Palynomorph Darkness Index (PDI) – a new technique for assessing thermal maturity

Abstract

Palynomorph Darkness Index (PDI) is proposed as a new thermal maturity indicator. PDI is calculated from measurement of the red, green and blue (RGB) intensities of light transmitted through palynomorphs, using standard palynological microscopes and digital cameras. PDIs determined by different microscope and camera combinations show excellent correlation, suggesting that the method is largely platform-independent, though calibration is required, preferably using photographic filters as standards. Investigation of PDI from experimentally heated Tasmanites reveals a progressive increase with increasing temperature, suggesting that the technique is applicable through a broad temperature range encompassing the whole of the oil window and at least part of the zone of dry gas generation. Potential applications of this inexpensive method include the estimation of thermal maturity of rocks deficient in vitrinite, such as the Lower Palaeozoic rocks of Arabia and the Upper Devonian black shales of North America.

Keywords:

• palynomorph colour
• thermal maturity
• quantitative RGB
• transmitted light

Acknowledgements

This publication has emanated from research conducted with the financial support of Science Foundation Ireland (Research Frontiers Programme Grant GEOF152). The authors are grateful to Dr Clive Burnett for kindly providing Tasmanian material and data and Drs Catherine Duggan, David Naylor and Charles Wellman for their constructive comments. We are also grateful to Professor John Marshall and Dr Cortland Eble for their constructive reviews of the manuscript. The authors also wish to thank the Natural Resources Authority of Jordan and Petrel Resources Plc. for access to Jordanian samples and for permission to publish.