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Vitrinite-like macerals (VLM) are ubiquitous in many pre-Devonian marine shales. The thermal maturity of these sediments, which are devoid of classical vitrinite, has been commonly evaluated using the reflectance of VLM as a substitute for vitrinite reflectance, but the molecular structure of VLM itself has seldom been examined. In this study, the immature VLM-rich Cambrian Alum shales from Scandinavia were selected and pyrolyzed at 300–550 ℃ to acquire a series of artificial pyrolysis samples with different ranks. The molecular structures and thermal evolution of VLM in natural and artificial shales were investigated by Raman spectroscopy. These Raman spectra and optical properties were compared with those of vitrinite, graptolite, and solid bitumen. The Raman spectra of VLM in natural samples of variable thermal maturities and immature samples altered by pyrolysis were generally comparable. The reduction of WD (D band position) and FWHM-G (full width at half maximum of the G band) and the increase of WG (G band position) and AD/AG (area ratio) in VLM spectra, which parallels the increasing thermal maturity of the samples, indicated an increased degree of aromatic conjugation and more structurally ordered carbon moieties. The increase in wavenumber difference between the D and G bands positions in Raman spectra (Raman band separation, RBS) demonstrated the gradual removal of oxygen-containing functional groups, the loss of aliphatic side chains, and the increased degree of aromatic ring condensation. In this way, with progressing maturity, the molecular structure of VLM gradually evolved from heterocyclic aromatic compounds and amorphous carbon structures at low maturity levels to structurally better-ordered, larger, aromatic fused-ring units characteristic of post-to-over mature samples. These molecular structures and their changes associated with thermal maturation were similar to those known from classical vitrinite. Aside from its reflectance, the thermal maturity of VLM-rich sediments could be assessed by Raman spectroscopy, notably through the RBS parameter of VLM Raman spectra, which closely correlated with VLM random reflectance throughout the reflectance interval between 0.5% and 5.5%.

The Cretaceous Second White Specks Formation is an organic-rich marine mudstone rich in calcareous coccoliths and fecal pellets that form an immature oil shale deposit. Corehole and outcrop oil shale samples, taken from western Manitoba and across Saskatchewan, Canada, were analyzed by organic petrography (reflected white and UV light), Rock-Eval pyrolysis, and elemental analysis (boron and sulfur) to characterize the oil shales in relation to their depositional environment and hydrocarbon generating potential. This study revealed that the oil shales all contain small amounts of sapropelite/huminite and inertinite. Inertinite consists of particles deposited from a terrigenous environment and have higher %Ro > 0.4, referred to as secondary inertinite. Primary inertinite is associated with the roots of marine grass and seaweeds. Fluorescing marine telalginite, Nostochopsis alginite, dinoflagellates, liptodetrinite, matrix bituminite, and chlorophyllinite are also present. Based on a detailed geological, sedimentological, geochemical, and organic petrological investigation, the oil shales were divided into three depositional facies: coastal marine (B:60 ppm), shallow marine (B: ∼80 ppm), and open marine (B:113 ppm). The oil shales are immature (Tmax = 398–425 °C) across the entire study area with %Ro of 0.20–0.27. The coastal marine facies oil shales have high TOC content (ave. = 10.4 wt%), S2 ave. = 54.9 mg HC/g TOC, HI ave. = 480 mg HC/g TOC, low Boron (ave. = 47 ppm) content and can generate an average of 43 lit/tonne of hydrocarbons. On the other hand, the shallow marine facies oil shales have average TOC of 8.1 wt%, average S2 of 41.2 mg HC/g TOC, and are much higher in B content (ave. = 80 ppm). The open marine facies oil shales have low TOC (ave. = 0.94 wt%), S2 averages 1.3 mg HC/g TOC, and HI averages 189 mg HC/g TOC. They have much lower hydrocarbon potential (ave. = 1.50 lit/tonne) but are high in B content (ave. = 109 ppm). Overall, it was concluded that in one specific area in the Pasquia Hills of northeastern Saskatchewan, the Second White Specks oil shales are shallow (7 wt%, and can generate >80 lit/tonne of hydrocarbon. This makes them suitable for ex-situ extraction.

Thermal maturation is traditionally evaluated based on vitrinite reflectance (VRo) measurements and its relationship to oil and gas generation and diagenetic transformation are ingrained in many basin modeling tools. However, vitrinite derives from higher land plants that evolved in Devonian. In pre-Devonian rocks graptolite reflectance (GR) is the most significant thermal index for establishing thermal maturation. Currently, conversions of GR to VRo equivalent rely on several established linear relationships. This study investigates a continuous thermal evolution of GR during artificial maturation of the Lower Ordovician (Tremadocian) Alum Shale of Estonia. We observe an anomalous breakdown in the gradient of GR versus thermal maturity. The anomaly trend is characterized as a suppressed GR gradient throughout the entire gas window (VRo: 1.0–2.0%). We attribute the suppressed measured GR trend to surface imperfection caused by the generation and evasion of hydrocarbon gases that generate nano-porosity vacuolation of the graptolite tissues. After the gas window, GR resumes its increasing trend with a similar gradient as observed in the pre-gas window, due to continued aromatization and condensation of the organic molecules. The GR anomaly indicates a potential underestimation of thermal maturity up to 0.52%VRo when applying a linear conversion formula between random GR and VRo directly. Therefore, a significant maturity correction should be applied to all legacy GR-based maturity measurements that indicate a gas or post-gas maturity rank.