Applying SANS technique to characterize nano-scale pore structure of Longmaxi shale, Sichuan Basin (China)

- SANS technique is applied to study pore structure of Longmaxi shale.
- Scattering intensity follows a power-law relation with an exponent D < 3, indicative of mass fractal nature.
- Surface area, pore size distribution and porosity calculated using PDSP and Porod models are compared to MIP method.
- Porosity variation is mainly related to the content of TOC and mineral composition.

Longmaxi Formation is the leading target play of shale gas development in Sichuan Basin (China) with a large amount of natural gas produced from this organic-rich overmature shale since 2014. In order to investigate the nano-scale pore system, small-angle neutron scattering (SANS) technique was applied on four Longmaxi shale samples (JY-01 to -04) with a range of different lithology. The scattering length density (SLD) for each sample is calculated as a volume average using the chemical composition of different mineral components, with values from 3.58 × 10−10 cm−2 in JY-02 to 3.81 × 10−10 cm−2 in JY-04. Scattering results and analyses reveal Longmaxi shales are mass fractal with an exponent D less than 3. Pore size distribution, specific surface area and porosity are calculated using Porod invariant method and Polydisperse spherical pore (PDSP) model, and are comparable to mercury intrusion porosimetry (MIP) results, suggesting SANS is valuable to study nano-scale pore space from microscopic to nearly micrometer scale. In addition, with the increasing depth and the scattering vector (Q) values, the scattering intensity decrease over five orders of magnitude, reflecting the differences in the certain scale structure of scatters at various burial depths of Longmaxi shale. For a certain Q value, with an increasing depth, the scattering intensity values initially increase slowly first, and then increase rapidly. The fractions of closed pore also increase with depth and reach the maximum in JY-04, which is related to the TOC content and mineral composition and is partially developed during the development of very complicated pore network.