Comparative study on micro-pore structure of marine, terrestrial, and transitional shales in key areas, China

The purpose of this article is to make a comparative study on the pore structure of different sedimentary facies shales developed in typical areas, China. Results show that the surface area (SA) and pore volume (PV) of the over-mature marine shales are in the range of 17.83-29.49 m2/g and 3.65-18.26 ml/kg, respectively, which are much higher than that of the highly mature transitional and less mature terrestrial shales in the range of 0.9-1.9 m2/g and 2.5-10.9 ml/kg, respectively. In the view of the pore-size distribution (PSD), marine shales concentrate at the fine mesopores < 10 nm, while the terrestrial and transitional shales focus on the large-mesopores to fine-macropores (30-70 nm). Additionally, the qualitative analysis of hysteresis loops from the nitrogen isotherms reveals that the marine shales are dominated by the inkbottle-shaped pores, which are further confirmed to be the organic pores. While the terrestrial and transitional shales investigated give priority to the slit-shaped pores or wedge-shaped pores, which are further identified as the interlayer pores within clay minerals. Furthermore, different shale components have great differences in the contribution to the SA and PV in different sedimentary facies shales due to the different development of nanopores within (or associated with) different shale compositions. In post-mature marine shales, organic matters (OMs) bearing a large amount of smaller organic nanopores contribute mostly to the SA and PV. For the highly mature transitional shales, it is the illite-smectite mixed clay (I/S) that contributes mostly to the SA and PV because the I/S-hosted nanopores are in the dominated position of the pore system and OMs are almost free of organic pores. As to the less mature terrestrial shales, the I/S minerals together with the OMs jointly contribute to the SA and PV due to the large amount of I/S-hosted pores and some existence of organic pores. Finally, the differences among the three sedimentary facies shales in terms of the pore space, suggest that the marine shales in the South China likely possess greater potential for shale gas than do the terrestrial and transitional shales in North China.