Thermal evolution of graptolite and solid bitumen properties at high maturity under natural and artificial conditions

To refine the use of graptolite and solid bitumen properties as thermal proxies at overmature conditions, we evaluated their evolution via Raman and infrared spectroscopies, reflectance, and geochemical screening in high-temperature hydrous and anhydrous experiments in comparison to naturally matured samples. Naturally matured samples included four overmature Wufeng-Longmaxi marine shales from the Sichuan Basin, China, with solid bitumen reflectance (BRo) values of 2.5-3.6% and graptolite reflectance (GRo) values of 2.3-3.9%. Immature starting materials for pyrolysis experiments included Mesoproterozoic (Ectasian) Xiamaling marine shale from Hebei, China (0.34% BRo, graptolite absent) and graptolite-bearing Ordovician (Tremadocian) Alum marine shale from Grönhögen, Sweden (0.50% BRo, 1.00% GRo). Pyrolysis experiments were carried out at 360°C for 10 days and 27 days (hydrous conditions), and 450°C 3 days, 550°C 3 days, and 550°C 10 days (all anhydrous). The experiments created Xiamaling and Alum residues with solid bitumen and graptolite, respectively, of similar and higher maturity (1.8-5.7% BRo, 1.8-5.3% GRo) compared to the naturally matured Wufeng-Longmaxi samples. Compositional proxies from infrared spectroscopy were inconclusive. Raman spectral properties including Raman band separation (RBS) and the full-width at half-maximum of the G-band (G-FWHM) exhibited robust positive and inverse relationships with increasing reflectance in pyrolysis residues, respectively. Similar to previous work, RBS and G-FWHM values were systematically lower and higher, respectively, in artificially matured samples versus naturally matured samples with equivalent reflectance values. This observation indicates the limited duration or low internal reactor pressure of pyrolysis experiments is insufficient to allow alignment of aromatic carbon arrays, or the ordering of aromatic clusters present in both organic matter types, relative to natural maturation occurring at geologic time scales. Reflectance of graptolite was usually but not always higher than co-occurring solid bitumen in all samples, suggesting inherently higher aromaticity. However, aromaticity in pyrolysis residues was systematically higher in solid bitumen compared to graptolite with equivalent reflectance values, indicating a higher kinetic barrier to molecular rearrangement in graptolite versus solid bitumen. These data further clarify differences in the molecular evolution of sedimentary organic matter in natural versus analogue laboratory environments and distinguish the properties of individual organic matter types in response to thermal stress.