Publications

The Mundu Formation is best described as the latest Miocene to Pliocene age of Globigerina rich sediment in the East Java Basin. In four fields that Santos (now Ophir Energy) currently operates, this reservoir performs very well. For example, in the Oyong Field, four production wells have produced 118 Bcf of gas in 9 years. In addition, the Mundu Formation has become a prolific target for exploration in the region. Since 1980, 25 exploration wells have targeted this reservoir in the Madura Straits region with 18 technical discoveries for a success ratio of 70%. The depositional environment of the Pliocene bioclastic limestones that predominantly consist of foraminiferal tests of planktonic genus Globigerina has been subject to debate. Previous researchers have observed a wide variety of planktonic and benthonic presence in the sediments. These, combined with sedimentary structures lead to different opinions whether these sediments were deposited in a shallow or deepwater environment. The reservoir pore space provided by foraminiferal tests created not only inter-granular but also intra-granular porosity and matric micropores in which, in clean grainstones facies, can reach up to 50% total porosity. The permeability of this reservoir is observed to be governed mainly by the presence of mud cement/matrix content which is in turn geologically related to rock facies. Clean grainstones facies can have permeability in the range of 100 – 3,000 md, however in packstones/wackestones facies that contain more matrix/cement, permeability is in magnitude of 5 – 50mD. As the connection of interparticle pores is served by the punctae in the globigerinid walls, when matrix content rises, these punctae are blocked and hence permeability becomes lower. These sediments are mainly cemented by calcite with minor dolomite that was precipitated shortly after deposition. There is no significant diagenesis observed in the reservoir rock, possibly due to hydrocarbon migration at a relatively early stage which inhibited mineral precipitation and created buoyancy. The lack of diagenesis is also supported by the fact that the reservoir has not been introduced to high temperature conditions due to a relatively thin overburden through time as a result of continuous regional uplift.