Some sedimentological and geochemical characters of the late Triassic Noto formation, source rock in the Ragusa basin (Sicily)

Abstract : In the Ragusa basin (southeastern Sicily), the late Triassic Noto formation is considered as the main oil source rock. We provide a detailed description of the sedimentary facies determined from core samples, and discuss geochemical results, obtained for both kerogens and chloroform extractable hydrocarbons from samples where sedimentary organic matter is immature. Two main sedimentary sequences were encountered: (i) layers of limestones and of marls (or shales) alternating at a metric scale, and (ii) laminites having various types and carbonate contents. The high petroleum potentials (S2 up to 100 kg HC/t rock) are related to sedimentary lithologies that contain a minimum of clay. Two types of algal organic matter are probably mixed in the sediments studied. These two types can be distinguished by the oxygen content. They also do not have the same kinetic behavior in thermal evolution. Special attention has been paid to the possible use of bitumens and of pyrolysates obtained from kerogens for understanding “organic facies”. Bulk properties together with several biomarker distributions were examined through different facies. But most of the parameters measured or computed are more clearly related to the maturity of the sequences analyzed rather than to the intrinsic nature of the in situ organic matter. A modeling approach is proposed, taking into account geological and geochemical data. Different situations of the Noto source rock lead to different timings of oil formation. The importance of source rock heterogeneities at different scales in basin evaluation is underlined.