Energy Geoscience (Jan 2023)
Biodegradation of occluded hydrocarbons and kerogen macromolecules of the Permian Lucaogou shales, Junggar Basin, NW China
Abstract
Three kerogen samples (JJZG-1, JJZG-2 and JJZG-3) isolated from the Permian Lucaogou shales of varying biodegradation levels (BLs ≈ 0, 3 and 7, respectively) were subjected to sequential stepwise pyrolysis combined with on-line detection of gas chromatography-mass spectrometry (GC-MS). Occluded fractions (bitumen II) released at low-temperature steps (≤410 °C) show consistent biodegradative signatures with that reported for solvent-extracted fractions (bitumen I) of the original shales, e.g., broad range of abundant n-alkanes, isoprenoids and regular hopanes for the non-biodegraded JJZG-1; trace n-alkanes and abundant hopanes for the moderately biodegraded JJZG-2; and no n-alkanes but still prominent hopanes including the microbially produced 25-nohopanes for the severely biodegraded JJZG-3. This consistency between bitumen II and bitumen I fractions indicates the biodegradability of the kerogen-occluded bitumen II with limited protection from host kerogen. A minor level of protection was suggested by the trace distribution of n-alkanes in the bitumen II of JJZG-2, whereas the bitumen I had no n-alkanes. The kerogen itself was more resistant to biodegradation as reflected by the persistence of high abundances of both n-alkanes and hopanes in the high temperature (≥460 °C) products of all three kerogen samples. However, the relative abundances of these product groups did show some evidence of biodegradation alteration, e.g., ratios of n-C15 alkene/C27 hop-17(21)-ene at 510 °C pyrolysis decreased by order of magnitude from the non-biodegraded (JJZG-1 = 27.4) to highly biodegraded (0.3 for JJZG-3) samples. The reduced biodegradation impact on the kerogen fraction (Cf. bitumen fractions) was also evident by the absence of 25-norhopanes in the high-temperature analysis of the JJZG-3 kerogen.
