Pertamina’s devastating oil spill off Java, Indonesia last year was caused by a technical error during drilling operations, according to Ministry of Energy & Mineral Resources oil and gas environment director, Adhi Wibowo.

Adhi said Tuesday that the leak occurred following a “premature explosion” during drilling on the YYA-1 well on the state-owned operator’s Offshore North West Java block.

He told reporters the explosive to perforate the well bore pipe should have been detonated at a subsurface depth of 6600 feet however it was set off at a depth of less than 1000 feet, where downhole pressures were lower.

“It damaged the bore pipe and the geological formation such that the drill rig became tilted,” Adhi was quoted by the Jakarta Post.

The ministry’s investigation team determined that the rig drilling the YYA-1 well had tilted 13 degrees following the blast.

The damaged well is one of three below a platform on Pertamina’s ONWJ block, just two kilometres off Karawang – one of several nearby villages that was inundated by oil.

“We are still investigating why this premature explosion happened. There might have been unexpected pressure from the rock formation that triggered the explosive," Adhi said.

He added that the investigative team, consisting of academics and industry practitioners, was expected to have its final report in February.

Pertamina in October finally managed to stop the leak on the YYA-1 well, which had spilled crude since the gas kick on 12 July last year.

The operator earlier said that the clean up of crude from affected beaches in Karawang, Bekasi and Thousand Islands regencies could take until March this year.

Upwards of 40,000 barrels of oil from the offending YYA-1 well have already been recovered from the Java Sea.