The Lower Cretaceous J sandstone is the principal reservoir for oil and gas in the Denver basin of Colorado, Wyoming, and Nebraska. Net pay of the J sandstone depends strongly on sandstone depositional environments, but other important aspects of reservoir quality reflect the burial history. Most notable of these are porosity, permeability, depth, and degree of thermal maturation (as indicated by vitrinite reflectance). An understanding of the regional interrelationships between these variables is important in predicting reservoir quality and in estimating undiscovered petroleum resources in the Denver basin.
Statistical treatment of the core analysis and well-log data from 134 widespread boreholes across the basin, for which the U.S. Geological Survey has core, reveal the following. (1) Thermal maturity increases exponentially with depth, indicating increased temperature with burial. (2) Porosity decreases linearly with increasing Ro and depth. The presence of authigenic clays and carbonate cements are important to porosity reduction. In many examples across the basin, however, quartz pressure solution and precipitation processes are the main causes of porosity reduction, and these phenomena may be temperature-limited. (3) Permeability decreases exponentially with increasing depth. The permeability data exhibit more scatter than porosity, indicating a less direct relationship to depth and reflecting the effects of both porosity loss and increased surface area of the pore network. Authigenic clays, especially ordered illite-smectite, control the specific surface area of the pore network in the J sandstone.