Abstract

Low-contrast oil and gas reservoirs are becoming increasingly important exploration targets. These reservoirs contain significant reserves, but are difficult to evaluate petrophysically because their water-saturation (Sw) calculations seem high using conventional analyses. Recent technological advances in logging have improved interpretation and evaluation of these zones. High-resolution resistivity and porosity logs accurately represent rock properties in beds greater than 1 -foot thick. Formation evaluation can be refined further in some situations using core-calibrated micro-imaging logs. These logs detect lithologic and diagenetic changes to the inch-scale and can be used to identify reservoir and seal zones.

A low resistivity/low contrast LR/LC gas reservoir was analyzed using an integrated approach involving detailed core analysis, production data, and various petrophysical methods. Wireline logs were correlated to other data to evaluate the effectiveness of various tools in determining lithology and measuring rock and pore fluid properties in shaly sandstones. A variety of methods were used to determine clay content. Water-saturation (Sw) calculations from core analyses were compared with those from several models designed to evaluate shaly sandstones. The models analyzed show high variability in predicting water saturation. This suggests successful Sw calculations in shaly LR/LC reservoirs may depend on modifying these equations so their components reflect rock constituents.