Fears that the cost of rehabilitating old gas fields and platforms could be higher than accounted for are emerging as a risk to the Abu Dhabi National Oil Company’s $36.4 billion takeover proposal for Santos.

The ASX-listed oil and gas giant’s liabilities for the removal of disused wells, pipelines and other heavy equipment, along with the cost of dealing with waste and rehabilitating land, are currently estimated at $6.3 billion.