Palaeofluid evolution in a fractured basalt hosted reservoir in the Üllés-Ruzsa-Bordány area, southern sector of the Pannonian Basin

Extensive Miocene volcanic activity produced basaltic and pyroclastic successions, which were penetrated by many wells in the area of Üllés-Ruzsa-Bordány, in the western and central part of the Great Hungarian Plain. The Kecel Basalt comprises primary porosity from a high proportion of vesicles and significant secondary fracture porosity, as well. Due to the significant fracture porosity, some depth intervals show good reservoir characteristics, although the fractures crosscutting the rock body are partly or entirely cemented by various minerals. Based on the textural and mineralogical features, four distinct vein types can be distinguished, named after their volumetrically most abundant cement phases, i.e. potassium-feldspar (Kfp-), calcite (Cal-), laumontite (Lmt-) and analcime (Anl-) types. This study focuses on probably the youngest of these vein types, the Anl-type. Based on the study of veins and mineral sequences, the direction of temperature changes cannot be given unequivocally for every stage of cementation, but crystallization of the Anl-type veins might have occurred at lower temperatures than the formation of the Lmt-type veins. Fluid inclusion studies suggest that hydrocarbon migration and accumulation took place after cementation of the first three vein types (Kfp-, Caland Lmt-types). In the newly opened fracture system, two types of hydrocarbon (HC1 and HC2)-bearing fluid inclusion assemblages were captured during precipitation of analcime and later zeolites. This refers to two stages of hydrocarbon migration in the fracture system. Observations of the fluorescence colours and low temperature behaviours of the hydrocarbon-bearing inclusions, the earlier HC1 petroleum-inclusions captured heavier (presumably less mature oils), while the later ones (HC2) lighter (presumably more mature) oils. The HC2 petroleum seems to be very similar to the crude oil sampled in a well in the area based on their fluorescence parameters. evolution of the first three types were discussed by SZABÓ et al. (2009b). Nevertheless, from a petroleum geology point of view, the most important vein generation is the youngest one, since hydrocarbon inclusions were identified exclusively in its vein filling mineral phases (SZABÓ et al., 2009a). It was previously proven (SZABÓ et al., 2009a) that rocks of the KBF are characterized by two types of porosity comprising an extended fracture system and partially-filled vesicles.The occurrence of similar petroleum inclusions both in the cemented Anl-veins and in the adjacent wall-rock vesicles suggests that filling of the reservoir took place not only along localized fractures but also along altered basaltic zones around them. The evolution of the youngest vein generation and its role in the hydrocarbon system in question is the focus of the present paper. The key questions concern the subsidence history of the area through estimating temperature and pres sure change during the formation of the reservoir, as well as the stages and the timing of petroleum migration. Utilising geochemical data, SZABÓ et al. (2012) evidenced a genetic relationship between the potential source rock (marl) and the hydrocarbons produced. From this aspect the area in question is a hy dro carbon system based on the definition of MAGOON (2004). The study of the vein-filling minerals and their fluid inclusion assemblages could provide valuable information about the post-volcanic evolution of the area, the circumstances of hydroArticle history: Manuscript received May 19, 2015 Revised manuscript accepted August 22, 2016 Available online October 07, 2016