Organic matter pore characteristics of over-mature marine black shale: a comparison of organic fractions with different densities

Organic matter pores are considered to be the most important type of pore for preserving hydrocarbon gases in shale gas reservoirs. The organic matter in each over-mature marine shale sample was separated into two organic fractions with densities of greater than and less than 1.25 g/cm3, and then their molecular compositions and pore characteristics were quantitatively evaluated using solid state 13C-nuclear magnetic resonance (NMR) and gas (N2 and CO2) adsorption analyses, respectively. The results revealed that aromatic carbon is the dominant molecular composition of the over-mature organic matter in the Lower Cambrian Niutitang shale. During the overmature stage, the organic fractions with densities of greater than and less than 1.25 g/cm3 have no significant differences in molecular composition. The organic fractions with densities of greater than and less than 1.25 g/cm3 do have significant differences in their organic pore characteristics. In contrast to the high density organic fraction, the low density fraction contained abundant micropores and lacked mesopores and macropores. The organic pore structures of the different occurrence states of organic matter were significantly different. The C/O of organic matter in different occurrence states are obviously different, which proves that the organic pore structure is closely related to both the occurrence state and density of the organic matter. However, these relationships are still unclear and require further study.