Vertical and lateral changes in organic matter from the Mesozoic, eastern Paris Basin (France): Variability of sources and burial history

The Callovian–Oxfordian claystones located at 500 m depth in the eastern Paris Basin are currently being investigated by ANDRA (the French National Radioactive Waste Management Agency) for long-term geological nuclear waste disposal. For this purpose, a recent 2000 m deep well was drilled close to the ANDRA underground research laboratory. This borehole penetrated the Mesozoic sedimentary series for a detailed study of the sediments located beneath the target formation. It also provides the opportunity to study the vertical evolution of the sedimentary organic matter and its thermal maturity in this part of the Basin, where the thickness of sedimentary eroded cover is poorly constrained. To extend this study at the scale of the eastern Paris Basin, 5 more wells and numerous outcrops were sampled. Sedimentary organic matter characterization was performed on kerogen (Rock-Eval pyrolysis, organic petrography and mean huminite and vitrinite reflectance measurements) and on the extractible organic fraction (Gas Chromatography–Mass Spectrometry). Results show that inputs of organic matter during the Mesozoic were essentially of terrigenous origin, except during the Toarcian where marine contributions are predominant. The major macerals present are huminite (ulminite), vitrinite (collotelinite) and inertinite (inertodetrinite and fusinite). Vitrinite shows a regular increase of reflectance with depth in the 2000 m depth well. Correlations between Tmax, vitrinite reflectance, and biomarker data define the thermal maturity of the Mesozoic sediments: maturity is reached at 1300–1500 m present day depth in the 2000 m well, in the Upper Triassic. At the regional scale, the organic matter maturity of each sedimentary formation decreases towards the eastern part of the basin, probably in relation to a decrease of the Jurassic and/or Cretaceous cover thickness, which is now eroded. All results indicate the lack of hydrocarbon generation in the Mesozoic series in this part of the basin.

► Thermal maturity of organic matter in the area studied for radioactive waste disposal.
► Thermal maturity is exclusively linked to burial depth.
► Sedimentary OM is essentially of terrigenous origin.
► The thickness of the eroded cover decreases towards the eastern part of the basin.