Aromatic hydrocarbon evolution patterns and maturity indication significance of marine shale: a comparative study of naturally evolved and thermally simulated samples from the Second White Specks Formation of Cretaceous Colorado Group, Western Canada Basin

Abstract

A pyrolysis hydrocarbon generation experiment was conducted on low-maturity marine shale from the Second White Specks (2WS) Formation of the Cretaceous Colorado Group in the Western Canada Basin. Using GC-MS, quantitative analysis of aromatics in naturally evolved and thermally simulated samples was performed to systematically compare the geochemical characteristics of aromatics in both sets of samples. The results indicated that: (1) In the naturally evolved series of 2WS shale, the absolute contents of trimethylnaphthalene (TMN), tetramethylnaphthalene (TeMN), phenanthrene (P) and methylphenanthrene (MP) were relatively higher andincreased with the degree of evolution. In the thermally simulated series, the absolute contents and trends of P and MP remained consistent with those of naturally evolved series at higher thermal evolution levels, whereas the absolute contents of TMN and TeMN were relatively lower and showed different trends, increasing initially and then decreasing with maturity. (2) The trimethylnaphthalene ratio (TMNR) values of the naturally evolved series increased gradually with burial depth, while the TMNR values of the thermally simulated series decreased initially and then increased. The tetramethylnaphthalene ratio (TeMNR) and methylphenanthrene index (MPI) values of both series showed a consistent pattern: TeMNR values decreased initially and then increased with maturity, while MPI values increased with maturity, suggesting that phenanthrene series compounds effectively indicated shale maturity under both thermal simulation and natural evolution conditions. (3) The thermal simulation experiment was capable of effectively replicating the thermal evolution process of aromatics within a certain temperature range. Specifically, TMNR values of the simulated shale deviated from the naturally evolved shale before 350 ℃ but became consistent after 350 ℃. Similarly, alkyl phenanthrenes-related parameters aligned with those of the naturally evolved shale before 425 ℃ but diverged significantly beyond 425 ℃. This divergence was primarily influenced by the change in aromatic evolution mechanisms when the temperature reached a critical value, as well as by factors such as heating rate and the state of organic matter.

Keywords

• aromatic hydrocarbon evolution
• maturity
• thermal simulation
• marine shale
• cretaceous
• western canada basin