The spill, which happened on Saturday, occurred during pressure testing of a new valve, the spokesperson added.
The area affected by the spill is believed to be small and the spokesperson described it as a methanol/water oil substance as opposed to straight oil.
“We will determine the cause in due course," added the spokesperson who was not willing to reveal the amount of substance spilled.
BP is a 26% owner of the Lisburne field with rivals ConocoPhillips and ExxonMobil each taking 36%. US supermajor Chevron holds the remaining 2%.
The Lisburne field is a complex, fractured carbonate reservoir which lies underneath and adjacent to the main Ivishak reservoir at Prudhoe Bay. It was discovered in 1968 and came on-stream in 1986.
Cumulative production from the field is nearly 189 million barrels of oil equivalent. Source