Stratigraphic Framework and Sandstone Facies Distribution of the Pannonian Sequence in the Békés Basin

Abstract

The Békés basin is a late Tertiary structural basin containing as much as 6500 m of Neogene sandstone, siltstone, claystone, and marl, most of which comprise the Pannonian sequence or group. Interpretation of well data and seismic reflection data indicates that these rocks were deposited in a wide range of depositional environments ranging from deep-basinal brackish/lacustrine to basin slope to shallow lake and delta plain to alluvial plain. Seismic reflection profiles clearly show topset, foreset (clinoform), and bottomset (basinal) depositional architecture and that the different faciès are highly diachronous. Correlations of seismic reflections with well logs indicate that the reflection patterns within each of the three main reflection groups contain characteristics that can be related to lithologies and depositional environments, which in turn reflect formational terminologies. In ascending order, these formations are (1) the Tótkomlós and Nagykörü Formations, a basal clayey marl and marl unit; (2) the Szolnok Formation, a basinal unit consisting of thick amalgamated beds of very fine grained turbidite sandstone and interbedded marl; (3) the Algyö Formation, consisting of prodelta-slope clay and marl and lesser amounts of turbidite or slumped sandstone in the lower part; (4) the Törtel Formation, consisting of delta-plain and delta-front very fine- to medium-grained sandstone and interbedded clay and clayey marl; and (5) the Zagyva and Nagyalfold Formations, consisting of upper delta-plain or alluvial-plain and inland-swamp sandstone and clay. The lower three formations are referred to as lower Pannonian, and the upper formations as upper Pannonian. Sandstones in the lower Pannonian are more compacted, and the reservoir properties are not as good as the generally friable sandstones in the upper Pannonian.