| Popis: |
In the established production province of the Panonian basin, an operator is in an appraisal phase of a recent discovery that focuses on a series of radioactive sandstone reservoirs. During the exploration phase, the operator could not clearly identify and quantify the source of radioactivity in the sandstones ; the source could be clay, silt, or the sand fraction of the mineral succession. A petrographic analysis of cores identified the primary minerals as illite/chlorite (clay representatives), potassium feldspars, mica minerals (muscovite main representative), and quartz. In general, the conventional (GR/SP/crossplots) clay volume calculations can mix up clay with radioactive matrix minerals and vice versa in radioactive sandstones. Consequently, the operator elected to use nuclear magnetic resonance (NMR) as part of the well logging program to acquire additional information regarding the clay presence. The goal of the acquisition was to integrate and calibrate porosity distribution (total and effective) and permeability information from NMR with core data, rather than simply to identify and quantify the clay content. Two sandstone reservoirs were evaluated within the same stratigraphic formation. The data set consisted of conventional well log data, NMR, and routine core analysis (RCAL) from cores of both reservoirs. The T2 distribution of NMR showed that the clay content is very low and that most of the radioactivity originates from the silt- or sand- sized particles. This information enabled the operator accurate calculation of the clay volume. This way underestimation of the effective porosity because of overestimated clay volume is minimized. Laboratory measurements from the core data were used to compare and perform quality control of the NMR logging. The laboratory porosity was used for NMR quality control, while the permeability was used to calibrate Coates equation. |
