Reuters reported the incident started on July 12, when natural gas was released during drilling at one of its wells in the ONWJ platform on the Java sea, Pertamina's upstream director Dharmawan Samsu told a news conference.
Three days later the state-owned oil company declared an emergency and on July 16, a layer of oil began to rise to the surface of the sea in addition to the gas bubbles, he said.
The oil spill has reached villages on the coast of the Karawang area, West Java, 2km away from the facility, he said.
It will take an estimated 10 weeks from the declaration of emergency to stop the oil and gas leakage.
"For Pertamina, the most important thing is the safety of our employees and residents (in the affected area) and the environment, to make sure there is as little environmental impact as possible from this," Samsu said.
Twenty-nine ships have been deployed to patrol the area that are also on standby for firefighting, he said, adding that the firm has put up a 3.5km containment boom at sea and another 3km boom and 700m of fish nets along the shoreline, Reuters reported.
Boots and Coots, a well control company that handled a similar spill in the Gulf of Mexico, has been hired to help control the situation, Samsu said.
Pertamina has set up an emergency centre in Karawang and promised to compensate fishermen, he said.
"The cause is still being thoroughly and deeply investigated. Early indications showed pressure anomalies that resulted in gas bubbles, followed by oil spill," Samsu said.
The incident happened at a well that was yet to come into production, which had been expected to produce 3000 barrels per day of crude and 20 to 40 million standard cubic feet per day of natural gas in September, Samsu said.
There were two other wells in the field that Pertamina initially wanted to reactivate, but the company has now isolated them awaiting the result of the investigation, he said.