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Geological Society, London, Special Publications; 1987; v. 26; p. 69-78;
DOI: 10.1144/GSL.SP.1987.026.01.04
© 1987 Geological Society of London

Part I: Concepts and Methods

Pyrite formation in Jurassic shales of contrasting biofacies

I. St. J. Fisher & J. D. Hudson

Department of Geology, University of Leicester, Leicester LE1 7RH

The quantity and composition of organic matter passing into the hydrocarbon window is dependent on the extent of organic degradation occurring during early diagenesis. An important early diagenetic reaction in which organic matter is consumed is sulphate reduction. Relationships exist between pyrite sulphur (a product of sulphate reduction), organic carbon, and degree of pyritization of available iron, which can be used to help characterize the source-rock potential. In Jurassic, marine, organic-rich shales, these parameters are related to the environment of deposition, as indicated by biofacies analysis, and to variations in bottom water oxygenation and supplies of organic carbon, sulphate and iron. The best preservation of organic matter, for reaction in subsequent diagenetic zones, does not necessarily occur in the most anoxic sediments, but in those in which the organic matter buried in the sediment is more than sufficient to satisfy the supply of sulphate by diffusion. In the shales studied this is indicated by the most positive (> –28{per thousand}) {delta}34S values for fine grained pyrite.