Sedimentary facies and their diagenetic equivalents in Zechstein carbonates and adjacent evaporates are determined from wireline logs using a petrophysical database calibrated in14 key wells. The selected key wells have between 8and100% core cover with49% on average, and a logging suite consisting of density, neutron, photo-electric factor (Pe), sonic, gamma ray, deep, shallow and micro-resistivity logs. The database was constructed in three major steps:
A volumetric analysis incluing mineralogical composition, porosity and saturation was computed usin an iterative minimisation program and checked against petrophysical core data, petrographic data provided by core description, and mineraolgical data based on microscopy and X-ray analyses. Principal minerals were calcite, dolomite, anhydrite, and halite with occasionally significant amounts of potassium salts, pyrite, clay, sulfur, and bitumen. Gas corrections based on the calculated saturations were used to normalized the logs for each key well.
The gas-corrected log data were then grouped using a cluster analysis of their principal components to obtain electrofacies for each single well. These electrofacies were matched with geological facies defined from the petrographic core descriptions.
Log data values of similar electrofacies from the individual key wells were plotted in 10 two-dimensional multiwell-crossplots.
To avoid ambiguities produced by the cluster analyses an independent five-dimensional ellipsoid was created from the multiwell-crossplots for each geological facies. The defined ellipsoids were combined to forma petrophysical database. In general the following carbonate facies with significant differences in log responses are distinguished based on different mineralogies and porosity types:" replacement dolomites such as mudstones, mouldic mudstones, wacke-, grain- and packstones, which show a curved density/neutron response," recrystallized dolomites derived from successive microsolution, accretive crystallization and subsequent compaction, which show a straight line response in the density/neutron crossplot;" replacement and recrystallized dolomites as above, but evaporite cemented with anhydrite and/or halite;" dedolomites including layered, nodular (concretionary), vuggy types and calcite-cemented grainstones;" limestones divided into micrites, sparitic mudstones, wacke- and grainstones. Petrophysically significant traces of minerals such as pyrite and evaporite mixes also require separate ellipsoids, resulting in 48 carbonate and 24 evaporite lithofacies. The correctness of the database was verified on three cored test wells. Evaluation using old logging suites consisting of density, sidewall or gamma-ray neutron, gamma ray, sonic and laterolog seven is now also performed. Routine application of the database provides complete facies descriptions in uncored sections and wells for use in geological profiles and facies maps.
