Tunu Field is a giant gas field located in the Mahakam Delta, East Kalimantan, Indonesia. Tunu Main Zone is the biggest gas contributor of this field, which is dominated by delta front mouth bar sands with some delta plain distributary channel sands. One of the uncertainties in Initial Gas In-Place (IGIP) calculation is coming from water salinity used in the formation evaluation. Observation of the water salinity values over Tunu Main Zone shows a distinctive decreasing pattern (the deeper the burial depth, the smaller the water salinity value is). This condition attracts attention to get a better understanding on the genesis of water salinity. Any alteration related to the different genesis will have sensitive impact on the salinity values.

Integrated analysis of depositional environment facies, water chemistry and clay mineralogy is required to characterize the genesis. Therefore, the following available and valuable data of 75 Tunu wells have been used in this study, i.e.: water chemistry (water salinity, isotopes and anion-cation content), E-logs and core data.

The results show that there is a decrease in water salinity trend by burial depth in the Tunu Main Zone. It is caused by the contribution of water expelled from shale (because of compaction and transformation of Feldspar into Kaolinite) and entering the Tunu Main Zone reservoirs. This process is interpreted as a reverse osmosis, i.e. water flows through a semi-permeable membrane from higher salinity region in the eastern side of the field to lower salinity region in the western side. In the study area, the shale acts as the semi-permeable membrane. During the water expulsion, the salts that dissolved in the shale water cannot penetrate the shale because its molecular size is relatively bigger, thus only water molecules can be expelled from the shale. As the consequence, the expelled water has very low salinity, then mixed with reservoir water (originally more saline). This circumstance causes decreasing of water salinity in the reservoirs in order to balance the reservoir hydrodynamics.

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