--> Developing an Open Shelf Shale Depositional Model: the Early Ordovician Tøyen Shale Formation Example.
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Developing an Open Shelf Shale Depositional Model: the Early Ordovician Tøyen Shale Formation Example.

Abstract

Fine-grained organic-rich siliciclastic rocks referred to as ‘black shales’ have proven to perform excellently as self-sourced petroleum reservoirs. Oil and gas production from these organic-rich ‘tight’ shales significantly increased over the past decades due to the technological advancement in unconventional plays exploration. Despite the wide abundance and economic importance of shales, its depositional framework and processes leading to preservation and distribution of organic matter are not well understood. This study uses the Ordovician Tøyen Shale Formation of Scandinavia to build a comprehensive sedimentological model for shales deposited in a marine open-shelf environment.

The succession examination was performed on the field, drill core and thin section scales. Correlated stratigraphic sections from outcrops and core supported by graptolite biostratigraphy provide the sequence stratigraphic framework, while Optical and Scatter Electron Microscopy was used to determine fine-scale sediment characteristics and establish facies architecture. Zones of highest organic matter content and hydrocarbon potential were determined using TOC and Rock-Eval pyrolysis analysis.

The Tøyen Shale consists mostly of silt-dominated mudstones with much of the silt grains consisting of carbonates. The carbonate content varies: this unit is in places arranged in centimeter- to decimeter-scale normally graded beds with high amounts of carbonate at the base and successively more clay-size material and organic matter towards the top. Preliminary analysis show the organic matter content to be around 0-5% with the lowest values in the lower Hagastrand member abruptly increasing at the onset of the upper Galgeberg Member, and from there successively decreasing upsection.

The resulting model will contribute to the understanding of the processes operating on marine shelves and their effect on shale facies architecture and distribution of organic-rich high-TOC intervals. Even though this model is based on an Early Paleozoic example, the processes influenced specific to open shelf successions facies architecture should make it applicable to many unconventional systems worldwide. With modern progress in understanding shale plays and production technology, screening of sedimentary systems like the Tøyen Shale Formation may contribute to expanding less-explored potentially hydrocarbon-bearing zones in the Northern America and worldwide.