A comprehensive re-understanding of the OM-hosted nanopores in the marine Wufeng–Longmaxi shale formation in South China by organic petrology, gas adsorption, and X-ray diffraction studies

Abstract

The overmature marine Ordovician–Silurian Wufeng–Longmaxi (WF–LMX) formation shale is the most important exploration target for shale gas in South China. In this study, WF–LMX shale samples from Well XY-1 in the Northwestern Guizhou Province (NGP) were selected to comprehensively investigate organic matter (OM) assemblages and OM-hosted nanopores. Petrographic observations under an optical microscope and a scanning electron microscope (SEM) revealed that the OM in the WF–LMX shale is predominately solid bitumen (pyrobitumen), followed by organic zooclasts (predominantly as graptolites), with a minor amount of micrinites (as the heavier conversion products of the oil-prone kerogen macerals). Additionally, the degraded alginites were traced by their bright halo feature in reflected light and blurred fluorescence in blue light when enhancing the exposure intensity, which demonstrates that the previously deposited alginites in the WF–LMX shale were basically converted to hydrocarbons. SEM imaging revealed that the secondary OM pores associated with hydrocarbon generation are pervasive in pyrobitumen, while undeveloped in organic zooclasts and micrinites. Conversely, the good linear relationship between TOC contents and methane absorbed amounts indicates that there must be numerous SEM-invisible pores existed in both the solid bitumen and organic zooclasts in addition to the secondary pores.Aiming to certify and quantify the SEM-invisible pores, the OM were separated from the WF-LMX shale and then were subjected to a series physical and chemical measurements. The combined N2 and CO2 adsorption on the OM isolates of the WF–LMX shale further revealed two pore concentration sections, i.e., pores of 0.33–0.5 nm and 1.2–2.0 nm within the micropore regime, which contributed approximately 82% and 18% to the total surface area and pore volume of the OM, respectively. X-ray diffraction testing further correlated the micropores to the spacing of aromatic rings and macromolecular structural units that constitute the OM. As such, it is the micropores related to the chemical structure of the OM that essentially determine the sorptive capacity of OM and also the WF–LMX bulk shale, whereas the main contribution of SEM-visible OM pores is affording the effective porosity for gas storage. Overall, the clarification of OM assemblages, carriers of secondary OM pores, and existence and origin of OM micropores in the WF–LMX shale provide a further understanding of the generation and occurrence of shale gas in South China.