Ieva Kaminskaite-Baranauskiene , this is a great question in need of a detailed case study or review! There is literature on the characterization of these types of plays, e.g. Article Mineralogical and morphological characterization of Older Ca...

that suggest low fractability and clay content would contribute to fracture healing. However, in this study, it is also suggested that areas of the Barnett and Marcellus shales have similar characteristics....suggesting a little more research might identify a more clear answer/case study.

I would say that Marcellus is consist of up to 60% of clay and still brittle because brittle-ductile transition in rock mechanics is driven by stress tensor and is not a material property of the rock. I would say that clay itself is brittle up to certain confinement pressure.

Your question should be reformulated for looking into low conductive unpropped fractures under effective closure stress. That may be driven by clay, TOC, Shmin, pressure drawdown, temperature, chemical effects of fracturing fluid, non-elastic properties (e.g., creep), or 10,000 other physical reasons.

Hi Ieva Kaminskaite-Baranauskiene , I am not expert on this matter, but I tried my hands on the similar aspects from one of the highly discussed shales from India, which happens to be a proven source rock of Paleocene-Eocene and the reservoir is producing since the last many decades. What I have found is you can get similar clay volume or UCS or elastic properties in many shales globally, but they will differ in the kerogen quality, thermal maturity, organic content, mineralogical assemblages. I am not very fond of comparing one shale with the another, does not make sense. I would have simply gone with the 'Brittleness Index' calc, both from elastic properties as well as mineralogy., compare results from both the approaches. Best if you have core based mineralogy details, also elastic properties measured on core plugs. Brittleness index will guide you on fracability aspects.