Oil- and gas-bearing sediments of the Main Dolomite (Ca2) in the Międzychód region: a depositional model and the problem of the boundary between the second and third depositional sequences in the Polish Zechstein Basin

Abstract

A b s t r a c t. The Polish Zechstein Basin was a tideless sea dominated by storms. Main Dolomite deposits of the Międzychód region were deposited: a) on the carbonate platform (in the environments of the outer barrier, inner barrier and high- and low-energy platform flat); b) on the platform slope; c) at the toe-of slope; d) on the basin floor. The best reservoir properties are recorded in shallow-marine deposits of the outer and inner barriers and in deep-sea sediments of the toe-of-slope (turbidites and debrites). Rich reserves of crude oil and natural gas were discovered both on the carbonate platform (the Międzychód and Grotów deposits) and at its toe-of-slope (the Lubiatów deposit). The Main Dolomite sediments are wholly included in the second depositional sequence (PZS2 sensu Wagner & Peryt, 1997). The maximum flooding surface of the PZS2 sequence within the platform, its slope and toe-of-slope, runs along the A1g/Ca2 boundary. In the basinal zone, its correlative equivalent is a hard ground observed within the Main Dolomite carbonate rhythmites. The boundary between the second and third (PZS2/PZS3) depositional sequences (corresponding to the ZS3/ZS4 sequence boundary in the German Basin) runs on top of the Main Dolomite carbonates (on the platform slope, at the toe-of-slope and on the basin floor) and above top of the Main Dolomite carbonates, within the lower part of the Basal Anhydrite (on the platform).