Characteristics of Organic Matter Graphitization in Over- Mature Lower Paleozoic Marine Shales, South China

Abstract

The lower Paleozoic marine shales in South China are currently in over- mature stage with vitrinite reflectance (Ro %) ranging from 2.5% to 5.0%. The kerogen and bitumen are supposed to have experienced different degrees of graphitization. Graphitization will definitely change the original organic matter structure, leading to reduced reservoir pore spaces and declined gas storage capacity. This study attempts to characterize the graphitization degree of organic matter in over- mature shales. We investigate electric properties of the Lower Cambrian and the Upper Ordovician-Lower Silurian graphitized organic-rich shales by integrating resistivity test on core samples and wire-line resistivity log data. Laser Raman microprobe (LRM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and transmission electron microscope (TEM) were applied to characterize the organic matter including both kerogen and solid bitumen structure of shale samples with different thermal maturity. Our results suggest a negative correlation between TOC content and resistivity of dry samples and wire-line resistivity data, and a positive correlation between TOC content and polarizability. The graphitized shale samples are characterized by ultra-low resistivity on downhole logs (less than 1 Ω) and high polarizability (high than 10%). The FTIR analysis suggests that there are relatively high aromatics C=C chains and low aromatic C-H chains in the graphitized shales. Comparing with the non-graphitized samples, the Laser Raman spectra of the graphitized shales show narrower half-peak width of the vibration peaks G and D, and higher ratio of peak height (Dh/Gh). The equivalent vitrinite reflectance of the graphitized shale is generally higher than 3.8% converted by LRM. XRD analysis indicates that the distance of the carbon layer (d002) of the graphitized shales ranges from 0.359 to 0.429 nm, more close to that of graphite (0.334 nm), and shorter than non-graphitized shales (between 0.389 and 0.485 nm). TEM analysis suggests that the carbon layer of shales with high graphitization degree shows better ordering and continuity than that of the non-graphitized shales. In summary, graphitized shale is characterized by ultra-low resistivity, more stable chemistry structure, more condensed and orderly carbon layer constructure.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018