Catagenetic trends in composition and structure parameters of asphaltenes

Abstract

Сatagenetic transformations of asphaltenes of organic matter (OM) from recent sediments and fossil rocks were studied on rocks sampled from different sedimentary basins (Timan-Pechora, Mezen, West Siberian, Kuznetsk, Aldan-Maya and Middle Amur) of Russia. Changes in the elemental composition of asphaltenes were analyzed with the Veselovsky’s model of the formation of major groups of fossil fuels using cO parameter (cO = (O/8 – N)/(C/3)) as compared with the total carbon based on elemental analysis. Just as the Van-Krevelen diagram, the plot built in C–cO coordinates for OM of various genetic forms demonstrates a significant difference in the asphaltenes composition evolution during catagenesis. At the same time, the cO parameter has notably increased in asphaltenes of both types of OM. Structural transformations of asphaltenes during catagenesis were identified in the course of high-resolution NMR (nuclear magnetic resonance) spectroscopy. The research results comprise data on carbon distributions in the aromatic and aliphatic groups in the investigated asphaltenes of different type OM of sediments and rocks at different stages of lithogenesis. A correlation between the asphaltene composition and structural parameters depending on thermocatalytic transformations is revealed. The asphaltenes of terrestrial organic matter are distinguished by higher cO values and different carbon yields of aromatics (higher) and aliphatics (lower) throughout the catagenesis phase. The highly transformed marine OM is marked by surge condensation of aromatic rings in asphaltenes, which makes them similar to asphaltenes of the terrestrial type OM.