Publications

The quartz sands of the early Middle Miocene Ngrayong Formation are the most productive reservoir in the onshore East Java Basin. In the complex of fields in the vicinity of Cepu, such as the Kawengan Field, and well known outcrops to the north and west, this formation is present as thickly bedded, commonly medium-grained, cross-bedded sandstones.The Joint Operating Body Pertamina - Trend. Tuban (P-TT) drilled three wells on pod-like seismic anomalies in an area further south than any known Ngrayong sands. These features were-correlated as being equivalent in age to the Ngrayong Formation, and were hoped to be sand-rich fans or related sedimentary systems. Cores taken from early Middle Miocene sections in these wells (Ngasin-1, Gondang-1 and Grigis Barat-1) contained much finer grained clastics than expected. Grain sizes are typically silt to fine sand, although some medium-grained quartz is recorded in some beds.Palaeontology conclusively places all these cores in a very deep marine, probably fully bathyal depositional setting. These sediments are very thinly bedded and locally have good flow characteristics. The Ngasin-1 well kicked and flowed salt water from a sandstone at TD at an estimated rate of 2,000 barrels of fluid a day. The Gondang-1 well tested 538 BOPD from a 25 foot thick sandy pelagic carbonate. Two cores, totaling 40 feet, from the Grigis Barat-1 well, have porosities of 8.2 to 37.6 percent and permeabilities of up to 193 millidarcies.The sedimentary facies represented by these cores appears to be part of a single depositional system even though the wells drilled different seismic “pods. The chief differences between the cores are the amount of bioturbation, and variations in the proportions of clay, silt and sand grains. There are apparently no significant differences in the clastic components or depositional processes involved. Detailed sedimentological studies suggest that although the clastics required an initial process to originally bring them into the deep marine setting, the dominant depositional system involved was controlled by currents on the deep sea floor, that is, a contourite type process. The Grigis Barat-1 section appears to have some features indicative of a distal turbidite with apparently less modification by bottom currents.The proof of productive quartz sands in the central and southern part of the JOB Tuban Block, south of the previously interpreted southern limit of the Ngrayong sands, has important implications for exploration in East Java, and possibly elsewhere. A more detailed understanding of the unusual sedimentary facies is needed to develop play types and predict their characteristics. Such a detailed study has been undertaken by JOB geologists and appears to successfully incorporate the facies seen in existing fields and outcrops. Success in deep marine sandstone plays of East Java may spur efforts to locate similar plays in other Indonesian basins.