SEQUENCE STRATIGRAPHY OF THE UPPER PERMIAN ZECHSTEIN MAIN DOLOMITE CARBONATES IN WESTERN POLAND: A NEW APPROACH

Abstract

The Upper Permian Main Dolomite in the Zechstein 2 cyclothem in the Gorzów Block (part of the Zechstein Basin in western Poland) contains both hydrocarbon source and reservoir rocks, and is sealed both above and below by evaporites. In this paper we propose a new sequence stratigraphic model for the development of potential reservoir rocks in toe‐of‐slope locations. Data came from detailed analyses of 35 cores from wells in and at the margins of the Wielkopolska platform, a palaeogeographic element composed of Main Dolomite carbonates.In basinal areas, the Main Dolomite carbonates begin with a transgressive interval overlain by laminated dolomudstones interpreted as transgressive facies. The TST begins in the upper part of the underlying Alg anhydrites. The dolostones are underlain by a ravinement surface on the platform, and by a maximum regressive surface in toe‐of‐slope and basinal locations. In well Gorzów Wielkopolski‐2, a hardground marks the maximum flooding surface. Overlying the TST deposits are thick intervals of intraclast‐oolitic grainstones and floatstones which are interpreted as highstand deposits and indicate “highstand shedding”. Toe‐of‐slope facies are composed of alternating laminated dolomudstones, intraclast‐oolitic grainstones, packstones and floatstones which make up submarine fans (prisms) interpreted as falling stage facies which are capped by dolomudstones. A subaerial unconformity was recognized on the platform, and a slope onlap surface on the slope and toe‐of‐slope, respectively.In platform areas, the Main Dolomite begins with thin intervals containing microbial complexes deposited during the early HST, which pass into thick oolitic grainstones (HST to late HST) and terminate as microbial‐to‐oolitic wackestone and mudstone complexes interpreted as falling stage facies. Thrombolitic bioherms constitute a reference horizon which can be correlated between wells in the study area. The beginning of the LST occurs in the upper part of the Main Dolomite. The boundary between lowstand and transgressive deposits was identified in the lower part of the Basal Anhydrite and is marked by sabkha and salina facies, respectively, where an erosional ravinement surface and maximum regressive surface were identified. Thus, the upper part of the underlying Upper Anhydrite and the upper part of the Main Dolomite deposits form a second depositional sequence in the study area.The depositional environment of the Main Dolomite platform carbonates was variable, and was influenced by the topography of the pre‐existing evaporitic platform. The newly proposed sequence stratigraphic model emphasises the role of forced regressive submarine fans as potential hydrocarbon accumulations and traps in the toe‐of‐slope area.