Occurrence of stable and mobile organic matter in the clay-sized fraction of shale: Significance for petroleum geology and carbon cycle

• TOC in clay-sized fraction can be categorized into stable and mobile OCs.
• Different OCs varied in correlation with different mineral surface areas.
• The forms of OM occurrence determine the stability and mobility of OC.
• This study is of great significance for petroleum geology and carbon cycle.

Stability and mobility of organic matter (OM) in shale is of great significance for the carbon cycle and petroleum exploration and exploitation. To examine the stability and mobility of OM occurrence in shales, clay-sized fractions (< 2 μm) were separated from shales physically and chemically treated with different reagents. These fractions were followed by pyrolysis and measurements of specific surface area (SSA) and Fourier transform infrared spectrophotometry (FTIR). It is revealed that after wet chemical oxidation and organic solvent extraction, there exist: a) changes in total organic carbon (TOC), hydrogen index (HI) and oxygen index (OI); b) appreciable decrease in absorption intensity of –CH2– stretching vibration bands at 2924 and 2853 cm− 1; and c) dramatic increase of SSAs after wet chemical oxidation. These characteristics suggest that the TOC in the clay-sized fraction can be categorized into physical mobile-OC (PmOC), chemical mobile-OC (CmOC) and stable-OC (SOC), which account for 43.3%, 17.1% and 39.6% of TOC on average in the studied samples, respectively. The OM in the clay-sized fraction mainly occurs on mineral internal surface, and the occurrence of OM determines the stability and mobility of OC. The PmOC mainly accumulates in the pores and at the mouth and/or edge of the interlayer spaces of clay minerals, the CmOC chiefly adsorbs on the mineral external surface, and the SOC mainly occurs on the mineral internal surface. These occurrence characteristics of different OCs indicate that the mobility and stability of OM occurrence in a sample have different levels. The occurrence of stable and mobile OM in the clay-sized fraction of shale as investigated can improve the understanding of the occurrence of OM in shales, and provide a new insight for carbon cycle research and petroleum exploration and exploitation.