Improved Understanding of Hydrocarbon Expulsion and Associated Fracturing During Successive Stages of Maturation: Insights from the Artificial Maturation of Organic-Rich, Immature to Early Mature Source Rocks

Organic-rich carbonate mudrocks are among the most important petroleum source rocks in the world, sourcing most of the hydrocarbons of Arabia and many other oil provinces. Hydrocarbon generation and migration in organic-rich carbonate source rocks and the governing geological processes have previously been widely discussed. However, only a few studies focus on the early mobilisation of initial bitumen and its relationship to pore space and fracture development during early maturation. We investigated these initial stages in petroleum generation and primary migration in the organic-rich, immature to early mature (<0.6 VRE) Upper Cretaceous source rocks of Jordan, both observationally on samples from 15 subsurface cores and experimentally through artificial maturation and pyrolysis of carefully selected unfractured samples. Both sets of samples (untreated and artificially matured) were examined using optical microscopy, BIB-SEM (Broad Ion Beam-Scanning Electron Microscopy) and micro-CT scanning and analysed for their geochemistry (Total Organic Carbon; TOC, sulphur content and maturity). The rocks are mainly composed of carbonate minerals and Type II-S kerogen, with a TOC of 16 wt.% on average and relatively high total sulphur contents (2.08-7.39 wt.%). The organic matter has a high capacity for oil generation, based on its high Hydrogen Index (HI) of up to 852 mg HC/gTOC. Observations from the untreated core samples show that early-expelled hydrocarbons either disseminate in the host rock filling primary matrix porosity or existing fractures/microfractures. Our sequential artificial maturation experiments provide insights into the mechanisms and pathways of both the hydrocarbon expulsion and fracturing that could be expected under continued burial and natural maturation. After one week of pyrolysis, a significant part of the organic matter has been converted to hydrocarbons and mobilised through the remaining primary porosity, preexisting microfractures and pore space generated within organic laminae. Thin section and BIB-SEM characterisation indicate that the mobilised bitumen passively invades pre-maturation created fractures or the pore network generated in organic matter during conversion, disturbing rock fabric and creating new migration pathways. This artificially generated bitumen forms bridges between existing kerogen particles and partly create new fractures. The migration pathways mainly propagate parallel to bedding, based on thin sections and BIB-SEM images. The innovative experimental approach utilised in this study creates an improved understanding of hydrocarbon expulsion processes and associated migration during the successive stages of maturation. This, in turn, provides insights into the thermal maturity evolution of source rock sequences in Arabia and elsewhere.