alongside the industry.
Five CCS Systems
Using the current industry-standard stratigraphy we identified five CO2 storage systems, or reservoir sets separated by major capillary seals, common to three basins: Anadarko, Ardmore and Arkoma basins (figure 2). Although the scope of this article is limited to system-level description, it is easy to see how local seals will create basin-specific sub-systems.
Potential leakage pathways from wellbores and faults are the primary risks associated with CO2 containment, and wellbores from the early days of Oklahoma’s exploration now pose a challenge to storage potential. Boreholes provide geological constraints and puncture an otherwise intact formation. It is easy to see how a propagating CO2 plume can leak into the atmosphere if the well lacks integrity or is not plugged. However, even when plugged, the wellbore’s cement casing can be corroded by the propagating plume. Thus, existing wellbores should be monitored or ideally avoided in CO2 storage projects. While Oklahoma’s “orphan” wells are plugged with government and private initiatives as they are discovered, it is challenging to know when this effort can be considered “complete.”
The oldest structures in Oklahoma are associated with a Precambrian rift-related transform system, an approximately 1,200-mile-long failed rift in the center of North America, with least 20,000 feet of detrital clastics, basic sills and basalt flows. This feature is mostly interpreted from gravity anomalies and does not directly manifest in the morphology of the state’s basins, although it is likely to have influenced their later orogenic evolution. Most recent structures, including several giant oilfields, are from Early Pennsylvanian orogeny. Controversy remains on the sealing nature of the faults associated with these large structures. While oilfields such as Oklahoma City and Deep Crescent are fault-bounded favoring their sealing ability, the earthquakes that started in 2008 indicate a level of risk. The seismicity trend, likely from the reactivation of deep basement faults, correlates to high-volume, high-pressure injection in the early Ordovician Arbuckle Group. Like boreholes, we recommend avoiding known faults regardless of their present-day orientation. We have limited our discussion in this article to stratigraphy younger than Arbuckle because, although experience from wastewater injection shows ample storage, open questions surrounding the risks of containment and induced seismicity remain.
The first and deepest CO2 system includes Middle to Late Ordovician strata of the sandstone-rich Simpson Group, limestone-dominated Viola Group and Sylvan Shale. CO2 injected at the base of the Simpson would be trapped in the sandstones, with subsequent mineralization in the Simpson and Viola carbonates and eventual structural trapping by the Sylvan Shale. While significant faulting exists, wellbore density is low outside the Anadarko Basin.
The next system comprises the limestone and dolomite-rich Late Ordovician to Middle Devonian Hunton Group, capped by the organic-rich Late Devonian Woodford Shale, Oklahoma’s primary source rock. The Hunton varies from 100 to 500 feet in thickness, while the Woodford typically ranges from 50 to 200 feet. Both units have considerable mineralization potential but are heavily explored, complicating the search for injection sites away from existing wells.
The third system dominantly comprises Mississippian-age rocks mostly deposited in shallow seas. Key formations include the Keokuk and Reeds Spring Formations in the Ozarks and Sycamore Limestone in southern Oklahoma. These rocks, naturally fractured and altered, often have low permeability and require hydraulic stimulation for injectivity. In the Mid to Late Mississippian period, southern Oklahoma experienced subsiding basins resulting in thick deposits of the Chester Group comprising shale interbedded with limestone and sandstone. This system is best developed for storage in the Anadarko Basin, and is unsuitable in the northeast where it becomes a groundwater resource.
The penultimate system, dominantly Pennsylvanian, offers the most diverse storage reservoirs and possibly the greatest risk from legacy wells. The Pennsylvanian age spans five epochs: Morrowan, Atokan, Desmoinesian, Missourian and Virgilian, each linked to an orogenic event. Major mountain systems, such as the Wichita, Arbuckle and Ouachita (figure 1a), as well as several of the giant fields (figure 1b), formed during this era. The Mississippian-Desmoinesian Wichita Orogeny caused 10,000 to 15,000 feet of uplift in the Wichita Mountains (figure 1a), depositing unique conglomeratic facies (granite wash) with interfingering seals and reservoirs near the uplifts in western Oklahoma and the panhandle (figure 2). During this period, the Nemaha Uplift also formed across central Oklahoma, resulting in pinching-out of the Morrowan and Atokan-age rocks in the north-central part of the state, creating large-scale stratigraphic traps. The Anadarko and Arkoma Basins additionally have a Late Mississippian to Early Pennsylvanian sandstone-dominated, prolific reservoir known as the Springerian series. Pennsylvanian rocks are mainly marine shale with sandstone, limestone, conglomerate and coal, ranging from 2,000 to 5,000 feet in northern shelf areas to more than 10,000 feet in the Arkoma Basin. The system is unsuitable in central and north-central areas where the Virgilian Vamoosa Formation is a major aquifer.
The shallowest system in the state is Permian Age, which is divided into four epochs: Wolfcampian, Leonardian, Guadalupian and Ochoan. During the Wolfcampian, a shallow inland sea deposited limestones, gray shales and red beds. In the Leonardian, evaporating seawater left salt, gypsum and other deposits – a trend that continued through the Guadalupian and Ochoan epochs. The thickness of the sequence varies from 1,000 feet in the northeast to 6,000 feet in the Anadarko Basin. This system is best developed for CO2 storage in the Panhandle and Western Oklahoma, as central Oklahoma formations are major groundwater sources. Post-Permian sediments are irrelevant to CO2 storage.
Existing Infrastructure
The underpressure formations, widely present in the state, offer a unique reservoir type for CO2 storage. These units have lower pore pressure than hydrostatic (standing freshwater columns) and are generally perceived as compartments isolated by sealing rocks. In Oklahoma, the early Ordovician Arbuckle formation is an archetype. Routinely used for wastewater disposal, the Arbuckle operates under gravity flow in many parts of the state. Figure 1b shows the distribution of Class II commercial disposal wells (both enhanced oil recovery and wastewater), and figure 3 is a compilation of their injectivities. The reader will note that in figure 3, in addition to the Arbuckle Group, several other units in the stratigraphic column exhibit sub-hydrostatic pore pressure (cluster labeled gravity; figure 3). Injection into underpressure units may alleviate leakage concerns.
The state currently has 337 miles of CO2 pipelines (figure 1b). A decommissioned ammonia pipeline running east to west between Tulsa and Enid could be of particular value to future CCS deployments (figure 1b). Upgraded to CO2 standards and tied to the 120-kilometer north-south Enid-Purdy line, it would connect Oklahoma City and Tulsa, the two main metros. Several pressure-depleted giant oilfields (figure 1b) are available for both Class II and VI injections within a 10-mile swath of the connected corridor. In our opinion, the biggest hurdle to scaling up carbon injection in the state isn’t pore space or pipelines but rather securing a consistent and adequate CO2 supply at low enough costs. The state has ample 45Q-eligible emitters, but because of the differences in flue gas CO2 concentration, the cost of capture varies greatly from industry to industry. A recent compilation by Great Plains Institute showed that 45Q-eligible facilities in the state collectively emit 23.4 Mt CO2e per year, out of which only 3.7 metric tons of CO2 per year is capturable in the near- to medium-term. Missed targets at the Petra Nova (Texas) and San Juan (N.M.) coal plant facilities show that capturing, even from the purest CO2 streams, can be daunting.
Social Considerations
The historic presence of pipelines and pumpjacks in Oklahoma’s landscape and the industry’s contributions to the state’s economy have helped create an overall positive orientation to subsurface activities in the minds of the public and legislature. The post-2008 induced seismicity trend challenged this sentiment, though. Also, a recent motion picture about malfeasance during Oklahoma’s early exploration days has further stirred strong conversations around social justice and mineral rights in the state. Public response in Mississippi and Louisiana shows that, despite a state’s physical readiness, unless safety and equity concerns are addressed, the CCS industry is unlikely to take off. These perception barriers must be addressed through stakeholder engagement and education. In Oklahoma, surface owners own pore space, and 43 percent of the land is in tribal reservations. Therefore, stakeholder engagement is all the more important, particularly for federally-funded projects with Justice40 mandates. Finally, we believe Oklahoma has the requisite source, storage, and transport for launching CCS in place or within reach. The future of Oklahoma’s CCS economy will depend on obtaining and maintaining a state-wide social license.
Acknowledgment: Thanks to Gary Steward of Oklahoma State University and Nick Hayman of the Oklahoma Geological Survey)for their valuable feedback. This material is based upon work supported by the Department of Energy under Award Number(s) DE-FE0032362.
Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States government. Neither the U.S. government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. government or any agency thereof.