Pertamina upstream subsidiary PT Pertamina Hulu Rokan (PHR) took over the block from Chevron’s local arm, PT Chevron Pacific Indonesia (CPI), on Sunday night, marking the end of the US company’s 97-year reign over the Rokan Block located in Riau.

The Rokan Block is currently Indonesia’s second-most productive oil block after the Cepu Block but was, in its heyday, the country’s most productive oil site and the backbone of the country’s membership in the powerful Organization of the Petroleum Exporting Countries (OPEC).

“[This takeover] marks a historic milestone in Indonesia's upstream oil and gas history. PT CPI has managed the block well, thus we hope PHR can continue [CPI’s] success in developing the block,” said Energy and Mineral Resources Minister Arifin Tasrif during a livestream takeover ceremony on Sunday night.

Pertamina won operatorship rights over the Rokan Block in July 2018 after beating CPI at an auction. Pertamina inked a production sharing contract (PSC) with the government in May 2018, marking the beginning of the transition to be completed by the time CPI’s PSC expired this month.

Read also: Pertamina wins lucrative Rokan Block contract

Critics previously said the government’s handover of Rokan to Pertamina exemplified how Indonesia’s energy sector had been run according to a nationalistic agenda, particularly ahead of the 2019 presidential election, but then-energy minister Arcandra Tahar hit back saying that Pertamina had fairly won the block because it made a “much better” offer than CPI.

Upstream Oil and Gas Special Regulatory Taskforce (SKKMigas) head Dwi Soetjipto said on Sunday that PHR aimed to raise block production to 165,000 barrels of oil per day (bopd) by the end of 2021.

The figure marks a 2.7 percent increase from the block’s output of 160,646 barrels bopd as of June 30, which accounted for 24 percent of the national output, ranking it second to the Cepu Block in East Java, according to SKKMigas data.

“We will ensure Rokan’s smooth takeover and maintain the level of oil production at the end of the CPI contract,” said Dwi.

The taskforce previously acknowledged that the transition might reduce oil production from the already aging block, which produced nearly 1 million bopd in 1973, a time when Indonesia was producing around 1.3 million bopd, an output sufficient for the Southeast Asian country to secure a trade surplus and a seat on OPEC.

A further decline in the block’s output would be detrimental to the government’s goal of producing 1 million bopd by 2030 to attain energy security.

The block also produced 41 million standard cubic feet per day (mmscfd) of gas as of June 30, which accounted for 7 percent of national output, the same SKKMigas data show.

SKKMigas and Chevron signed a heads of agreement (HoA) on the block’s handover on Sept. 29 last year. The regulator directly oversaw the transfer of manpower, power supply and technology, among nine aspects of the transition.

SKKMigas wrote in a statement on Sunday that CPI had drilled 103 development wells between the HoA-signing day and Aug. 8 in maintaining production levels.

“We hope the Rokan Block can continue contributing to the state and the nation,” said CPI managing director Albert Simanjuntak in a statement.

Read also: Pertamina gets Rokan oil block from Chevron. What does this tell us?

Pertamina president director Nicke Widyawati said that Pertamina planned to invest US$2 billion in developing the Rokan block until 2025.

The state-owned company is set to drill 161 new oil and gas wells by the end of this year and 500 new wells in 2022 to maintain production levels. The company has also formed a transition team to oversee production, finance and project and facility-engineering processes.

“We are committed to maintaining production of the Rokan block,” Nicke said, also on Sunday.

PHR is slated to manage the block for 20 years. The company is also required to transfer 10 percent control over the block to a region-owned enterprise as required by prevailing regulations.

Chevron’s role in the Rokan Block began in 1924 when the company, then called the Standard Oil Company of California (Socal), sent four geologists to study the area. The company discovered oil in the Duri field in 1941 and in the Minas field in 1944. It began production in 1951 and has cumulatively produced 11.69 billion barrels of oil.