Fracture-controlled paleofluid migration in the Óbánya Valley (Eastern Mecsek)

Abstract

Over the past two decades, traces of hydrocarbon-bearing paleofluid migration have been detected in several Triassic and Jurassic formations in the Mecsek Mountains. This study focuses on the easternmost known hydrocarbon occur­rence, represented by a partially cemented fracture network in the Óbánya Valley, with the aim of reconstructing the formation conditions of the fracture system. Classical structural geological methods were employed to characterize the evolution of this fracture system. As a result, two distinct deformation phases were identified: a) a folding event, and b) a subsequent left-lateral displacement. Theoretical orientation of fractures associated with the observed folds were deter­mined and compared with field measurements. This approach allowed us to define several fault orientations and fault breccias, as well as to link calcite veins to a folding process. Fluid inclusion petrography revealed the presence of both aqueous and hydrocarbon-bearing fluid inclusions within six distinct vein generations. These findings suggest that hydro­carbon-bearing fluids migrated along one of the four deformation-related fracture sets in this fracture system. Although the nature of the fluid inclusion assemblages provides only limited constraints on the thermal conditions during fluid migration, several vein structures suggest episodic increases in fluid pressure during the system’s evolution.