Publications

The study area is located in the North Madura Platform, offshore East Java Basin, with discoveries made in the Oligocene-Miocene carbonate, making it a proven prolific hydrocarbon reservoir. Following the discovery in 2001, three wells further appraised the field. Six development wells (horizontal wells) have been drilled to date and produced with a peak oil production of 16,000 BOPD. Well-X was spudded in 2004. The well was extensively cored from the top of the Oligo-Miocene Carbonate into the igneous basement with a total length of 241 feet of core. Four DSTs were run in succession. The reservoir zone with the most prolific oil production was displayed for the core showcase. The core recorded a nearly complete history of the Oligocene carbonate platform development. Detailed core description work was conducted by integrating geological and petrophysical analysis. It confirmed the geological model of stacked upward shallowing successions culminating in fine-grained platform-top carbonate sediments that were periodically subaerially exposed and subjected to karst processes. Active tectonism and relative sea level changes were critical in carbonate growth and development. Evidence of karstification in the core was characterized by natural fractures, brecciation of the host rock, rubbles (macroporosity), enlarged and touching vugs enhancing porosity, and infilling of overlying sediment (argillaceous wackestones/ mudstones) into open fractures. Besides the core data, karst intervals were also defined based on well-log data such as SPI, image log, and NMR. The study concluded that the principal reservoirs' pore structure mainly resulted from secondary porosity creation during karstification and that a karst channel network was responsible for the high flow rates (> 3,000 BOPD) observed during DSTs. Longer-duration DSTs were conducted to evaluate reservoir connectivity, flow potential, and volume of hydrocarbons in the karst system. Recognition of karst-related pore systems from the core and correlating this observation with surrounding cored wells paired with the results of long-term DSTs indicated that the karst pore systems were widespread and well connected. It was further proven during the development drilling, where several losses were encountered, indicating possible karstified intervals. Results from this finding were integrated into the static model to understand further the reservoir development, effectiveness, and karst distribution and for future production planning. The main challenge was mapping the karst extension and network significantly beyond the well and cored data. Identifying karst intervals from seismic data was also a limitation due to the seismic resolution. Thus, the modeling and mapping of the karst trend maps, depositional environment interpretation, and conceptual model was constructed by integrating multidisciplinary data such as seismic attributes, dynamic data, and advanced geo-steering tool image result of horizontal wells for identifying karst interval in the development wells.