Selection of the most appropriate interpolation method for sandstone reservoirs in the Kloštar oil and gas field

Abstract

The distribution of porosity data collected at the Kloštar field is analysed. This field is located in the north-western part of the Sava depression, and represents one of the most long-standing fields with respect to oil production in Croatia, where production has been ongoing for more than 50 years. Several reservoirs of very diverse lithology and stratigraphic age have been discovered. The oldest reservoirs (with minor oil reserves) occur in basement rocks of the Tertiary system (informally named the „temeljno gorje";;), then in Badenian coarse-grained clastics, as well as sandstones of Pannonian and Pontian ages. Major reservoirs, with respect to size, number and volume, are located in sediments of the Upper Pannonian and Lower Pontian. These are medium-grained sandstones, mostly quartz-mica grains, mutually separated by marls. Reservoirs of Lower Pontian age occur within whole structures (anticlines) at Kloštar, while the Upper Pannonian has been developed only in the south-west part. Earlier studies on the reservoirs have included only several measured values of reservoir porosity. These values have not been mapped, but only a unique average value, characteristic for the whole reservoir was calculated. Reinterpretation of e-logs increased the number of input values to 20 points of data. Rules are determined for the distribution of porosities in the largest reservoir of the Lower Pontian named ";;T";;. These regularities can be translated to the other sandstone reservoirs. Several interpolation methods were selected (inverse distance, nearest neighbourhood and moving average), with special emphasis on kriging. The interpolated maps are different. Based on a comparison of isoporosity, lines, shapes, and cross-validation results, kriging was evaluated as the most appropriate method for porosity interpolation.