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Organic Matter Matters! The Role of Organic Matter Composition on Shale Geomechanics


Shale reservoir geomechanics are a key control on unconventional play viability, primarily determined by mineralogy, maturity and depositional environment. Recent studies have shown that organic matter content is an important factor in rock frackability. Nanoscale analysis of shale geomechanics shows that the variability of organic matter Young’s modulus is dependent on depositional environment, provenance (organofacies), maceral content and burial history. Initial results indicate that all organic matter have a Young’s modulus value of <30GPa but has a different distribution depending on the organofacies or maturity. Here we show the first results of organic matter geomechanical analysis across many different depositional environments on the sub-micron scale. This has been enabled by use of Atomic Force Microscopy Quantitative Imaging (AFMQI), a technique established in the last decade, allowing even amorphous organic matter (AOM) to be analysed, a feat which wasn’t possible using other techniques such as nanoindentation. Shales with known depositional environments, maturities, organo-facies, and ages are used in this study to create a database of typical organic matter Young’s modulus values. Shales studied include the Barnett, Eagle Ford, Green River, Bowland (UK) and Tarfaya (Morocco) Shales, which have been measured not just with conventional Rock Eval pyrolysis but also with point counts for maceral type and palynofacies determination. Initial probability density functions show that the Young’s modulus distribution of all the shales studied are bi-modal. Mineral matter appears to have an approximately symmetrical peak with a high mode Young’s modulus (>50GPa) and a high frequency, but also a relatively large variance. The second peak at between 16-18GPa represents a smaller frequency of Young’s moduli measured and is distributed asymmetrically, skewing towards less stiff values (<10GPa) and appears to represent organic matter in most samples. In order to systematically address the relationship between maceral composition and organic matter Young’s modulus, samples of Cannel Coal, Indiana Paper Coal and New Albany Shale have been measured. These samples, both immature and rich in certain Liptinite macerals have been analysed palynologically and using AFMQI. The results of which can be integrated with results from the other shales to analyse which macerals have a significant effect on the Young’s modulus distribution, and what role AOM plays in this.