The Schistes Carton — the Lower Toarcian of the Paris Basin

Abstract

The Schistes Carton was deposited within an epicontinental sea during the falciferum zone (early Toarcian), during a period of approximately 500 000 years. Available data suggest that deposition occurred under oxygen-depleted conditions. These shales typically obtain thicknesses between 10 and 20 m, with maximum values in excess of 50 m. The shales are dominated by illite and kaolinite, with only minor amounts of calcite and quartz. Total organic carbon content ranges from ~ 1 to ~ 12 wt.% organic carbon, with total generation potentials ranging from 0.5 to 85 mg HC/g rock. Thermally immature samples display hydrogen index values of ~ 600 mg HC/g TOC and atomic H/C ratios ~ 1.25. Visual kerogen analysis reveals that the samples are dominated by finely disseminated amorphous organic matter. The kerogen displays only a minor terrestrial background signal, i.e., only trace quantities of vitrinite are present. Studies of the associated bitumen fractions also reveal a predominantly marine character, with only a minor terrestrial overprint. Differences in the character of extracted bitumens from outcrop samples commonly ascribed to provincial differences appear more likely due to differences in the level of thermal maturity. The lack of significant amounts of vitrinite precludes its use in the determination of thermal maturity. Other thermal maturity indices, however, suggest that the onset of thermal hydrocarbon generation occurs within the Toarcian shales at a depth between 1500 and 2100 m, with the main stage of hydrocarbon generation and expulsion occurring at a depth of ~ 2400 m. Petroleum expulsion occurred primarily between 35 and 20 Ma B.P. Exploration for Schistes Carton-derived oils is largely limited by the unit’s low level of thermal maturity.