An unknown amount of gas is seeping to the surface on Barrow Island, where Chevron's six decades of oil production ceased in May.
In addition to any local effects the pollution may have, methane has more than 80 times the climate-warming effect of carbon dioxide over a 20-year period.
A Chevron spokesman said the company has informed relevant regulators of the problem and will work with them to implement a comprehensive investigation.
“As part of detailed planning for the decommissioning of WA oil infrastructure, data analysis has indicated an environmental risk from the subsurface migration of hydrocarbons, primarily gas, to groundwater and the surface," he said.
Chevron has drilled 895 oil wells on Barrow Island since 1964, according to the WA Government's database of wells, with 696 more than 40 years old.
The southern half of Barrow Island is covered in well pads, access roads and oil pipelines. Source: Google Maps
A spokesman for the Department of Mines, Petroleum and Exploration (DMPE) said Chevron told it the gas was coming from the Gearle Formation, but the company had not determined the extent of the flow.
The Gearle Formation is a layer about 300m to 700m below the surface of Barrow Island, where some oil has been produced from, but most oil has been extracted from deeper areas.
Since 1967, 335 million barrels of oil produced on Barrow Island have produced more than $1 billion in royalties for the WA and Federal governments.
The DMPE spokesman said it could not provide details on any possible legal action it may take as its investigation is ongoing, but it has required Chevron to update its environmental plan "as an initial step in possible compliance action."
Conservation Council of WA executive director Matt Roberts said Chevron was privileged to be allowed to operate on a Class A Reserve and should work to the highest environmental standards.
Barrow Island is home to more than 100 species of birds, 13 mammal species, 44 different reptiles and almost 4000 different plants.
The 235-square-kilometre island is home to 24 animal and plant species found nowhere else.
"Chevron continues to fall short in its duty to manage its operations in a way that protects the unique environment of Barrow Island," Roberts said.
"This news is unsurprising given the history of Chevron on Barrow Island."
Barrow Island no longer pristine
The gas seepage adds to numerous other environmental problems on Barrow Island caused by Chevron's long-term oil production and the more recent Gorgon gas export plant.
The entire island is classified as a contaminated site under WA legislation, with public records listing sixteen areas contaminated with hydrocarbons and one area contaminated with asbestos.
The worst area near oil storage tanks is estimated to have contaminated the water table with up to 900,000 litres of crude oil, reaching depths of 11m, caused by numerous spills over many years up to 2006.
Remediation work over almost 20 years has stabilised the 18-hectare plume, and an independent audit found the hydrocarbons were naturally breaking down.
The $US54 billion ($82 billion) Gorgon project built between 2009 and 2017 is under investigation by the WA environmental regulator for PFAS contamination, erosion of beaches where vulnerable flatback turtles nest and quarantine failures.
Freedom of information requests revealed the existence of 111 documents from three investigations that the Department of Water and Environmental Regulation (DWER) would not release so as not to "prejudice an investigation relating to contravention or possible contravention of the laws.”
The regulator was sufficiently concerned to prepare contentious issue briefing notes for the environment minister in late 2022 for all three issues.
A DWER spokesman said the investigations into quarantine, beach erosion and PFAS issues on Barrow Island remain ongoing.
"There is no scheduled completion date for these investigations," he said.
The environmental license governing the Gorgon project was amended in February to include regulation of PFAS contamination. The Conservation Council of WA has appealed the conditions, arguing that they focus on monitoring the risk rather than reducing it.
Roberts said Chevron continued to fall short in its duty to protect the unique environment of Barrow Island, and DWER was not meeting its obligations as regulator.
"We continue to hold grave concerns about unresolved issues relating to quarantine, beach erosion and PFAS contamination," he said.
Chevron was only allowed to build the huge Gorgon gas plant on Barrow Island so it could inject underground the millions of tonnes of carbon dioxide that flow to the island with gas from offshore fields every year.
However, despite costing more than $3 billion, the CO2 injection effort has failed, with it operating at only one-third of its design capacity eight years after gas exports began.
The shuttered oil asset on Barrow Island is owned by Chevron (57 per cent), Santos (29 per cent), and ExxonMobil (14 per cent).
The Gorgon joint venture consists of Chevron (47 per cent), Shell and ExxonMobil, each owning 25 per cent, and three Japanese energy utilities with small stakes.
I worked in oil & gas in commercial and engineering roles for 20 years. Since 2016, I have written for The West Australian, WAtoday and Boiling Cold, winning five WA Media Awards.
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