Nanomechanical and chemical variations of inertinite and vitrinite within lacustrine shale during oil generation

Abstract

Inertinite and vitrinite in 3 samples during oil generation from a lacustrine shale, the third member of the Paleogene Shahejie Formation are investigated nanomechanical and chemical characterization by atomic force microscopy (AFM) and laser Raman spectroscopy. The kerogen of 3 samples are all lacustrine type II. With the increase of maturity (VRo = 0.78%–1.10%), the average elastic moduli of inertinites (macrinite) increases from 4.89 to 6.02 GPa; while the average elastic moduli of vitrinites are 3.04, 3.76, 2.47 GPa. When VRo = 1.10%, the average elastic moduli of two other inertinite macerals are 7.20 GPa for fusinite and 5.99 GPa for micrinite. Furthermore, Raman spectroscopy was found to be insensitive to inertinite thermal maturation while this was not the case for vitrinite. Inertinite (macrinite) gradually becomes stiffer with increasing maturity whereas vitrinite stiffness does not correlate with maturity which could be due to its diverse biological origin. At the same thermal maturity (VRo = 1.10%), stiffness of inertinite and vitrinite followed: fusinite > inertinite (macrinite) > micrinite > vitrinite. Chemical composition of inertinite was found relatively stable as maturity advanced while the vitrinite is gradually carbonized. Finally, it became evident that macerals with different biological origins exhibit variations in nanochemo-mechanical properties with respect to thermal maturity.