Pitfalls of Using Biomarker Maturity Parameters for Organic Matter Maturity Assessment Suggested by Coal Hydrous Pyrolysis

Crude oil maturity assessment is a vital goal for petroleum geochemistry, and equally important is the exploration of maturity indicators of sufficient credibility. While most molecular proxy parameters have been extensively used and have provided some useful insights; the component ratios approach is somewhat limited in validity regarding oil maturity characterization for variable reasons. Novel thermal trends of hopanes and steranes were observed in a series of hydrous pyrolysates of an immature coal (0.49 %Ro) generated at eight target temperatures ranging from 250–375 °C (measured vitrinite reflectance of 0.71–0.91 %Rm), which–further substantiated this idea. Expelled oil and extractable bitumen were combined as the total soluble organic material (tSOM) for each pyrolysis experiment to mitigate the effects of primary expulsion fractionation. While bitumen extracted from the original coal—the 250 °C tSOM—the 275 °C tSOM (0.49–0.73 %Rm) sequence recorded normal increases in C 31 αβ-hopane 22 S /(22 S + 22 R ) and decreases in C 29 –C 30 βα-moretane/αβ-hopane ratios, low values and continuous decreases in C 29 5α-sterane 20 S /(20 S + 20 R ) and ββ/(αα + ββ), Ts/(Ts + Tm) and C 29 Ts/(C 29 Ts + C 29 αβ-hopane) suggested no biomarker thermal isomerization but predominant control from precursor-to-biomarker transformation. Continuous increases in 22 S /(22 S + 22 R ) until 1.43 %Rm accorded with thermal isomerization, but a delayed ratio equilibration at 1.43 %Rm again suggested biomarker precursor interference, which also played a role in the reductions in 20 S /(20 S + 20 R ) and ββ/(αα + ββ) to 0.9 %Rm, whereas increasing and high values of C 29 –C 30 βα-moretane/αβ-hopane ratios occurring during 0.73–1.43 %Rm. Reversals in 22 S /(22 S + 22 R ) and fluctuations in 20 S /(20 S + 20 R ) and ββ/(αα + ββ) at elevated maturity levels with minimum yields of biomarker precursors were predominantly controlled by differential isomer degradative rates. These rarely reported thermal distribution patterns of biomarkers illustrated very complicated biomarker generation–destruction processes during maturation and suggested that the release of bond biomarker to the free status may govern the biomarker maturity ratios rather than thermal isomerization. While the rapid heating conditions and high temperatures in pyrolysis differ inevitably from natural evolution under geological conditions, our study unveiled that unusual biomarker ratios in geological samples could be the norm, contradictory to common beliefs. Accordingly, we propose that isomer concentration is an essential tool to validate maturity estimation of organic matter by isomer ratios, especially for highly mature oils and sediment extracts.