Well log interpretation of metamorphic hydrocarbon-bearing formations

Exploration for hydrocarbon reservoirs within fractured and weathered zones of the crystalline basement is of great importance in Hungary. The majority of hydrocarbon reserves found in recent years have come from metamorphic reservoirs in the basement. Metamorphic hydrocarbon-bearing formations are generally characterised by complex lithology, low porosity and a heterogeneous distribution of pore sizes and fractures. The interpretation of well logs from these formations is an especially difficult task because of the above-mentioned properties, yet the required standards of data quality are the same as in the case of sedimentary reservoirs. The interpretation of weil logs to locate reservoir zones and phase boundaries has been made even more difficult because many of the boreholes have been logged with only a few tools. It was therefore necessary to develop a novel method of well log interpretation which could produce useful results even in cases when the selection of well logs had been far from optimal. The low porosity has made it essential that reservoirs are not damaged by cementing in casing before logging. These circumstances and the need to obtain more accurate reservoir characteristics have led to the application of production well logging techniques. The interpretation of production well logging and open hole data has resulted in a better understanding of the nature of reservoirs of metamorphic lithology and poor reservoir characteristics, and in a significant improvement in their exploitability.