THE INFLUENCE OF SEDIMENTATION ON SALINITY VARIATION IN THE EAST KUTAI BASIN, EAST KALIMANTAN, INDONESIA

The East Kutai Basin represents a major deltaic system formed under strong river-dominated sedimentary processes, characterized by long-term progradation from marine to terrestrial environments. In such deltaic settings, salinity distributions are generally expected to exhibit systematic lateral and vertical trends, with freshwater dominating proximal facies and saline water occurring in distal marine facies. Vertically, salinity typically increases with depth as a result of progressive progradation through time. This study investigates lateral and vertical salinity variations within the East Kutai Basin and evaluates their relationship with sedimentary facies distribution. Salinity data were obtained from formation water analyses and apparent water resistivity (Rwa) calculations derived from resistivity logs using Archie’s equation. These datasets were integrated with net sand maps and electrofacies interpretations to examine salinity patterns across multiple stratigraphic intervals. The results show that while the general lateral trend of low salinity in channel facies transitioning to higher salinity in mouth-bar facies is preserved, significant anomalies are observed. Laterally, mouth-bar deposits locally exhibit unexpectedly low salinity values, approaching those of channel facies. Vertically, salinity decreases with increasing depth in several stratigraphic groups, contrary to the expected progradational model. These anomalies are interpreted to reflect lateral shifting of depositional environments during delta progradation and the expulsion of low-salinity compaction water from overpressured prodelta shales into adjacent sand bodies.