Elsevier

Chemical Geology

Volume 234, Issues 1–2, 30 October 2006, Pages 169-177
Chemical Geology

Effect of thermal maturation on the K–Ar, Rb–Sr and REE systematics of an organic-rich New Albany Shale as determined by hydrous pyrolysis

https://doi.org/10.1016/j.chemgeo.2006.04.008Get rights and content

Abstract

Hydrous-pyrolysis experiments were conducted on an organic-rich Devonian–Mississippian shale, which was also leached by dilute HCl before and after pyrolysis, to identify and quantify the induced chemical and isotopic changes in the rock. The experiments significantly affect the organic–mineral organization, which plays an important role in natural interactions during diagenetic hydrocarbon maturation in source rocks. They produce 10.5% of volatiles and the amount of HCl leachables almost doubles from about 6% to 11%. The Rb–Sr and K–Ar data are significantly modified, but not just by removal of radiogenic 40Ar and 87Sr, as described in many studies of natural samples at similar thermal and hydrous conditions. The determining reactions relate to alteration of the organic matter marked by a significant change in the heavy REEs in the HCl leachate after pyrolysis, underlining the potential effects of acidic fluids in natural environments. Pyrolysis induces also release from organics of some Sr with a very low 87Sr/86Sr ratio, as well as part of U. Both seem to have been volatilised during the experiment, whereas other metals such as Pb, Th and part of U appear to have been transferred from soluble phases into stable (silicate?) components.

Increase of the K2O and radiogenic 40Ar contents of the silicate minerals after pyrolysis is explained by removal of other elements that could only be volatilised, as the system remains strictly closed during the experiment. The observed increase in radiogenic 40Ar implies that it was not preferentially released as a volatile gas phase when escaping the altered mineral phases. It had to be re-incorporated into newly-formed soluble phases, which is opposite to the general knowledge about the behavior of Ar in supergene natural environments. Because of the strictly closed-system conditions, hydrous-pyrolysis experiments allow to better identify and even quantify the geochemical aspects of organic–inorganic interactions, such as elemental exchanges, transfers and volatilisation, in potential source-rock shales during natural diagenetic hydrocarbon maturation.

Introduction

Hydrous pyrolysis has proved useful in providing major information about the kinetic parameters influencing or even controlling petroleum generation in source rocks, as well as on the thermal maturation indices related to primary oil generation in producing sedimentary basins (Lewan et al., 1979, Lewan, 1997, Lewan and Ruble, 2002). However, there is much to be known about the impact of fluids on the organic maturation processes, and on the trace-elemental compositions and isotopic systematics of the mineral assemblages in petroleum source rocks since studies of Espitalié et al., 1980, Espitalié et al., 1984. In particular, little is known about the reciprocal impact of hydrous pyrolysis on the mineral–organic assemblage of natural rocks, as it is inducing specific closed-system reactions, that may occur in shales acting as source rocks of maturating hydrocarbons.

To evaluate the degree of mineral–organic exchanges in source rocks involved in petroleum generation and thermal maturation, a hydrous-pyrolysis experiment has been undertaken on an organic-rich shale to explore the behavior of major, trace and rare-earth elements (REEs), as well as of K–Ar and Rb–Sr isotope systems of a source rock, in order to evaluate the reciprocal effect induced by the experiment. The results obtained were thought to present some preliminary experimental constraints related to natural petroleum generation, as a basis for additional experimental work to formulate a coherent relationship between petroleum generation and mineral alteration. The aim is a better understanding of the evolution of the minerals of source rocks with high contents of organic matter, depending on the timing and extent of its maturation. To the best of our knowledge, no previous data are yet available to describe the effect of thermal maturation induced by hydrous pyrolysis on the isotopic and elemental behavior of such organic–inorganic relationships of sedimentary rocks.

Section snippets

Sample description and analytical procedure

The chemical and isotopic investigations after the pyrolysis experiments were carried out on a thermally immature sample of previously studied Devonian–Mississippian (∼ 360 Ma) New Albany Shale from Indiana, USA (Lewan and Ruble, 2002; Spl. No. 931026-3). This rock consists of quartz, micas including illite, pyrite and minor amounts of dolomite and plagioclase, as well as of 14.34  wt.% total organic carbon (TOC). The thermal immaturity of the sample is evidenced by its Rock-Eval high hydrogen

Results

The XRD study of the whole-rock powder from unheated and pyrolyzed aliquots does not show, as expected, noticeable changes in the patterns. The only changes are a slight broadening towards the low angles of the 10.05-Å peak that characterizes mica and illite-type components, suggesting a very slight change in the clay-type mixed layered illite–smectite by an increase of the smectite-like interlayers. The appearance of a discrete peak at 7.06 Å was also found, which is attributed to

Discussion

Hydrous pyrolysis has a significant impact on the K–Ar and Rb–Sr systematics of the organic-rich shale rock, even if the final values appear not to be very different from initial ones. The changes in the respective isotopic data are not simply due to removal of radiogenic 40Ar or radiogenic 87Sr from altered silicates or from soluble minerals into solution, as reported in studies of natural materials. As no gain or loss of elements and radiogenic isotopes, except possibly for radiogenic 40Ar,

Conclusions

Hydrous-pyrolysis experiments significantly affect the organic–mineral organization of an organic-rich shale in producing 10.5% of volatiles and almost doubling the amount of HCl leachables from about 6% to 11%. The changes are recorded in the chemical and isotopic (Rb–Sr and K–Ar) data of the studied rock. However, the final results are not just removal of radiogenic 40Ar and 87Sr, as known from studies of natural samples at the same thermal and hydrous conditions. The reactions appear to be

Acknowledgments

We would like to acknowledge the technical assistance of Ray. Wendling, B. Kiefel and J. Samuel of the Centre de Géochimie de la Surface (CNRS/ULP) during analytical acquisition. S.C. would also like to thank the French Academy of Sciences and the TOTAL oil company for funding a one-year professorship at the Centre de Géochimie de la Surface.

References (19)

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