The Impact of Water Salinity in Deltaic Environment to the Resistivity-Based Pore Pressure Analysis: A Study Case in Exploration Well in Sanga Sanga Axis, Kutei Basin, Indonesia
Authors: Savitri Cindy Dhevayani , Sakti Kurniawan Giga , Manurung M. Syukur , Nursasono Danny , Said Dwi Kurniawan , Pasaribu Hendra Halomoan , Raharjo Budi , Berto Albertus Ditya , Melkybudiantoro Didi , Apranda Yoseph Robby , Putranto Asnanto Multa 
Published: January 2025

ABSTRACT:
Various geological factors, such as, bed thickness, grain size, mineralogy, clay distribution, and water salinity contribute to high or low resistivity reading in the subsurface formation. This study focused on the effect of water salinity on the normal compaction trend (NCT) determination. A study was conducted in a wildcat exploration well in the Sanga Sanga Axis, Kutei Basin, Indonesia. Logging While Drilling (LWD) using gamma ray and resistivity was employed for subsurface evaluation. The objective was to identify hydrocarbons trapped within a deltaic sedimentary sub-thrust geological setting. The pore pressure was real time monitored by employing resistivity log and calculated by using Eaton method. NCT from resistivity log was determined prior to the pore pressure analysis to observe the normal trend of hydrostatic pressure. Based on the observation, there were various change in the resistivity log due to water salinity. The variation of salinity including fresh water, slightly saline, and highly saline water, were influenced by progradation and retrogradation resulting from sea level changes during sediment deposition. This paper shows the necessity of water salinity study prior to conduct the pore pressure analysis. This study suggests the improvement of resistivity-based pore pressure analysis by considering water salinity in the deltaic environment.

1. INTRODUCTION
Various geological factors, such as bed thickness, grain size, mineralogy, clay distribution, and water salinity, play critical roles in determining resistivity readings in subsurface formations. These parameters are not only essential for interpreting logging responses but are also vital for accurately identifying hydrocarbon-bearing zones during oil and gas exploration. Furthermore, they hold significant importance in analyzing pore pressure, which is crucial for predicting drilling hazards and ensuring safe extraction operations.
Among these factors, water salinity is particularly influential. Salinity variations may have a pronounced impact on the electrical properties of the formation, leading to changes in resistivity values. Such fluctuations complicate the interpretation of well logs and necessitate careful calibration when determining formation properties. This study focused on water salinity and its relationship with the Normal Compaction Trend (NCT), a key reference used in pore pressure analysis. The findings suggest that multiple changes in salinity disrupt the expected resistivity pattern along the NCT, highlighting the need for accurate salinity assessments to ensure reliable compaction trend analysis. Understanding these interactions may lead to more accurate subsurface evaluations, reducing risks associated with overpressure zones and improving resource recovery efficiency.