Facies variations in the Upper Jurassic source rocks of the Norwegian North Sea; from micro- to macro scale

Abstract

In the northern North Sea, the Upper Jurassic Kimmeridge Clay (KC) and its equivalents in the Norwegian sector (i.e., Mandal, Tau, and Draupne) are the most prolific source rocks. These source rock units are known to record both lateral and vertical heterogeneity in organofacies with increasing terrigenous organic inputs near highs and basin flanks. Existing interpretations rely predominantly on Rock-Eval data, but only a few authors validate their geochemical interpretations with maceral assessments. In this study, Upper Jurassic source rocks from fourteen wells in the northern North Sea were analysed for both organic petrography and geochemistry. A potential correlation between the maceral compositions and Rock-Eval, total organic carbon (TOC), and stable carbon isotopes (saturate and aromatic fractions) of source rock extracts was established. This correlation suggests that carbon isotopes mirror the organofacies, and inferred redox conditions during sedimentation, described for the studied source rocks.Stable carbon isotopes of the saturate and aromatic fractions for 152 oils distributed across the Norwegian North Sea were compared to biomarker compounds sensitive to precursor organic material and oxygen abundance in the sedimentary environment. A correlation consistent with that observed between stable carbon isotopes and organofacies documented for the investigated source rocks is also observable in the oil dataset. Hence, the stable carbon isotope composition of the oils in the North Sea dataset appears to carry information related to the bulk organofacies and redox conditions prevailing during the sedimentation of their source. Oils often reflect the compositional attributes of their parental sources more efficiently than a single, or a set of rock extracts. Therefore, the stable carbon isotopes of migrated oils provide a means of inferring gross organofacies and redox conditions within different source rock kitchens in the study area.