Dispersed organic matter from pre-Devonian marine shales: A review on its composition, origin, evolution, and potential for hydrocarbon prospecting

Abstract

The pre-Devonian shales are important natural archives, preserving extensive information on paleobiology, paleoenvironment, and paleoecology, and also acting as important hydrocarbon source rocks for both unconventional and conventional petroleum systems. In China, however, most of the pre-Devonian shales are postmature, resulting in a gradual convergence in the chemical composition and structure of organic matter (OM). This convergence complicates OM identification under optical microscope, making it challenging to trace its genesis and evolution due to similar optical characteristics. Additionally, the absence of true vitrinite particles poses an additional challenge in the evaluation of thermal maturity of these shales, a challenge that has persisted for decades. This study compares detailed optical characteristics and genesis of selected OM types, namely, graptolite, vitrinite-like maceral (VLM), solid bitumen (SB) in the pre-Devonian shales to improve identification and differentiation between these macerals. The physicochemical evolution of OM is reviewed based on the naturally and artificially matured samples, with emphasis on the organic petrology and petroleum production in some of the most extensively studied pre-Devonian shales globally. VLM and graptolites exhibit higher reflectance and stronger anisotropy than co-occurring in-source SB, with their reflectance as indicators to evaluate OM maturity in these marine shales. VLM can be distinguished from SB by its elongate and syndepositional occurrence, and from graptolites by the characteristic biological structures of graptolite. The origin of VLM is hypothesized to be a product of early diagenetic anaerobic biodegradation of lamalginite and/or bituminite in the Precambrian shales, although its origin in the Cambrian shales remains unclear. On the other hand, in-source SB is derived from the transformation of liptinites or from the cracking of the residual petroleum, or a combination thereof. Graptolites are associated with high total organic carbon (TOC), S₁, S₂, and hydrogen index (HI) contents, with kerogen predominantly classified as Type II-III, indicating significant hydrocarbon generation potential. Notably, in-source SB plays a significant role in gas generation. In the early mature pre-Devonian shales, OM mainly consists of lamalginite and bituminite; whereas in-source SB becomes the dominant OM in the postmature pre-Devonian shales. Non-granular graptolites are commonly observed in the Ordovician-Silurian Wufeng–Longmaxi shales, contributing significantly to OM. A comparison of naturally and artificially matured samples suggests that low-maturity liptinites transform into SB as thermal maturity increases. This finding is essential for reconstructing the original composition and hydrocarbon potential of these economically significant marine shales.