Paleofluid evolution of the fractured basalt hydrocarbon reservoir in the Üllés-Ruzsa-Bordány area, SE Hungary

Abstract

Volcanic successions of the Kecel Basalt Formation (KBF) occur in the southern part of the Pannonian Basin. As a result of periodic submarine eruptions, the basaltic and pyroclastic rock horizons were intercalated with layers of the Late Miocene Endrod Marl Formation, which is regarded as one of the most important hydrocarbon source rocks in the area. The KBF was discovered through almost 30 wells between 2,200 and 2,900 meters of depth. Due to the high fracture porosity, some parts of the formation show good reservoir characteristics and act as important migration pathways of hydrocarbon-bearing fluids. Since the reservoir is presumably fracture-controlled, this study concentrates on the evolution of fractures crosscutting the rock body. Based on textural and mineralogical features, four distinct vein types can be distinguished, of which the first three types are discussed in this paper. Beside calcite, quartz, feldspar, and chlorite, the veins are cemented by various zeolite minerals. The vertical dimension of the dominant zeolite zone indicates the burial-diagenetic type of zeolite zonation and suggests subsidence of the subaqueous basalt after formation.

Pervasive early diagenetic dolomitization, subsequent hydrothermal alteration, and late stage hydrocarbon accumulation in a Middle Triassic carbonate sequence (Szeged Basin, SE Hungary)

“…Throughout the Szeged Basin, anticlinal features (particularly compactional anticlines over uplifted basement blocks), paleogeographic highs, growth faults, and rollover structures are the main trap types in conventional plays (Kókai and Pogácsás, 1991). The reservoir rocks are highly variable (e.g., fractured Variscan metamorphites, Mesozoic sandstones and carbonates, Upper Miocene sandstones and fractured basalts) (see Dank, 1988;Kókai and Pogácsás, 1991;Szabó et al, 2009Szabó et al, , 2016.…”

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

“…1). The volcanic rocks were penetrated between 2200 and 2900 m with a maximum thickness of 185 m. These basaltic rocks show intense chloritization and carbonatization as the result of metasomatism following the subaqueous volcanism (SZABÓ et al, 2009b). Fractures are often cemented by minerals (prehnite, chlorite, quartz, calcite), in particular by different zeolites (laumontite, thomsonite, analcime, mesolite, heulandite, stilbite).…”

“…The Laumontite (Lmt)-type veins formed later, during formation of the early fracture network and are composed of the prehnite-laumontite-thomsonite-laumontitechlorite-calcite mineral sequence. Details on the fracture fillings of the first three of the four vein types were given in SZABÓ et al (2009b).…”

“…In Lmt-type veins, the primary inclusions in the prehnite contain methane in the vapour phase at room temperature, but the rare single two-phase aqueous inclusions in calcite with no identifiable genetics do not contain any methane. For the fluid inclusion petrography and microthermometry of the first three vein types (Kfp-, Cal-and Lmt-types) see SZABÓ et al (2009b).…”

“…Named after their volumetrically most abundant cement phases, these are the potassium-feldspar (Kfp-), calcite (Cal-), laumontite (Lmt-) and the analcime (Anl-) types, respectively. A summary of the characteristics of all these vein types was published in SZABÓ et al (2009a), while the detailed description and circumstances of the Palaeofluid evolution in a fractured basalt hosted reservoir in the Üllés-Ruzsa-Bordány area, southern sector of the Pannonian Basin 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).…”