Publications

Formation water salinity of Tunu giant gas field located in Mahakam area has been proved to have significantly different value between hydrocarbon zone and water zone. Water salinity value in water zone measured from water sample and computed from openhole logs is significantly lower than water salinity in hydrocarbon zone derived from log to core calibration using capillary pressure data. The difference of salinity value is increasing with depth over Tunu Main Zone which most likely related to burial compaction and clay diagenesis. Clay digenesis expels fresh water through ion filtration processes. The initial formation water salinity during deposition in delta is depending on depositional environment and tidal effect. Delta plain with low tide will have very low salinity, otherwise it will have very high salinity during high tide in prodelta. Lateral variation of formation water salinity is ranging from 1 to 33 kppm. The fresh water expelled from clay is then charging the reservoir replacing the movable water resulting in the reduction of water salinity in water bearing reservoir. This is not the case in hydrocarbon bearing reservoir due to moveable water already replaced by hydrocarbon during migration process. The remaining water is only irreducible water held by capillary force which has initial water salinity as deposited. This process makes water salinity in water zone different from water salinity in hydrocarbon zone. The difference is increasing by depth since clay digenesis process increases by depth as temperature increases. Besides expelling water, clay diagenesis also changes clay mineral type from montmorilonite to illite by reducing mineral size which increases pore pressure. The decreasing of water salinity over Tunu Main Zone is very important information for regional overpressure study. The charging of expelled water from clay diagenesis to reservoir is suspected as one of the causes of overpressure in Mahakam area.