Estimation of remaining oil saturation and monitoring the time-lapse changes plays critical roles in hydrocarbon reservoir management. This saturation information is used for tracking reservoir depletion, water-oil contact movement, deciding workover strategies, water-injection/waterflood management and enhanced recovery.

World’s most oil reservoirs are in low-salinity environment where Carbon-Oxygen (C/O) logging is invariably used to establish cased-hole oil saturation. The objective of a C/O logging is to locate bypassed or left-over oil reserves and to estimate remaining oil saturation. Mineralogy of the formation significantly influences the C/O log processing for saturation estimation. Quantitative knowledge of formation mineralogy is a key to accurate saturation evaluation form C/O log. Inaccurate lithology information leads to wrong saturation estimation. This uncertainty in saturation calculation can be addressed by including a multi-mineral model generated from traditional petrophysical “elemental analysis” of open-hole logs with C/O measurements.

In cases where a detailed petrophysical analysis is not available for the well, spectral stripping of the capture gamma-ray spectrum, obtained from the C/O log, can define the elemental yields of some characteristic elements. These relative elemental yields are then used in an oxide closure model to derive the dry weight elemental concentration. Ultimately the approach gives a basic mineral model of the matrix in terms of clay, carbonate, anhydrite, coal, evaporite and quartz-feldspar-mica (the sand fraction). This basic model can be used very well in absence of petrophysical elemental analysis to reduce saturation uncertainty.

This paper details a methodology to mitigate uncertainty in saturation estimation due to uncertainty in lithology. Reducing uncertainty is essential for an efficient reservoir management. A suitable mineral model is worth to incorporate in saturation evaluation and can lead to an informed decision making.