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Tunu is a giant gas field located below the recent Mahakam Delta, East Kalimantan, Indonesia. The field is divided into two intervals, Tunu Shallow Zone with a depth of 500 - 2500mSS and Tunu Main Zone between 2500 - 5000mSS. Since the first gas in 1990, more than 1100 development wells have been drilled in this field with the cumulative production more than 9 Tcf in 2019. The main gas production is coming from Tunu Main Zone reservoirs which were deposited by dominantly deltaic sand-shale lamination. Thin layer reservoirs, wide range variation of formation water salinity in deltaic system and matured field development are the reason why the fluid interpretation in the new wells of this field becomes more challenging. Gas bearing reservoirs with high clay content usually have high value of water saturation computed from electrical logs. It is generally related with high irreducible water retained in clay-associated microporosity that causes resistivity depression and increasing e-logs derived water saturation. Consequently, direct use of water saturation cut-off for hydrocarbon identification can eliminate some potential hydrocarbon reservoirs. There is plenty of irreducible water saturation data that can be derived from first drainage capillary pressure measurements on core sample in Stratigraphy Unit 4 to 6 of Tunu Main zone. In general, these irreducible water saturation values are decreasing in non linear relationship with porosity measured on the same core samples. A comparison of irreducible water saturation from core samples and total water saturation computed from e-logs can be used for hydrocarbon identification. This approach is one of the complement techniques for hydrocarbon identification in high clay content reservoirs. It has been regularly applied in Tunu Main Zone and validated by using down-hole fluid analysis and fluid interpretation with very convincing results around 75 � 75% success ratio. This method could be applied as well to the other field in Mahakam area.