Microbial methane emissions from inactive oil wells 1,000 times higher than estimated: McGill study

Microbial methane leaking from non-producing oil and gas wells is being emitted at rates nearly 1,000 times higher than previously estimated, according to a new study led by researchers at McGill University.

Microbial methane (or biogenic methane) is methane gas produced by microorganisms, specifically archaea known as methanogens — through the decomposition of organic matter in oxygen-depleted (anaerobic) environments. It is a major component of the global carbon cycle, generated in wetlands, rice paddies, landfills and digestive systems of ruminants. 

“Methane is a powerful greenhouse gas when released into the atmosphere, regardless of its origin. In particular, this study implies that non-producing oil and gas wells could continue to emit microbial methane long after the targeted formation has been fully depleted,” said Mary Kang, associate professor of civil engineering and co-author of the study.

The research is based on samples collected from 401 non-producing wells across Canada, particularly in Western Canada, which hosts more than 90 per cent of such wells. The category includes inactive wells, those that never produced, and those that have ceased production, according to the report published in the journal Environmental Science and Technology.

“For this study, we looked at chemical properties such as gas composition and stable isotopic signatures, which enable a better understanding of the origins of the leaking methane. This analysis is highly sensitive, and we were able to reliably characterize the origins of emissions from 100 of the 401 wells sampled,” said Gianni Micucci, co-author and postdoctoral researcher in civil engineering.

Canada has nearly 500,000 non-producing oil and gas wells. While not all emit methane, earlier research by the same team showed that the top 12 per cent of emitting wells account for 98 per cent of emissions from this source.

The new study titled Origins of Subsurface Methane Leaking from Nonproducing Oil and Gas Wells in Canada found that most methane leaks originate from deep underground thermogenic sources, formed under high temperatures in petroleum reservoirs. However, it also highlighted that the contribution of microbial methane, typically associated with shallow formations, has been significantly underestimated.

“Our results raise the question of whether the studied wells were above microbial methane-containing formations, and whether they provide a pathway for this microbial methane to migrate into the atmosphere,” Micucci said.

The findings also indicate that half of the studied wells emit a mix of thermogenic and microbial methane. While thermogenic methane remains a key indicator of high-emission wells — with average emission magnitudes 500 times higher than microbial sources — the newly identified scale of microbial emissions adds complexity to mitigation efforts.

Kang noted that the subsurface remains a “complex system with multiple gas-bearing formations,” making it difficult to pinpoint exact methane sources. She added that the findings could help improve well integrity assessments and guide more effective mitigation and repair strategies.
The study applies a geochemical framework to distinguish methane origins, offering new insights into how emissions travel underground and escape through well infrastructure.

Researchers say understanding the nature and pathways of these emissions will be critical as countries grapple with reducing methane — a potent greenhouse gas driving near-term climate change.