Heterogeneities of organic matter and its occurrence forms in mudrocks: Evidence from comparisons of palynofacies. (January 2020)

• Record Type: Journal Article
• Authors: Cai, Jingong; Zhu, Xiaojun; Zhang, Jingqiao; Song, Mingshui; Wang, Yongshi
• Is Part Of: Marine and petroleum geology. Volume 111(2020)
• Page Start: 21
• Page End: 32
• Publication Date: 2020-01
• Subjects: Organic matter occurrence -- Free organic matter -- Organo-clay composites -- Palynofacies -- Mudrocks; Submarine geology -- Periodicals; Petroleum -- Geology -- Periodicals; Géologie sous-marine -- Périodiques; Pétrole -- Géologie -- Périodiques; Petroleum -- Geology; Submarine geology; Periodicals; Electronic journals; 551.468
• DOI: 10.1016/j.marpetgeo.2019.08.004
• Languages: English
• ISSNs: 0264-8172

Abstract

Abstract: Mudrocks buried at different depths in a single well exhibiting different sedimentary environments and lithologies were collected from the Dongying Sag of the Jiyang Depression of eastern China to investigate the heterogeneities of organic matter (OM) and its occurrence forms. In this study, both bulk rocks and their organo-clay composites (i.e., <2 μm clay-sized fractions) were subjected to palynofacies analysis. The results demonstrate that palynological OM abundance and total organic carbon (TOC) content for bulk rocks are positively correlated with those in organo-clay composites respectively, suggesting that the palynological OM in organo-clay composites is intimately related to that in bulk rocks. However, other numerous variations between them exist: a) OM in the organo-clay composites is aquatic and prone to type I, while it is both terrestrial and aquatic and has multiple types of OM in bulk rocks; b) the palynological organic components in the organo-clay composites, which are mostly amorphous OM (AOM) with an average abundance exceeding 90%, are substantially different from those in bulk rocks, which possess both AOM and terrestrial OM (palynomorphs and structured OM); and c) at different depths, sedimentary environments and lithologies, the percentages of individual palynological organic components are relatively stable in organo-clay composites, whereas they vary over a wide range in bulk rocks. As many previous studies have indicated that OM occurrence is dominantly in the form of organo-clay composites, our results reveal that OM in mudrocks is heterogeneous with respect to its components and occurrence forms, namely, free OM and organo-clay composites, within which OM is greatly varied in association with minerals and its palynofacies. Free OM is physically coexisted with minerals and has different palynological organic components, while OM in organo-clay composites is combined with minerals and is dominantly composed of AOM. Our work demonstrates that OM occurrence in mudrocks has two distinct forms and their palynological organic components are greatly varied, which should be concerned in studies of source rock for further understanding the source material of petroleum. Highlights: Comparisons of palynofacies between mudrocks and their organo-clay composites were performed. Components, types and origins of palynological OM are diverse in bulk rocks while are simplex in organo-clay composites. OM in mudrocks has two occurrence forms: free OM and organo-clay composites.