On the changing petroleum generation properties of Alum Shale over geological time caused by uranium irradiation

Abstract

An interdisciplinary study was carried out to unravel organic-inorganic interactions caused by the radiogenic decay of uranium in the immature organic-rich Alum Shale (Middle Cambrian-Lower Ordovician). Based on pyrolysis experiments, uranium content is positively correlated with the gas-oil ratios and the aromaticities of both the free hydrocarbons residing in the rock and the pyrolysis products from its kerogen, indicating that irradiation has had a strong influence on organic matter composition overall and hence on petroleum potential. The Fourier Transform Ion Cyclotron Resonance mass spectrometry data reveal that macro-molecules in the uranium-rich Alum Shale samples are less alkylated than less irradiated counterparts, providing further evidence for structural alteration by αparticle bombardment. In addition, oxygen containing-compounds are enriched in the uranium-rich samples but are not easily degradable into low-molecular-weight products due to irradiation-induced crosslinking. Irradiation has induced changes in organic matter composition throughout the shale's entire ca. 500 Ma history, irrespective of thermal history. This factor has to be taken into account when reconstructing petroleum generation history. The Alum Shale's kerogen underwent catagenesis in the main petroleum kitchen area 420 to 340 Ma bp. Our calculations suggest the kerogen was much more aliphatic and oil-prone after deposition than that after extensive exposure to radiation. In addition, the gas sorption capacity of the organic matter in the Alum Shale can be assumed to have been less developed during Palaeozoic times, in contrast to results gained by sorption experiments performed at the present day, for the same reason. The kerogen reconstruction method developed here precludes overestimations of gas generation and gas retention in the Alum Shale by taking irradiation exposure into account and can thus significantly mitigate charge risk when applied in the explorations for both conventional and unconventional hydrocarbons.

Thermovaporisation: A screening tool for the gas-sorptive properties of source rocks

“…1e). Focusing only on immature samples (squares), the Alum Shales exhibit highest aromaticity levels, most likely caused by uranium irradiation (Yang et al, 2018), followed by Barnett Shales and lowest for the Posidonia Shales. This directly confirms results of Zhang et al (2012), who reported that sorption capacity decreases in the order Type III Cameo Coal, Type II Woodford Kerogen, Type I Green River Kerogen, and Ziegs et al (2017), who showed for Mandal Fm.…”

The Furongian to Lower Ordovician Alum Shale Formation in conventional and unconventional petroleum systems in the Baltic Basin – A review

“…A correlation based on a larger sample set showed that the uranium content generally correlates well with pyrolysate composition (Fig. 13) in that U-richer samples generate pyrolysates with higher GOR and higher aromatic compounds content (Yang et al, 2018). In the same publication Fourier Transform Ion Cyclotron Resonance mass spectrometry was used to evidence the alteration of the kerogen structure.…”

“…Analytical pyrolysis (Horsfield et al, 1992;Lewan and Buchardt, 1989;Yang et al, 2018) suggests a rather gassy petroleum composition throughout the classical oil window development, even for samples which were less affected due to lower uranium content. For example, the PhaseKinetics approach (di Primio and Horsfield, 2006) was applied for one immature, Middle…”

“…23). The high gas retention capacity of the Alum Shale Formation is very likely related to the increased aromaticity of the kerogen structure which is induced by the uranium irradiation (Yang et al, 2018). Mahlstedt and Horsfield (2019) have shown that vertical profiles of Tvap gas yields (C 1-5 , actually an S0 equivalent) can be used as a screening tool for the assessment of gas-sorptive properties of shales (Fig.…”

“…1) was deposited from the Middle Cambrian (Miaolingian) to the Early Ordovician (Tremadoc) in the Baltic Basin and surrounding countries (Nielsen and Schovsbo, 2006). Due to its high total organic carbon (TOC) content, widespread occurrence and varying maturity, it is a well-known conventional source rock for hydrocarbons found in Cambrian sandstones (Schleicher et al, 1998) and Ordovician and Silurian carbonates in the Baltic Basin (Sivhed et al, 2004, Yang et al, 2018. Also, these black shales raised attention for its atypical, i.e., gas generation characteristics (Bharati et al, 1992;Horsfield et al, 1992).…”