Impact of natural weathering on source rocks: Organic and inorganic geochemical evidence from the Triassic Chang 7 outcrop profile in Tongchuan of the Southern Ordos Basin (China)

Abstract

Exposure of buried source rocks at or near the surface causes organic matter (OM) to be subjected to loss through weathering impacts. A detailed organic and inorganic investigation of the natural weathering of the Triassic Chang 7 Yishicun Profile in Tongchuan City of the Ordos Basin revealed pronounced changes in bulk, molecular, mineral, and elemental compositions. In this study, systematic and visible variations caused by natural weathering were shown in the amount and composition of both organic (kerogen and extractable organic matter [EOM]) and inorganic matter (minerals and elements). The results indicate that natural weathering could substantially affect the characteristics of source rocks at a depth of 2 m from the surface, resulting in a 41% decrease in total organic carbon (TOC) and a 75% loss of EOM. Furthermore, changes were noted in kerogen elemental compositions, fractional compositions of EOM, and some geochemical parameters from Rock-Eval pyrolysis. The mineral and major, trace, and rare earth element (REE) compositions of the outcrop samples were also analysed and compared. The weathering of source rocks was mainly due to the influence of surface water leaching and biodegradation of the OM. This resulted in the oxidation of kerogen and the preferential depletion of low-carbon-number n-alkanes and low-molecular-weight high-water-solubility biomarkers. This has led to bias in petroleum geochemical interpretation based on these organic geochemical parameters. The major elemental composition indicated that the outcrop profile was moderately weathered. Moreover, some commonly used palaeoenvironmental geochemical proxies, including U/Th, AU, Mo, Sr/Ba, Sr/Cu, and P/Ti, display varying degrees of weathering-related alteration. However, other palaeoenvironmental proxies such as V/(V + Ni), Ni/Co, V/Cr, δU, and Ceanom experienced little variation and remained reliable for environmental interpretation. Thus, petroleum resource evaluation and sedimentary environmental analysis of source rocks based on outcrop samples should consider the effects of natural weathering.