Stochastic simulations of dependent geological variables in sandstone reservoirs of Neogene age: A case study of the Klostar Field, Sava Depression

摘要

The research presented herein is the fi rst attempt to perform geostatistical simulations on three geological variables,porosity, thickness, and depth to reservoir, in the Croatian part of the Pannonian Basin. The data were collected from a reservoir of Lower Pontian age in the Klostar Field, located in the western part of the Sava Depression.All three variables were analyzed using sequential Gaussian simulations (SGS). Information regarding present-daydepth, thickness, and locations of areas with higher porosity values were used to reconstruct palaeo-depositional environments and the distribution of different lithotypes, ranging from medium-grained, to mostly clean sandstones and to pure, basin marls. Estimates of present-day thickness and depth can help to defi ne areas of gross tectonic displacementand the role of major faults that have been mapped in the fi eld. However, since mapping of the raw data (includingporosities) does not allow the reconstruction of palaeo-depositional environments, sequential indicator simulations (SIS) were applied as a secondary analytical tool. For this purpose, several cut-off values for thickness weredefi ned in an effort to distinguish the orientation of depositional hannels (main and transitional). This was accomplished by transforming porosities to indicator values (0 and 1) and by applying a non-linear indicator kriging technique such as the “zero” map for obtaining numerous indicator realizations by SIS.In the SGS and SIS approaches, the simulations encompassed 100 realizations. A representative realization was then selected using purely statistical criteria, i.e., two realizations were almost always chosen in accordance with the order of the calculation. The 1st and 100th realizations were selected for each variable in the SGS and SIS and fi ve indicator kriging maps were chosen for the thicknesses cut-offs.