The Sibaru PSC was signed with the Government of Indonesia in January 2007. The block lies in the East Java Sea in 50-100m water depths, and covers an area of 3,180 square kilometres, equivalent to 138 Gulf of Mexico blocks, or 12 Central North Sea blocks, following mandatory partial relinquishment in early 2010. Mitra Energy (Indonesia Sibaru) Ltd. holds 60% equity and operatorship, with Pearl Energy holding the remaining 40%.

Exploration in the East Java Sea began in the 1960s, culminating in a prolific drilling campaign by Cities Services in the early 1970s, with one well drilled in the present day Sibaru PSC area. A second well in the current PSC area was drilled by Agip. Since that time there was no exploration activity in the area until the PSC was awarded to Mitra.

As a result of the earlier exploration, more than 3,000 line kilometres of 2D seismic data are available over the Sibaru PSC. The first of the two wells on the block, JS5-1, was drilled by Cities Service in 1971, and targeted the culmination of a large, approximately 120 square kilometres, Kujung reef structure. A second well, NSA-1C, was drilled by Agip in 1982, on a deep reef-like structure in the basin to the southeast. A recent fluid inclusion study using drill cuttings obtained from that well identified liquid and gaseous hydrocarbon inclusions, indicating access to mature source rocks.

The JS5-1 well flowed 2.0 mmscf/d gas from a basal Ngimbang clastic section. The overlying Kujung Limestone section suffered heavy losses during drilling that necessitated large volumes of seawater being pumped into the well. Some oil shows were recorded during drilling, and Schlumberger’s petrophysical analysis in 1971 suggested that at least one hydrocarbon column had been penetrated. Nine DSTs in the Kujung Limestone section recovered only mud, filtrate or water, with only one test achieving flow to surface. The results of the JS5-1 well tests are believed to be inconclusive as a result of tool plugging and formation damage. Cities Services planned to re-drill the well but a number of factors meant that they never returned.

In order to delineate the prospectivity, Mitra has reprocessed over 2,800 line kilometres of vintage 2D seismic data and acquired 1,271 line kilometres of new 2D seismic in June 2008. A 2008 petrophysical study undertaken by Schlumberger on newly digitised well data now suggests that up to three hydrocarbon-bearing zones are present in the Kujung Limestone section of the JS5-1 well, in addition to the gas-bearing Ngimbang section.

Re-mapping of the structure using both the reprocessed and new 2D seismic data and incorporating the new petrophysical information gives an audited net mean estimate (Pmean) of 263 mmbo and a net high estimate (P10) of 620 mmbo Prospective Resource for the JS5-1 re-drill prospect (RISC, 2009). In addition, RISC calculated a net mean estimate of 200 mmbo and a net high estimate of 470 mmbo Prospective Resource for three other leads on the block. Mojo-1, a high impact re-drill of the JS5-1 well was spudded on 26th July 2010. The well was subsequently plugged and abandoned as dry hole after reaching a total depth of 5920 feet.