Sedimentological and pore-scale characterisation of the Bockfließ Formation, Central Vienna Basin: implications for sealing potential

The Bockflies Formation represents parts of the Ottnangian Stage of the Lower Miocene in the Central Vienna Basin and is characterized by the type section from the Matzen 269 well. However, a detailed sedimentological characterisation of the type section has not been published. Previous studies on the Bockflies Formation predominantly focused on fossil assemblages, which indicated a lagoonal depositional environment with riverine input. This study intends to complement previous paleontological studies with a detailed sedimentological analysis, involving lithological, petrographical, ichnological, and macrofossil content descriptions of the cored sections from the Matzen 269 well. The sedimentological data was thereafter used to establish lithofacies and determine depositional environments represented by the cores. This suggests that the Bockflies Formation in the study area largely represents a coastal lagoon depositional setting, with identified depositional environments include lagoon, washover fan, channel fills (inlet) and shoreface. The lagoon facies is the most dominant and relevant sealing facies in the Bockflies Formation and could span tens of kilometres at its widest point and most likely formed due to coastal sea flooding. However, a connection was maintained between the lagoon and the sea, representing an inundation model, with modern analogues found in the Venice Lagoon in Mediterranean Sea and the Curonian Lagoon in the Baltic Sea.

The selected core sections were also evaluated for their sealing potential as the formation is a regional caprock for the underlying fractured carbonate reservoirs. The sealing potential of a caprock system depends primarily on its geometry, storage capacity, and structural integrity. To evaluate the sealing potential, lagoon facies were further subjected to XRD, QEMSCAN, MICP and coreDNA analyses. Based on these, the lagoon facies would most likely act as an intraformational seal, given the presence of other non-sealing facies. Rock strength data obtained through scratch testing indicate a subtle variation in strength within the lagoon facies, attributed to clay content. Moreover, the compositional make-up of the lagoon facies is akin to ‘sealing shales’ reported elsewhere. Additionally, XRD analysis revealed that illite is the dominant clay mineral across the lagoon facies, which is a non-reactive clay mineral.