A study on pore systems of Silurian highly mature marine shale in Southern Sichuan Basin, China

Abstract

The porosity system in shale, which is a combination of organic-hosted pore and inorganic pore, plays a pivotal role in adsorbing methane for gas shale reservoir. In order to identify the porosity system with different properties and to give a quantitative characterization of pore structure for each individual porosity system, the dual liquid NMR method was conducted on six shale samples from the Lower Silurian Longmaxi Formation in the Southern Sichuan Basin. In shale reservoir, hydrophilic pore and oleophilic pore are respectively related to hydrophilic minerals and oleophilic organic matter. The NMR method detected the signal of water penetrating the water-wetting porosity system when saturated with water, which can reflect the pore size distribution (PSD) of hydrophilic porosity. The PSD of oleophilic pore can also be reflected when saturated with kerosene, vice versa. Generally, the NMR T2 spectra of sample saturated with DI water and kerosene, T2_water and T2_kerosene exhibit bimodal pattern, a dominant peak with a relaxation time range of 0.1–10 ms and a minor peak with a relaxation time ranging 10–100 ms. Generally, higher TOC content corresponds to large amounts of cumulative organic nanoscale pore. The organic-rich shale shows high content of organic-hosted pores and large amounts of inorganic pores which are validated by plane porosity statistics from the observation of SEM photographs. By correlating the two pore structure characterizing methods, the relaxivity of shale can be calculated to be 0.055–0.092 μm/ms when saturated with water. The quantitative estimation of organic-hosted porosity system and inorganic porosity system in the shale will provide a pivotal basis for elucidating gas occurrence states in shale.