The Role of Clay Minerals in the Preservation of Organic Matter; A Study of The Late Neoproterozoic Doushantuo Formation, South China

Thomas Bristow
Department of Earth Sciences, University of California, Riverside.
Riverside, California
tbris001@student.ucr.edu

Studies of the Upper Cretaceous, Pierre Shale show that the amount of organic matter (OM) in these rocks co-varies with the quantity of expandable clays. In suitable conditions organic compounds may enter the interlayer of expandable smectitic clays, and this has been suggested as a mechanism by which OM is shielded against breakdown (Kennedy et al., 2002). This mechanism helps explain the long observed trend that finer-grained rocks are host to greater concentrations of OM.

The Doushantuo Formation is part of the organic rich, ‘black rock’ series that spans the Late Precambrian to Cambrian succession of South China. Mineralogical analysis of these Late Neoproterozoic age, carbonaceous black-shales, using XRD, shows the presence of mixed-layer clays with a smectitic component of 80-90%. The aim of this investigation is to determine if these clays enhanced the preservation of OM in these sediments. Demonstration of interlayer shielding as a mechanism for OM preservation would challenge traditional assumptions regarding conditions necessary for deposition of petroleum source rock. Identifying sources and distribution of detrital smectite may be as important as paleomarine conditions when determining the distribution of source lithologies.

It is also strange to see such a high smectite component in mixed-layer clays of rocks this age. The illitization reaction normally runs to completion at relatively low temperatures (~120°C). Determining why illitization did not proceed in this succession will increase understanding of factors that control this reaction. This is important because water released during illitization is thought to drive hydrocarbon migration in other basins.