CRYSTAL GROWTH OF ILLITE IN PENNSYLVANIAN SHALE OF THE AMERICAN MID-CONTINENT REGION – SMECTITE PRECURSOR AND POSSIBLE IMPLICATIONS FOR PHYSICOCHEMICAL PROPERTIES OF UNCONVENTIONAL RESERVOIRS

Paleozoic shale successions of Anadarko Basin in the central United States have historically been known as major hydrocarbon sources. Lately, their importance arises from the perspective of unconventional reservoir exploitation. Despite the better understanding of shale diagenesis and the agreement on many aspects of the transition of smectite to illite through a series of mixed-layer illite-smectite (I-S), the origin and composition of the material, and exact processes related to mechanisms of illitization remain controversial in shale. While nowadays most of researchers consider shales to be consisted of “illitic” clays, which comprises both the non-expandable 10Å layers as well as mixed I-S, some authors exclude the presence of smectite in mature shale attributing its appearance in XRD patterns by peculiar diffraction artefacts. The question of the nature of shaley matrix influences greatly rocks’ physicochemical parameters such as porosity/permeability and brittleness, which is essential for amelioration of the unconventional reservoir exploitation practices.

In order to shed more light on crystal growth processes and clay mineral composition of shale matrix, an integrated study using different methodology was designed. Samples of Paleozoic shale of the Anadarko Basin were cored at depths from 2000 to 4200 m. Based on comparative mineralogical data - X-ray diffraction, electron microscopy and Infrared Attenuated Total Reflectance - and K-Ar age investigation performed on four illite fractions (2-1 μm , <2 μm, <1 μm, and <0.2 μm), we report on the mineralogical composition of shale matrix at different scales and mechanism of crystal growth of illite. Our integrated results show the finest fraction to be younger and to contain more smectite component than the coarser ones.

Our mineralogical and isotopic data suggest I-S as the dominant clay phase with up to 20% of smectite component, which underwent progressive illitization upon burial, essentially controlled by solid-state transformation processes. Such findings must be considered in the context of exploitation purposes, since characteristics that are inherent to these clay materials (i.e. I-S), including morphology and particle size, ion exchange properties, swelling and dispersion noticeably differ from pure illitic phase.