Lithology identification using open-hole well-log data in the metamorphic Kiskunhalas-NE hydrocarbon reservoir, South Hungary

There are four main rock types along the ideal rock column of the Kiskunhalas-NE field; in order from the bottom upwards, orthogneiss, orthogneiss mylonite, graphitic gneiss mylonite and graphitic carbonate phyllite. These main rock types are characterized by significantly different reservoir features, as was proved by previous rock mechanical investigation. The geophysical information of well-logs (gamma, resistivity, neutron, density and acoustic logs) allows an understanding of the spatial extension of the good reservoir blocks. In the course of the examination conventionally applied plots, MN plots and discriminant function analysis were used. Three rock types were successfully identified along the wells, the graphitic carbonate phyllite, the mylonite and the orthogneiss. On the basis of the results, lithological boundaries could be estimated in numerous wells. These boundaries were presented along geological sections. Taking also the independent hydraulic regimes of the reservoir into account, a series of south-dipping normal fault-bounded blocks are assumed. Inside each block shallow-dipping mylonite/gneiss boundaries with a north-northeast dip direction are typical. The existence of this low-angled (less than 5°) mylonitized zone refers to a presence of a one-time detachment fault linked to the formation of a metamorphic core complex.