engleski

Crystal growth of illite in Pennsylvanian shale of the American Mid‐Continent region – inferences from clay mineralogy and K‐ Ar ages

Paleozoic successions of Anadarko Basin in the central United States are largely composed of shale intervals and have historically been known as major hydrocarbon sources. Lately their importance arises from the perspective of unconventional reservoir exploitation. Clay mineral evolution in these sediments and, in particular, the processes related to illite growth have, however, remained relatively under‐ researched. K‐Ar data on illite has been proven to be very useful for the understanding of illitization processes in shale. Still, dating of mudstone by the K‐Ar method may reveal a range of ages that are older, younger or coincide with stratigraphic ages. Illite particle nucleation‐ growth process acts as a major control on K‐Ar ages that occasionally may be significantly different than expected. This research examines Paleozoic shale of the Anadarko Basin cored at depths from 2980 to 3160 m. Based on comparative mineralogical data, X‐ray diffraction (XRD) and electron microscopy (SEM‐EDS) performed on three illite fractions (<2 μm, <1 μm, and <0.2 μm), and K‐Ar age investigation, in this contribution we report on the mechanism of crystal growth of illite. Our preliminary results showed that the finest illite fraction yields about 20 Ma younger K‐Ar ages than coarser particles. Furthermore, XRD and SEM‐EDS data showed a dominance of smectite component in <0.2 μm fraction, while the <2 μm fraction has abundant illite. It is generally accepted that K‐Ar ages younger than the sedimentation age may result from (1) the diffusional loss of radiogenic 40Ar, (2) loss of 40Ar derived from the K initially found in expandable layers of mixed‐layer phases, and (3) addition of K induced by burial diagenesis that in turn increases the K‐Ar ratio and lowers measured ages. Detrital illite and feldspar are considered as feedstock of K needed for the illitization to proceed. Taking into account our mineralogical and isotopic data we hypothesize that the illitization of the illite‐smectite mixed layers of progressively buried shale was essentially controlled by solid‐state transformation processes followed by the fast precipitation and crystal ripening of illite particles.

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