Determining water saturation in low resistivity - low contrast formations using the Leverett J - Function: A case study of well X, field Y, Cuu Long basin

Abstract

The Field Y, located in the Cuu Long Basin, Vietnam, is currently being explored and developed for its potential in the Lower Miocene reservoirs. However, one of the key challenges faced by geologists and well log analysts is the presence of low - resistivity, low - contrast (LRLC) pay zones, which hinder the accuracy of log interpretation and evaluation. Calculating water saturation in these low - resistivity zones using conventional interpretation techniques often results in significant errors, potentially leading to the misidentification of productive intervals. This paper determines the primary causes of low resistivity, including the influence of invasion zones, well and formation dip, limitations of logging tools, grain size, clay content and distribution, and formation water salinity. The authors propose the Leverett (J - function) method, independent of resistivity, to calculate water saturation. This method aims to improve the calculation of net pay thickness in the Well X, Field Y. Results show that in the Lower Miocene formation, the net pay thickness of the BI.2.20 interval increased by 2,773 meters and the BI.1.20 interval increased by 1,981 meters. This improvement helps to avoid misidentification of productive zones and ultimately increase recoverable reserves.