Influence of geochemical features on the mechanical properties of organic matter in shale

Abstract

Organic matter is an important constituent in organic-rich shale, which influences the hydrocarbon generation, as well as the mechanical behavior, of shale reservoirs. The physical, chemical, and mechanical properties of organic matter depend on the source material and the thermal evolution process. Previous works attempted to investigate the impact of thermal maturation on the mechanical properties of organic matter. However, owing to the lack of maceral classification and the limitation of data volume during the mechanical measurement, no consistent trend has been identified. In this work, vitrinite reflectance test, scanning electron microscope observation, nanoindentation, and micro-Raman analysis were combined for geochemical and mechanical characterization. A total of 114 test areas were selected for testing, enhancing reliability of the test results. The Young's moduli of organic matter are from 3.57 to 8.32 GPa. With the same thermal maturity, inertinite has the highest Young's modulus, while the modulus of bitumen is the lowest. The Young's moduli of different organic types all increase with vitrinite reflectance. When vitrinite reflectance increases from 0.62% to 1.13%, the modulus of inertinite and vitrinite is increased by 57% and 78%, respectively. The indentation creep of different organic matter suggests that the stiffer organic type demonstrates less creep influence. In addition, with the increase of thermal maturity, the micro-Raman test results show a decrease of intensity ratio of D peak to G peak, indicating an increase of the ordered structure in organic matter. From the characterization results, it was found that organic type and thermal maturity reflect the diversity of the source material and the chemical structure change during the thermal evolution process, and together they influence the mechanical properties of organic matter.