Abstract
Significant natural gas resources are known to exist in the United States in tight, low-permeability sandstones that cover a prospective area of 1,000,000 mi2 (2,590,000 km2). Characterization and reliable estimation of their production potential based on well logs are important although difficult task.
Proper evaluation of low permeability sands based on conventional log-interpretation techniques is frequently inadequate. Furthermore, while empirical rules of thumb assist in the evaluation of localized conditions, they only provide guidelines.
Recent developments in quantitative log-analysis techniques incorporate natural-gamma-ray spectral data and application of the Waxman-Smits model for detailed reservoir description. Quantitative correlations of cation exchange capacity (CEC), water salinity, porosity, and conductivity of water-and hydrocarbon-bearing shaly sand reservoirs are based on resistivity, density, neutron and natural-gamma-ray spectral data. These correlations provide important information about clay volume, reservoir porosities (total, effective) and fluid-saturation distribution (total, effective), type of clay minerals (smectite, illite, chlorite/kaolinite), their distribution in the reservoir (dispersed, laminated, structural), and log-derived indicators of potential formation damage.
Field experiences are reviewed for logging and evaluating tight formations in south Texas; the Jurassic Cotton Valley trend in east Texas, Louisiana, and Arkansas; and the Tertiary Fort Union and Cretaceous Mesaverde Formations of the Piceance basin in Colorado.