Characteristics and controlling factors of pore structure of the Permian shale in southern Anhui province, East China

The Permian shale reservoir in southern Anhui province, East China is regarded as a promising target for shale gas exploration. In order to investigate the characteristics of shale pore structures and their controlling factors, total organic carbon (TOC), Rock-eval, organic petrology, X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), nitrogen gas adsorption (N2GA), mercury intrusion porosimetry (MIP) and helium pycnometry were conducted on the Permian shales collected from two shale gas parameter wells. The results indicate that the BET surface areas determined by N2GA method vary between 1.05 and 49.25 m2/g. The porosities derived from MIP and helium pycnometry tests are in the range of 0.68%-8.9% and 1.15%-9.79%, respectively. FE-SEM reveals that organic matter (OM) pores and cracks are well developed in the Permian shales, though some OM grains contain few pores, which might be related to the maceral composition. At a high maturity stage, vitrinite do not develop secondary OM pores, and sapropelinite generally develop abundant OM pores. However, solid bitumen occupies interparticle space between minerals grains, and generally contains a small amount of pores documented in studied samples. The TOC contents have a positive relationship with the BET surface areas, suggesting OM is a primary factor in micropore and fine mesopore (<10 nm) development. TOC content has a positive relationship with porosity for samples with TOC<6.16%, but samples with TOC>6.16% usually have a low porosity probably due to compaction and/or different organic fractions. Residual bitumen (S1) is weakly and negatively correlated with Hg-porosity, due to residual bitumen filling in OM and mineral pores and reducing the total porosity. In addition, BET surface area decreases with increasing clay mineral content and Hg-porosity decreases with increasing quartz content, illustrating that clay mineral is unfavorable to the development of micropores and fine mesopores and high content of quartz may reduce macropore space. Finally, the shales of the Gufeng and Dalong Formations display a higher TOC content and a better physical property than the Longtan Formation shales and appear to be superior prospective shale gas exploration potential.