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Trace Element Geochemistry of Late Pennsylvanian Shales of the Anadarko Basin (OK) – Implications for Sediment Provenance and Shale Diagenesis


This study focuses on Late Pennsylvanian shales from the Anadarko basin in Oklahoma. They have historically been known as major hydrocarbon sources, and lately their importance continuously arises from the perspective of unconventional reservoir exploitation. Yet, these shales remained relatively under-researched with respect to their clay mineral content and the role clay diagenesis exercised over shale reservoir characteristics. While diagenetic studies commonly rely on XRD and SEM-EDS inquiry, in this research, in addition to provenance investigation, we demonstrate a potential of trace element geochemistry in reconstructing the burial history of analyzed mudrocks.

Some 25 m thick layer of black shales was drilled at a depth of ~3 km. The recovered sequence is composed of illite and mixed layered illite-smectites, with minor chlorite, kaolinite and quartz. Black shales are enclosed between thicker strata of interbedded sandstone and shale that gradually change into argillaceous sandstone. Normalized trace element values are featured by smooth parallel curves of both shale varieties (black shales vs. shaly intervals in sandstones) with concentration levels in the range of 10 to 300x chondrite. Igneous rocks of acid to intermediate composition were likely the source of detrital phyllosilicates that made most of analyzed shales.

Despite the similarity in the shapes of normalized trace element curves, the two shale types differ in their trace element budget, where black shales are on average 10 to 30 times poorer compared to shaly interlayers in sandstones. Most of hydrocarbon unconventional reservoirs in the USA show a great uniformity with regard to their clay mineral composition and our results are in line with such findings, confirming a dominance of detrital illite. However, an ordered illite-smectite (R1 or R3 type) was found to be present uniquely in black shales emerging in SEM images in form of laths that bridge the pores defined by detrital platy illite. Being clearly authigenic, the appearance of such newly formed clays may have resulted in an overall trace element impoverishment arising from low trace-element content of aqueous solutions from which they originated. Alternatively, higher organic content in black shales may lead to lower trace element abundances retained by illite. Further LA-ICP-MS investigation will likely help to give additional insights of the role of diagenesis in distributing trace elements during mudrocks burial.