Understanding EPA’s OOOOb/OOOOc Rules for Methane Monitoring

The U.S. Environmental Protection Agency (EPA) finalized the OOOOb and OOOOc rules to reduce methane and other harmful emissions from the oil and natural gas sector. Published in the Federal Register in March 2024, these regulations are part of a broader effort to cut greenhouse gas pollution and improve air quality. For operators, understanding how the two rules work—and how they differ—is essential to staying compliant and avoiding penalties.

What Are the OOOOb/OOOOc Rules?

OOOOb and OOOOc are companion rules under the Clean Air Act that target methane and volatile organic compound (VOC) emissions from oil and gas operations, including wells, compressors, storage vessels, pneumatic controllers, and processing equipment. Methane is a potent greenhouse gas, and the oil and gas sector is one of the largest sources of human-caused methane emissions in the United States.

The two rules are not interchangeable—they cover different sources:

• Subpart OOOOb is a New Source Performance Standard (NSPS). It applies to facilities that were newly constructed, modified, or reconstructed after December 6, 2022. These standards are enforced at the federal level.
• Subpart OOOOc is a set of Emissions Guidelines for existing sources—those built on or before December 6, 2022. Rather than applying directly, OOOOc functions as a model rule that states use to develop their own plans, which EPA must then approve.

This new-versus-existing split, and the federal-versus-state implementation that comes with it, is the core structure of the regulation.

How Emissions Are Monitored

A common misconception is that these rules require continuous, real-time monitoring across the board. In practice, the baseline compliance regime is periodic leak detection and repair (LDAR). Operators must inspect equipment on a recurring schedule to find and fix fugitive emissions, with the frequency and method depending on the type and size of the facility.

For example, optical gas imaging (OGI) surveys are generally conducted on a quarterly to semi-annual basis, while audible, visual, and olfactory (AVO) inspections occur more frequently—often monthly or quarterly—depending on the site. Single-wellhead and small sites typically face lighter requirements than larger multi-wellhead sites or compressor stations.

Advanced and Continuous Monitoring as an Alternative

What makes these rules notable is that, for the first time, EPA allows operators to use advanced detection technologies—such as continuous on-site sensor networks and aerial or satellite remote sensing—in place of traditional OGI or Method 21 surveys. To do this, an operator submits a monitoring plan for EPA approval, and the required screening frequency scales with the sensitivity of the technology being used.

This is an opt-in pathway, not a mandate. But for many operators, continuous monitoring offers a way to detect leaks far sooner than a periodic survey schedule allows, which can reduce both emissions and the risk of a large, costly release going undetected between inspections.

The Super-Emitter Program

The rules also establish a super-emitter program, which uses credible third-party data (including remote sensing) to flag unusually large emission events so operators can investigate and respond. This is a separate mechanism from routine LDAR and is aimed specifically at the small share of leaks that account for a disproportionate amount of total emissions.

Other Key Requirements

Beyond leak detection, the rules address several major emission sources:

• Routine flaring of associated gas from new oil wells is being phased out under OOOOb, with gas required to be routed to a sales line, used onsite, or reinjected rather than flared.
• Process controllers and pumps face zero-emission or low-emission standards in many cases.
• Storage vessels with significant emissions must be controlled.

Compliance Strategies for Operators

Meeting these requirements involves a combination of technology, process, and people:

Technology selection. Operators need to decide whether to comply through traditional periodic surveys or to adopt an EPA-approved advanced monitoring approach. The right choice depends on site count, facility type, terrain, and how much value continuous detection adds for a given operation.

Data management. Whether monitoring is periodic or continuous, facilities generate substantial inspection and emissions records. Efficient systems for capturing, analyzing, and reporting that data are essential—both for catching issues quickly and for demonstrating compliance.

Staff training. Personnel need to understand inspection procedures, operate detection equipment correctly, and know how to act on the data. Regular training keeps response times short and compliance consistent.

Challenges in Implementation

Despite clear federal guidelines, compliance can be demanding:

• Cost. Installing and maintaining monitoring systems—particularly advanced sensor networks—requires meaningful investment, though it is often offset over time by avoided penalties and reduced product loss.
• Specialized expertise. Interpreting monitoring data and managing approved alternative-technology plans calls for trained personnel.
• A shifting regulatory landscape. The rules have continued to evolve since 2024, with EPA finalizing compliance-deadline extensions and revisions to certain flaring and vent-gas provisions. Operators should confirm current deadlines and requirements rather than relying on the rule exactly as originally published.

Conclusion

The EPA OOOOb and OOOOc rules are central to federal efforts to cut methane emissions from oil and gas operations. The key distinctions to keep in mind are that OOOOb governs new and modified sources at the federal level while OOOOc guides state plans for existing sources, and that periodic LDAR is the default compliance path—with advanced, continuous monitoring available as an EPA-approved alternative rather than a universal requirement. Operators who understand this structure, invest appropriately, and stay current with evolving deadlines will be best positioned to comply while contributing to lower emissions.