Organic geochemistry of the British Kimmeridge Clay: 1. Composition of shale oils produced from Kimmeridge sediments

Abstract

A series of shale oils produced by Fischer assay of Upper Jurassic Kimmeridgian oil shales, claystones and cementstones from on-shore Britain has been analysed. Oil yields are related to lithology via the quantity of sedimentary organic matter present in each lithologic unit, and also to stratigraphic horizon for certain rich, laterally uniform and persistent units. Shale oils are dark and sulphurous, containing < 50% hydrocarbons. NSO compounds are relatively important in the chromatographically separable fractions, while aliphatic hydrocarbons are usually subordinate. Unimodal normal alkane distributions maximize around nC16, while bimodal distributions have a secondary maximum around nC29. Many alkane gas chromatograms are characterized by a prominent peak due to the nC23 alkane. Certain compositional features are of pyrolytic origin (aromatics, heterocompounds, alkenes), while others are determined by the organic sedimentary input (alkane distributions, prominent nC23 alkane). Broad similarity in geochemical parameters suggests a consistency in the composition of sedimented organic matter. Smaller-scale compositional variations are thought to result from minor fluctuations in the composition of the organic sedimentary input, for example changes in the amount of terrestrial organic detritus reaching the depocentre. Results suggest a dominantly algal or bacterial organic matter source continuously operative throughout much of the Upper Jurassic over the entire region, with a smaller, more variable magnitude contribution from terrestrial sources, probably deposited in a near-shore marine shelf-sea environment under anoxic conditions.