Publications

The Offshore North-West Java, Production Sharing Contract (ONWJ block) is a mature area that has produced largely from conventional prospects within the Parigi, Main, Massive, Baturaja, and Talangakar Formations. Due to decreasing production in the shallow productive zones, new exploration concepts and ideas are needed to find significant replacement hydrocarbon resources, specifically in basement targets which have not been fully explored. Different than other analog basins, most of the deep ONWJ wells, that have hydrocarbon indications in the basement, are predominantly metamorphic rock. The objective of this study was to re-visit an extensive well data base and build localized basement characterizations for ONWJ. Data components such as basement lithology, oil and gas shows, mud losses, cores, and rates of penetration from 220 wells were fundamental to the evaluation of a prospective fractured basement reservoir. This study collated all the above basement information and built a basement distribution map which contained hydrocarbon information as the first guidance to determine the prospective areas for future ONWJ basement exploration targeting. Combined seismic and well log data were used to support the argument for possible petroleum system connections to the possible basement play. Based on hydrocarbon potential mapping, basement lithology was not the main control of hydrocarbon occurrence, since all lithologies such as schist, gneiss, quartzite, meta-carbonate, and granite each had hydrocarbon indications. Existing hydrocarbon indications strongly tend to lie on basement highs which have both structural closure and faces toward the basin in the Seribu Basins, the Ardjuna Basin, the E-15 Graben. This has been proved by the following: i) Four wells, via DST data, in the Seribu Basin that flowed 2603 BOPD oil and 0.450 MMCFPD gas from schist, ii) Two wells in the Ardjuna Basin that flowed 545 BOPD oil and 18.55 MMCFPD gas from meta-carbonate, and iii) Gas at a rate of 1.2 MMCFPD from quartzite in the E-15 Graben. Several migration paths are postulated for hydrocarbon's entry into fractured basement reservoirs. Structural and stratigraphic correlations indicate that the Talangakar Formation plays a role as productive source rock and/or as a possible migration pathway when in contact with fractured basement. Hydrocarbons also likely migrated through major faults which enter the fractured basement often with an unconformity as a vertical seal. Regional fault reactivations most likely coincided with the time of hydrocarbon migration in the Middle-Late Miocene which help create the basement fractures and are then filled by hydrocarbons. Applying these concepts to fractured basement reservoirs and their timing, highlights the possibility that new exploration targets are still to be found in ONWJ.