Controls of Organofacies and Redox Conditions on Organic Matter Quality in the Toarcian Black Shales of the Paris Basin

This study presents a new geochemical and petrographic data from investigation on 41 lower to middle Toarcian organic-rich shale to understand how redox conditions across the eastern half of the Paris Basin. Rock-Eval pyrolysis, time and space control organofacies and organic matter quality in the Paris basin. Kerogen types range from type II to type III, with marked lateral and vertical variations from the eastern basin margin toward the depocenter. Four organofacies associations were distinguished, showing a progressive shift from AOM-dominated assemblages, interpreted as reflecting anoxic to suboxic conditions and efficient preservation of marine organic matter, to phytoclasts- and terrestrial palynomorphs-rich assemblages, consistent with dysoxic to oxic conditions and poorer organic matter preservation. Reconstructed original TOC and HI values reach approximately15 wt.% and 700 mg HC/g TOC, respectively, in the eastern sector, indicate a first phase where restricted circulation and oxygen-depleted bottom waters favoured organic-matter accumulation and preservation in the eastern sector while suboxic conditions promoted the preservation of AOM-rich organic matter but with lower TOC value in the depocenter. Later, dysoxic to oxic conditions reduced source-rock quality across the whole basin.These results show that organofacies composition and redox-controlled preservation exerted a primary control on source-rock heterogeneity in the Toarcian black shales of the Paris Basin and can be used as an analogue for similar intracontinental basin.