Prospex Energy PLC (AIM:PXEN) shares fell in Wednesday's trade after it announced a delay to the start of gas production at the Viura field, in northern Spain, after problems during a test.
The operator, HEYCO Energia Iberia, reported a failure of monitoring equipment during testing.
The wireline logging tool and cable remain inside the well, the company said, noting that specialist equipment is being mobilised from Aberdeen to retrieve the tool and cable.
But, production cannot resume until the items are removed or secured below the well’s safety valve.
"This is a very unfortunate setback in re-instating production from this prolific gas field," chief executive Mark Routh said in a statement.
"Safety of production operations is paramount and the operator is working as quickly as it can to resume gas production from the field at stable and sustainable rates, but this has to be done safely."
Routh added: "I look forward to updating shareholders when the operator has brought the Viura gas field safely back into continuous production from the Viura-1B well."
In London, Prospex shares were down 4.5% changing hands at 3.8p, and earlier saw as low as 3.4p.