Advances in the study of ‘black shales’: source rocks for bricks and hydrocarbons

Abstract

SUMMARY Recent advances in methods and concepts in the study of organic-rich mudrocks are reviewed. Organic biomarkers, especially derivatives of isorenieratene, unambiguously identify anoxia within the photic zone of the water column in several well-known black shales. This demands a re-interpretation of much palaeoecological data in terms of intermittent anoxia, rather than only dysoxia, an endeavour aided by increased understanding of the life strategies of benthic invertebrates. The role of sulphur compounds in diagenesis, both organic-bound and as pyrite, is increasingly recognized. Pyrite studies afford anoxicity indices based on pyrite framboids and sulphur geochemistry. The role of organic sulphur is in stabilizing organic matter during kerogen formation, and in ‘fixing’ biomarker molecules. Recycling of nutrients is probably more important than previously thought in forming epicontinental black shales. Reworking of primary organic matter by heterotrophic organisms, especially bacteria, is indicated by isotopic studies on individual organic molecules. All these factors must be integrated with constraints from stratigraphical relationships and basin evolution. Some of these points are illustrated by reference to work by members of the Oxford Clay Working Group in understanding the palaeo-environment of the organic-rich Peterborough Member of that Jurassic formation, a famous source material for bricks and an analogue for hydrocarbon source-rock studies.