Chronohorizons Based on Resistivity Curve Variations - Upper Miocene Sediments of the Ivanić Grad Formation in the Sava Depression (NW Croatia)

Regional definition of chronohorizons represents a fundamental and most difficult problem in geology. Adequate precision of dating can only be obtained from biological evolution, radioactive decay and from the key surfaces of the sequence stratigraphic concept (MFS, TSE, and LSE). It is concluded in this paper that the variation of for- mation resistivity values (Ra curve), as measured in wells located in the Croatian part of the Pannonian Basin System, is also a criterion for the stratigraphic definition of concurrently deposited sediments. This enables the construction of a sequence of lithogenetic units relat - ed to very small time-intervals. The morphology of the resistivity curve measured in well sec- tions composed of massive marls (that were deposited in a quiet envi- ronment of moderate water depth), is regionally exceptionally well defined. This pattern consists of a succession of peaks of lower and higher resistivity values that are in the same relative position region- ally, which results in similar form of the resistivity curves of the wells in the study area. On the curve, one peak of higher values together with the next (overlying) peak of lower ones composes a single cycle (SC). Marls of a single cycle were regionally deposited at the same time, thus enabling the points of both the maximal and minimal resis- tivity values of each SC to be considered as time-markers (chrono- horizons). Variation of the resistivity reflects fine changes of petro- graphic and granulometric composition which can be associated with cyclic water-level fluctuations (Milankovitch cycles). For future exploration of the remaining, potential hydrocarbon accumulations in the sandstone reservoirs of the IvaniÊ Grad Forma- tion in the Sava Depression, spatial modelling of the distribution, geometry, facies and palaeocurrent structure of separated depositional episodes and genetic units is suggested. This should be done with special emphasis on the spatial definition of channel sandstones that are the most important hydrocarbon reservoirs (according to current interpretations).