Sedimentary characterization of the carbonate source rock of Upper Kimmeridgian Parnac Formation of the Aquitaine Basin (Quercy area)

The main source rock (SR) of the Aquitaine Basin, the most important oil province in France, corresponds to organic-rich marine limestones of Upper Kimmeridgian age (Lons Formation in the Béarn area and Parnac Formation in the Quercy area). In order to better characterize their depositional environment, in particular the conditions of accumulation and preservation of organic-matter, a sedimentological, micropaleontological and geochemical study of the Crayssac section (Quercy) has been performed. Organic-rich sediments are argillaceous limestones (65 to 99% CaCO3) organized in repetitive beds of up to 1-m thickness. Their total organic matter content reaches up to 15 wt.%, and in this SR kerogen is type of II and immature. Microfauna content, the lack of barrier facies and the control of the wave action base over the depositional environments reflect deposition in an open marine type homoclinal ramp. Strong similarities with Kimmeridgian organic-rich limestones of the Middle East (Hanifa Formation) suggest that the Parnac Formation could act as an analogue of this prolific SR.