LEVERAGING SPONTANEOUS POTENTIAL AND NEUTRON DENSITY POROSITY LOGS TO CONSTRUCT A GEOCELLULAR MODEL: AN EXAMPLE FROM THE THICK CYPRESS SANDSTONE AT NOBLE FIELD, ILLINOIS

Detailed geocellular models of reservoir architecture and geologic features reduce uncertainty in projections made for field developments such as CO₂ enhanced oil recovery (EOR) and storage. In mature oil producing basins, such as the Illinois Basin, the data required for the construction of such models is limited, due to the lack of wells with neutron-density porosity logs. A method was developed to derive porosity from widely available spontaneous potential (SP) logs. This method was enhanced with neutron-density porosity logs, which were available but much less abundant than the SP logs for this field example.

The volume of shale (Vsh) was calculated from SP logs using clean sandstone and shale baselines. Using 385 SP logs, a geocellular model of Vsh was made for the thick Cypress Sandstone at the Noble Oil Field in Richland County, IL. The spatial coverage of the SP logs was sufficient to detect a strong NE/SW trending anisotropy and condition simulations to model the distribution of sandstone and shale. Using the available neutron-density logs with SP from the same well, Vsh was cross plotted and regressed against porosity. This crossplot was used to convert the Vsh in the geocellular model into porosity.

From detailed evaluation of whole core and the porosity logs, thin (0.5-1 m thick) layers of low porosity calcite cement were found, which were not observed on the SP log. These were interpreted to have formed at the oil water contact and act as a laterally continuous baffle to vertical fluid flow. Data from 125 wells with neutron-density porosity logs was used to detect and incorporate the calcite cemented intervals into the model. There were too few of these logs to characterize the field-wide lateral anisotropy in sandstone and shale, but geostatistical analysis of the porosity logs did indicate two parallel layers of calcite cement; one at the oil water contact and one about 9 meters below it. The cells within these layers were assigned porosity values from the neutron-density logs and the remaining cells were assigned the porosity derived from the Vsh.

This study demonstrates the importance of understanding the strengths and weaknesses of data types, and how different log types can be leveraged to construct a geocellular model.