Nostrum is offering Tethys, which operates in central Asia, CA$0.2185 per share, around 10.7p, valuing the company at just over £36m.
Tethys had chosen to enter talks with AGR in July, spurning an offer from Nostrum.
The collapse of talks with AGR now leaves a deal with Nostrum between either administration or fundraising efforts likely to heavily dilute the company.
Nostrum, which has substantial operations in Kazakhstan where most of Tethys’ production assets are based, has now been granted exclusivity by Tethys until 25 August and will provide a $5m loan facility to Tethys to substitute for one AGR had previously made available.
A statement from Nostrum confirming the talks with Tethys emphasised that confirmation did not amount to “an announcement of a firm intention to make an offer for Tethys”. The company added that there “can be no certainty that any offer will be made, nor as to the terms on which any offer might be made”.
Nostrum achieved revenues worth $782m and net income of $146m in 2014.
The potential deal saw shares in Tethys rise by 5.08 per cent to close at 7.75p.
Shares in Nostrum also went up yesterday, increasing by 5.58 per cent in London.