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Application of chemostratigraphy and palynostratigraphy to the Mississippian of well Siciny-2, Poland

Abstract

This case study presents a multidiscipline approach applied to the Polish well Siciny-2, located in the Carboniferous Variscan foreland basin of the Fore-Sudetic Monocline, SW Poland. The area is structurally complex, being heavily affected by the Variscan orogeny, and later inversion events. Sequences are heavily faulted and may also even show repetition. Furthermore, the succession itself is characterised by featureless wireline logs making petrophysical correlation complicated. As a result it was deemed necessary to undertake a multidisciplinary stratigraphy methodology in order to unravel the structural complexity of this succession.

Palynostratigraphy divides assigns the Carboniferous (Mississippian) strata penetrated in Siciny-2 into six biozones ranging from Arnsbergian to Brigantian, with possible Asbian at TD. Overall the preservation is good towards the top of the section, although poorly preserved successions were encountered towards the bottom. Consequently a moderate to high level of confidence can be applied to the palynostratigraphy, providing a chronostratigraphic framework for the carboniferous succession. In addition, palynofacies analysis indicate open marine deposition with high terrestrial/freshwater input and dysoxic conditions at the sediment/water interface throughout the Mississippian. Parts of the section indicate changes in redox but these never achieve the suboxic or anoxic states associated with restricted marine basins.

Chemostratigraphy was also applied to mudstones in order to validate and enhance the palynostratigraphy. Samples collected every 10 metres over a 1730 metre interval were analysed by ICP-OES and ICP MS providing a full suite of major elements, trace elements and rare earth element data. The application of chemostratigraphy to the study interval has highlighted four chemostratigraphic sequences and eighteen chemostratigraphic packages. The geochemical zonations are applied to changes in; clay mineralogy (i.e. changes in weathering and paleoclimate), heavy minerals (i.e. provenance changes), feldspar abundances, and organic matter preservation. The chemostratigraphic zonation applied to the mudstone lithologies had the highest correlation potential. Furthermore, the chemostratigraphy also allows differentiation of the overlying Permian, revises the top Carboniferous pick, highlights candidate marine bands, based on changes in organic matter preservation, and highlights the occurrence altered trachyandesite volcanics within the Permian. In addition, a chemostratigraphic analysis of the coarser lithologies (turbidite facies) demonstrates a chemostratigraphic zonation, which most likely reflects a variation in provenance. While this zonation has lower correlation potential than that developed within the mudstone, should other wells be drilled in the basin which consisting of predominantly sandstone lithologies it has the potential to be used to characterise those wells and correlate to them.

The combination of biostratigraphy and chemostratigraphy in a multidiscipline approach has now developed a chronostratigraphic zonation for this study well, which as a type section can be applied to other wells drilled in this basin, possibly even at wellsite, developing a chronostratigraphic framework for this area.