Petrology and paleokarst features of the Gomba hydrocarbon reservoir (central Hungary)

The pre-Cenozoic basement of central Hungary is partly made up of different types of carbonate rocks. These carbonates are often good hydrocarbon reservoirs, and hydrocarbon production is significant in this region in Hungary. Nonetheless, the petrography of the reservoir rocks has not yet been investigated in detail. In this study, the results of the investigations of the lithology of a carbonate hydrocarbon reservoir from central Hungary (Gomba Field) are presented. Based on this work, two types of pure limestone, a dolomitic limestone and a polymictic breccia, could be distinguished in the study area. The limestone types are similar to the Kisfennsík Limestone Member and the Berva Limestone of the Bükk Mountains, but they contain significant amounts of framboidal pyrite and dead oil as vein fillings. The breccia is predominantly composed of angular carbonate clasts and minor metamorphic and sedimentary rock fragments in a chaotic pattern. The breccia has some grains that may be speleothems (e.g., stalactite or stalagmite) based on their structure and isotopic compositions. The fabric of the breccia suggests that it may have been formed by fluid-related processes. Cross-cutting relationships of the veins and petrography of the vein fillings suggest that there are four different fracture generations and two different hydrocarbon migration phases to be distinguished. The composition of the hydrocarbon-bearing fluid inclusions related to the second migration event is similar to the crude oil currently produced from the Gomba Field. During the Eocene, the Triassic basement was buried and brecciated. Subsequently, a primary hydrocarbon migration can be assumed, but the hydrocarbons became overmature, apparently due to the high temperatures of the burial environment. Finally, an uplift phase began and the youngest fracture generation formed, which serves as a primary pathway for the more recent hydrocarbon migration