Structural evolution of organic matter and implications for graphitization in over-mature marine shales, south China

To characterize the degree of organic matter graphitization in the over mature lower Paleozoic marine shales from South China, a suite of kerogen samples covering different maturity (R-o% ranging from 1.2% to 4.2%) were analyzed by Laser Raman microprobe (LRM), Transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR) techniques. Chemical structures and related major changes caused by thermal evolution history were systematically discussed. The results indicate that with the thermal maturity increasing, the aliphatic side chains fall off gradually, and the aromatic rings increase continuously. The ordered structure of shales continues to enhance, leading to the increasing of graphitization degree. The parameters of LRM (i.e. position of G and D bands, peak position difference (RBS), full width at half maximum (FWHM) and intensity ratio (I-D/I-G)) highlight the turning point at maturity level around R-Rmc(o)% = 3.5%, which represents the chemical structure jump of the kerogen. This chemical structure jump can be characterized by the continuous decreasing of amorphous carbon before the R-Rmc(o)% exceeded 3.5% and the sharp increasing degree of aromatic conjugation after that point. TEM analysis indicates that the carbon layers of shale samples with R-Rmc(o)% exceeds 3.5% show better ordering and continuity than that of low maturity shale samples. The FTIR analysis suggests that there are relatively high aromatics C=C chains and very few aromatic C-H chains in the high maturity samples. This critical chemical structure jump at R-Rmc(o)% = 3.5% could be interpreted as the structure of organic matter begins to transform from amorphous carbon to crystalline graphite in shales. Due to the increasing degree of graphitization, graphitized shale is characterized by stable chemical structure, condensed and orderly carbon layer constructure, and ultra-low resistivity.