Knowing the precise geochemistry of a non-producing oil and gas well’s emissions footprint is critical to understanding why and where it becomes an exit pathway for climate-wrecking methane, concludes a recent study from McGill’s Department of Civil Engineering.

Non-producing oil and gas wells (a category which includes inactive and exhausted sites, as well as those that have never produced) emit two types of methane: thermogenic and microbial. Thermogenic methane—the primary constituent of conventional natural gas and the target of natural gas extraction—is formed by geochemical processes occurring at great depth over millions of years. Microbial methane—not a target of extraction—is primarily produced by bacteria breaking down organic matter at much shallower depths and much more quickly.

While thermogenic methane remains the primary constituent of emissions from non-producing oil and gas wells, microbial methane is being emitted at rates 1,000 times higher than previously estimated, according to a new study led by Mary Kang, an associate professor of civil engineering at McGill, in her latest research into fugitive methane emissions.

The finding is one of several that suggest “well integrity failure” as a primary driver of severe methane leaks from non-producing wells.

In another study, published in April 2025 in the journal Energy & Climate, Kang and her team estimated that Canada’s non-producing oil and gas wells currently emit 230 kilotonnes of methane per year, or seven times the volume reported in the federal government’s 2024 National Inventory Report. 

While each of Canada’s some 427,000 non-producing wells is a potential pathway for subsurface methane to make its way to the surface and into the atmosphere, a relatively small proportion of these wells (about 12%) have been identified as supremely heavy leakers—contributing as much as 98% of total methane emissions from non-producing wells.

“The origins and drivers” of such high methane emissions “remain uncertain,” write Kang and co-author Gianni Micucci, a post-doctoral researcher,  in a new paper published in January in the journal Environmental Science & Technology.

Critical to reducing such uncertainty is the ability to consistently distinguish between thermogenic and microbial methane in the emissions footprint of non-producing wells. The McGill study takes a significant step toward reducing that uncertainty.

Using a first-of-its-kind “bespoke geochemical framework” to analyze methane samples collected from 401 non-producing wells across Canada, Kang and Micucci were able to precisely distinguish between the two kinds of methane at every site, and at both wellheads and surface casing vents (SCVs), the latter being pipes designed to prevent pressure buildup within a wellbore.

That level of precision allows them to begin to piece together some of the drivers of methane leakage.

For example, the team now hypothesizes that high thermogenic methane emissions at SCVs are “likely due to well integrity failures or uncemented zones [between the casing and the rock formation].”

“SCVs are not designed to be connected to the producing formation, which is generally a source of thermogenic gas; therefore, the detection of thermogenic gas may indicate a well integrity failure, which can lead to high emission rates,” they explain.

That roughly 23% of the studied wells, “or, three times earlier estimates” are emitting microbial methane, and at 1,000 times the previously estimated rate, raises critical questions about where this gas originates. Some of it might come from the oil and gas formation into which the non-producing well was originally drilled, making it “secondary” microbial methane produced during the slow breakdown of thermogenic methane within that formation.

Most of the mixed-origin samples (containing both thermogenic and microbial methane) came from depths between 600 and 1,500 metres—shallow reservoirs completely unrelated to the drilling where primary microbial methane would be typically found.

Here, Kang and Micucci hypothesize that well-integrity failure may also be “creating preferential pathways” between shallow reservoirs that were never destinations for drilling and deep target reservoirs, creating “conduits for microbial methane emissions that would otherwise remain sub-surface.”

More data and long-term studies will be needed to confirm these findings, they add.