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Oklahoma State Univ., Stillwater, OK 74078

Abstract: Improved Petrophysical Analysis of Shaley Sandstones Using Integrated Wire-Line Log Interpretation

Low-contrast oil and gas reservoirs are becoming increasingly important exploration targets. These reservoirs contain significant reserves, but are very difficult to evaluate petrophysically since their calculated water saturations appear very high using conventional logs. 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 further refined in some cases using micro-imaging and magnetic resonance logs. Core-calibrated micro-imaging logs detect lithologic and diagenetic changes to the inch-scale. Magnetic resonance tools can further improve evaluation of some shaley sandstones by providing lithology independent effective porosity, irreducible water saturation and permeability.

A low-contrast gas reservoir was interpreted using an integrated petrophysical method. This approach is accomplished by coupling detailed core analysis, wire-line logs, and existing water calculation methodologies available in the literature. In this study, we have used high-resolution resistivity and porosity, micro-imaging, and magnetic resonance tools. A variety of methods for calculating water saturation, including conventional Archie, Simandoux, Indonesia, Dual-Water and Waxman-Smits were evaluated. All water saturation data derived by these various methods were compared to those measured from core plug analysis. The most realistic data were obtained using methods emphasizing shale volume, cation exchange capacity and conductance of bound water. Integrating magnetic resonance data into these calculations drastically improved water saturation interpretations. This approach significantly enhanced petrophysical interpretation of these low-contrast shaley sandstone reservoirs and improved predictions of fluid producibility.

AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas