Geochemical and Biomarker Constraints from High-Pressure Hydrous Pyrolysis: Implications for Monitoring and Optimising In-Situ Conversion of Unconventional Resources

The sustainable exploration and management of unconventional resources, such as oil shale and low-maturity shale oil, require a predictive understanding of fluid-rock interactions under in-situ pressure-temperature conditions. This study integrates high-pressure hydrous pyrolysis with comprehensive geochemical and petrophysical analyses to unravel the coupled effects of thermal maturation, geological pressure, water, and rock composition on hydrocarbon generation, pore evolution, and the development of diagnostic geochemical tracers.

Sequential pyrolysis experiments (350–420 °C, up to ~600 bar) on immature lacustrine shales (Type I and II kerogen) simulated burial depths of 1.8–6.0 km. Results demonstrate that water and pressure are critical, non-passive factors. Water acts catalytically, significantly accelerating hydrocarbon generation, organic matter maturation, and nanopore development—the latter experiencing an additional 1.9–4.5-fold pore volume increase in wet gas stages compared to anhydrous systems. Pressure exerts a dual regulatory role, generally enhancing liquid yield and suppressing gasification, while also impeding expulsion efficiency, leading to viscous bitumen retention.

Crucially, biomarker systems evolve predictably under these simulated geo-conditions. Parameters such as C₂₉ and C₃₀ βα/αβ hopane ratios, C₃₁-C₃₂ 22S/(22S+22R) homohopane ratios, and C₂₉ ααα 20S/(20S+20R) sterane ratios show systematic progressions with maturity, providing robust, non-destructive proxies for monitoring thermal evolution. In contrast, Pr/Ph and Ts/(Ts+Tm) ratios are less reliable under these conditions. These geochemical signatures, alongside declining gas dryness indices, form a reliable tracer suite for assessing subsurface conversion progress.

Furthermore, pore network evolution is governed by a synergy of thermal maturity, kerogen type, and mineralogy (e.g., carbonate dissolution, clay stability), all mediated by the presence of water and internal pore pressure. This moves beyond maturity-centric models to a holistic shale-water-pressure framework.

Our findings establish that in-situ conversion (ISC) can be effective at temperatures (350–420 °C) lower than those used in ex-situ retorting, validating prolonged heating as a low-energy strategy. The integrated geochemical and petrophysical framework presented here provides essential constraints for optimizing ISC processes, enabling the use of advanced geo(bio)chemical tracers for real-time monitoring and contributing to the sustainable and efficient exploitation of deep unconventional resources.