“Jeruk is still in the phase of exploration but the reserves seem to be far from expected. It may be only half of a previous forecast of 170 million barrels,” BPMIGAS deputy chief Achmad Luthfi told Reuters in an interview.

“We don’t want any misunderstanding in future. Therefore, we want Santos to certify the reserves at Jeruk before developing it.”

With the Santos’ Cooper Basin heartland in decline and the Santos-operated offshore northwest Australia Mutineer-Exeter oil field already seeing big falls in production only a year after coming onstream, a major downgrade at Jeruk would be a big disappointment for the Adelaide-headquartered company.

Santos said last month it would probably cut its reserves estimate for Jeruk, which was discovered in 2004.

It said the height of the Jeruk hydrocarbon column was probably only 145m, compared with a previous estimate of 379m, but further testing was required to fully determine reserve levels.

There were initially very high hopes for Jeruk, but two appraisal wells have indicated the field’s geology is very complex.

Participants in the Sampang production sharing contract are operator Santos (Sampang), which holds a 40.5% stake, Singapore Petroleum Sampang (36%), Cue Sampang (13.5%) and PT Petrogas Oyong Jatim (10%).