Simulation of Thermal Maturity in Kerogen Type II Using Hydrous and Anhydrous Pyrolysis: A Case Study from the Bakken Shale, United States

A thermally immature specimen extracted from the Bakken Formation was exposed to both anhydrous and hydrous pyrolysis (AHP/HP) experiments spanning a temperature range of 300 to 450 degrees C. The objective was to investigate the potential discrepancies in geochemical properties, organic petrology, and chemical composition of the byproducts. The presence of unresolved complex mixtures (UCM) in gas chromatography (GC) resulted in fluctuations in the concentration of normal-alkanes as the temperature increased. Furthermore, it was observed that the ratios of Pr/Ph, Pr/nC(17), and Ph/nC(18) exhibited a significant reduction with temperature advancement during both the HP and AHP settings. This decline can be attributed to the enhanced maturation process. Significant reductions in C29Ts were observed in response to temperature, but a modest rise was observed in the GI. The ratio of Ts to Tm exhibited an increase during HP and a reduction during AHP conditions. The increase in temperature under both HP and AHP conditions resulted in an increase in the ratios of C-23 tricyclic/C-24 tetracyclic terpanes (C-23 tt/C-24 Tt), C-24 tetracyclic terpane and C-30 hopane, and C-30 rearranged hopane/C-30 hopane (C30RH/C30H). The C-29/C-30 hopanes (C-29 H/C30H) ratio exhibited a marginal increase from 10.11 to 10.5 when subjected to HP, and had a rising pattern when subjected to AHP. Based on petrographic investigations, it was observed that the physical alteration of telalginite was more pronounced when water was present in the system and temperature was increased. Furthermore, it was observed that the fluorescence of telalginite ceased to be detectable when exposed to temperatures exceeding 325 degrees C. The reflectance values of bitumen exhibited an increase when the temperature rose, regardless of whether it was under AHP or HP settings. However, the changes seen were more significant under HP conditions than under AHP conditions. The observation of a granular mosaic texture formed by pyrobitumen under HP conditions at a temperature of 450 degrees C indicates that it possesses graphitizing characteristics. The divergent outcomes of pyrolysis pathways in the presence and absence of water highlight the substantial effect exerted by water in the production and release of petroleum and its derivatives with particular emphasis on the potential contribution of exogenous hydrogen in this context.