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Storm Bed Sequences and Depositional Environments: Sedimentary Model and Well Correlation for the Utica/Point Pleasant Formations of Eastern Ohio, Appalachian Basin, Ordovician

Abstract

This study is focused on the Appalachian basin in the Eastern Ohio state. This paper discusses the sedimentological and palaeoenvironmental interpretation of shaly series recognized by two cored wells penetrating the Middle to Upper Ordovician Utica and Point Pleasant formations. This stratigraphic interval corresponds to a thick formation extending through many states in the NE of USA, extensively studied for its recoverable natural shale oil and gas resource potential. The underlying Trenton formation has also been described. Height distinct non reservoir and reservoir lithofacies are identified from cores. Studied samples allow detailed observation of storm bed deposits. Storm events seem to be the dominant sedimentary features, even if gravity-driven flows such as turbidity currents are not excluded as secondary reworking processes. Indeed, locally high energy event (such as storm) can trigger sediment suspension or downslope movement, initiating turbiditic deposit. Storm bed deposits are illustrated by fining upward graded bedding. Brachiopods and crinoids shell fragments are the main origin of coarse clasts. Their vertical occurrences are clues for bathymetry and palaeoenvironmental interpretation. Storm events are alternating with black shale facies associated to high TOC contents. Hence, palaeogeographic model and associated facies distribution understanding is the keystone of sweet spot delineation in this part of the Appalachian basin. First core descriptions show evidences for deposition under relatively shallow marine environment from middle shoreface to offshore, between typically 15m and 50m depth corresponding to the fair weather wave base (FWWB) and the storm wave base (SWB). Below storm wave base, the sea floor and suspended sediments are mostly influenced by density/gravity-induced flows. To conclude, this paper proposes a depositional model for Utica/Point Pleasant/Trenton formations supported by process evidences and diagnostic sequences. A stratigraphic framework is also displayed through the two cores correlation transect, showing polarity changes between depositional systems of Utica and Point Pleasant formations.