The Assessment of Organic Matter Young's Modulus Distribution With Depositional Environment and Maturity

Abstract

Quantification of risk to seal integrity in CCS, or gas extraction from hydraulic fracturing, is directly affected by the accessibility of organic pores within organic rich mudrocks. Knowledge of the host organic matter's mechanical properties, which are influenced by depositional environment and thermal maturity, are required to reduce operational risk. In this study we address the effect of both depositional environment and maturity on organic matter Young's modulus by means of Atomic Force Microscopy Quantitative Imaging TM , which is a nondestructive technique capable of nanomechanical measurements. Shales from varying marine depositional environments covering kerogen Types II (Barnett), IIS (Tarfaya), and II/III (Eagle Ford/ Bowland) are analyzed to capture variance in organic matter. The findings show organic matter has a Young's modulus ranging between 0.1 and 24 GPa. These marine shales have a bimodal distribution of Young's modulus to some degree, with peaks at between 3-10 and 19-24 GPa. These shales exhibit a trend with maturity, whereby Young's modulus values of <10 GPa are dominant in immature Tarfaya shale, becoming similar to the proportion of values above 15 GPa within the oil window, before the stiffer values dominate into the gas window. These peaks most likely represent soft heterogeneous aliphatic rich kerogen and stiff ordered aromatic rich kerogen, evidenced by the increase in the stiffer component with maturity and correlated with 13 C NMR spectrocopy. These findings enable increased realism in microscale geomechanical fracture tip propagation models and may allow direct comparison between Young's modulus and Rock-Eval parameters. Plain Language Summary Gas recovery from hydraulic fracturing and validating the top-seal integrity for carbon capture and storage require knowledge of key mechanical properties of the associate mudrock. One key property is elasticity (Young's modulus), which is relatively well constrained for each component of a shale, except for the organic matter. This has historically been due to the difficulties in analyzing elasticity at the resolution of organic matter particles, which can be <1 micron in diameter. Here we use a new technique to measure elasticity at a spatial resolution of between 10 and 50 nm. Organic matter elasticity measurements have been undertaken on a range of shales from marine and lacustrine depositional environments and a range of thermal maturities. The marine-derived organic matter exhibits a bimodal distribution with a peak at around 3-10 and 19-22 gigapascals (GPa). When comparing the shale samples with maturity, a clear bimodal trend is observed within the marine-derived organic matter, which becomes increasingly dominated by a stiffer (higher Young's modulus) peak with maturity.

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