UC Riverside

Thermal maturity parameters have traditionally focused around the peak-oil window while parameters for the late- to post-oil window are not well constrained. Previous studies have shown that 3-ring polycyclic aromatic hydrocarbons (PAHs) can be used to correctly predict the maturity order for sedimentary organic matter. PAHs are structurally stable molecules and, when bound within kerogen, the kinetics of thermal alteration are slowed. The utility and preservation systematics of kerogen-bound 5-ring PAH, as measured by the 252 Da fragment ion, have not been studied in depth. These molecules have the potential to extend the sensitivity of maturity parameters into the late-oil window and beyond.

Kerogen samples from five basins with a range of maturities and sedimentary environments were loaded with 5-wt% ammonium dioxydithiomolybdate [(NH4)2MoO2S2] and pyrolysed using catalytic hydropyrolysis (HyPy; 0C to 250C at 100C min-1 then 250C to 520C at 8C min-1) to extract covalently-bound compounds. Extracted compounds were analyzed by gas chromatography-mass spectrometry (GC-MS).

The relative order of stability of kerogen-bound 5-ring PAH was shown to be perylene < benzo(a)pyrene < benzo(e)pyrene < benzofluoranthenes. The ratio of benzo(a)pyrene.benzo(e)pyrene (BaP/BeP) values range from 0.02 to 1.20. The ratio of perylene/benzo(e)pyrene (Per/BeP) values range from 0.01 to 0.65. Kerogen-bound benzo(a)pyrene was found to be most sensitive in the late-oil window. Kerogen-bound perylene was found to be most sensitive to change in the peak-oil window though still present at very low abundances in the most mature kerogen suite. The use of HyPy can release kerogen-bound 5-ring PAH to extend maturity parameters beyond the late-oil window, regardless of the sedimentary environment at the time of deposition.