Investors piled into Ascent after a potential tie-up was announced on 23 March, sending its shares an eye watering 658 per cent higher.
Shares in Ascent tumbled 56.9 per cent to 2.58p per share on the news, while Cadogan dipped 1.3 per cent to 9.8p.
"Cadogan has conducted extensive due diligence on Ascent and the Petisovci Gas Project, however this share price rise has taken Ascent’s enterprise value above a level that Cadogan was prepared to consider for a potential transaction," Cadogan said in a statement to the London Stock Exchange.
Ascent added: "The board ... confirms that its view of the value of the company is considerably in excess of that contained in the preliminary approach received from Cadogan and as such the company and Cadogan were unable to agree appropriate terms."
Ascent also announced the appointment of Stockdale Securities as its nominated adviser and sole broker, with immediate effect.