A comparative study of the molecular and isotopic composition of biomarkers in immature oil shale (Aleksinac deposit, Serbia) and its liquid pyrolysis products (open and closed systems)

Abstract

The molecular and isotopic composition of biomarkers in initial bitumen isolated from immature (0.41% Rr) oil shale samples (Aleksinac deposit) and liquid products obtained by pyrolysis in open (OS) and closed (CS) systems are studied. The influence of pyrolysis type and variations of kerogen type on biomarkers composition and their isotopic signatures in liquid products is determined. The applicability of pyrolysis type, numerous biomarkers and carbon isotopic compositions (δ13C) of n-alkanes in liquid pyrolysates is established. Pyrolysis experiments were performed on two selected samples that showed high content of total organic carbon and hydrocarbon generation potential, but also certain variations in sources/depositional environment of organic matter (type I and mixed type I/II kerogen) within previous research of the oil shales sample set. The biomarker signatures were evaluated using gas chromatography-mass spectrometry (GC-MS) and δ13C of individual n-alkanes in bitumen and liquid pyrolysates. The molecular composition of liquid pyrolysates from the OS is very similar to those in initial bitumen, independently on kerogen type, confirming algal origin of organic matter (OM) deposited in lacustrine environment, even more apparently than results of initial bitumen. Therefore, OS can be useful for assessment of source and depositional environment of OM. Pyrolysis in the CS caused more intense thermal alterations, therefore the source fingerprints sometimes notably disappear. The liquid pyrolysates from the CS have the distributions of biomarkers similar to those in crude oils. The biomarker maturity parameters showed slightly higher values in the CS pyrolysate of mixed type I/II kerogen in relation to type I kerogen. δ13C of n-alkanes in liquid pyrolysates from the OS are isotopically lighter in comparison to bitumen, independently on kerogen type. Oppositely, in liquid pyrolysates from the CS, they become heavier than in bitumen, indicating the thermal influence on δ13C signatures, with more pronounced difference for type I kerogen.