Controls of Distinct Mineral Compositions on Pore Structure in Over-Mature Shales: A Case Study of Lower Cambrian Niutitang Shales in South China

Abstract

Investigating the impacts of rock composition on pore structure is of great significance to understand shale gas occurrence and gas accumulation mechanism. Shale samples from over-mature Niutitang formation of Lower Cambrian in south China were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption to elucidate the controls of distinct mineral composition on pore development. Two distinct lithofacies, namely siliceous shale and argillaceous shale, were ascertained based on their mineral composition. Due to the variability of mineral composition in different lithofacies, pore structure characteristics are not uniform. Pores in siliceous shales are dominated by interparticle pores and organic matter (OM) pores, among which the interparticle pores are mainly developed between authigenic quartz. Furthermore, most of these interparticle pores and cleavage-sheet intraparticle pores within clay minerals are usually filled by amorphous organic matter that is host to OM pores. Due to the lack of rigid minerals, argillaceous shale was cemented densely, resulting in few interparticle pores, while cleavage-sheet intraparticle pores within clay minerals are common. Comparing siliceous shales with argillaceous shales, specific surface areas and pore volumes are higher on the former than on the latter. The content of total organic carbon (TOC) and authigenic quartz have a great influence on micropore structures, but less on mesopore structure for siliceous shales. The rigid framework structure formed by authigenic quartz is believed to be able to prevent primary interparticle pores from mechanical compaction and facilitate the formation of organic matter-associated pores. In terms of argillaceous shales, due to the lack of authigenic quartz, interparticle pores were rarely developed and its pore structure is mainly controlled by illite content.

Influence of Mineral Compositions on Shale Pore Development of Longmaxi Formation in the Dingshan Area, Southeastern Sichuan Basin, China

“…Besides, quartz and feldspar, as the most important brittle minerals, are principal factors affecting the fracability of the shale. − Affected by tectonic stress in the later stage, a complex fracture network system might be formed, increasing the seepage capacity of shale gas. Organic matter and clay minerals present higher specific areas and strong capability of shale gas adsorption for their small constituent molecules, which can provide more adsorption potentials for the gas and increase the adsorption capacity of shale gas, principally affecting the content of adsorbed gas of the shale. − The difference in mineral composition is a critical factor affecting shale quality according to the analysis of the difference in shale gas adsorption capacity of different clay minerals and water contents and analysis concerning the influence of mineral composition on different shale levels gas contents. − …”

The accumulation model of organic matters for the Niutitang Formation shale and its control on the pore structure: a case study from Northern Guizhou

“…The intergranular pores and organic pores are developed in Niutitang Formation siliceous shale (Niu et al 2021), while Wang et al (2020a, b) found that the organic pores are rarely developed in the Niutitang Formation shale. For the argillaceous shale in the Niutitang Formation, the intergranular pores are less developed, and the cleavage-sheet intraparticle pores are developed in the clay minerals (Niu et al 2021). The content of TOC (< 6.5%) and quartz favor the development of pores for the siliceous shale, especially that for the micropores (Wang et al 2016b, c;Niu et al 2021).…”

“…For the Niutitang Formation shale in Guizhou, the content of quartz is also high, and the quartz comes from both continental detrital quartz and biogenetic quartz (Sun et al 2021). The intergranular pores and organic pores are developed in Niutitang Formation siliceous shale (Niu et al 2021), while Wang et al (2020a, b) found that the organic pores are rarely developed in the Niutitang Formation shale. For the argillaceous shale in the Niutitang Formation, the intergranular pores are less developed, and the cleavage-sheet intraparticle pores are developed in the clay minerals (Niu et al 2021).…”

“…For the argillaceous shale in the Niutitang Formation, the intergranular pores are less developed, and the cleavage-sheet intraparticle pores are developed in the clay minerals (Niu et al 2021). The content of TOC (< 6.5%) and quartz favor the development of pores for the siliceous shale, especially that for the micropores (Wang et al 2016b, c;Niu et al 2021). The quartz features as rigid mineral, it can resist a certain compaction, which will finally preserve the pore space (Xi et al 2019).…”

“…Besides, the dissolution of feldspar could also promote the development of pores (Guo et al 2021). When it comes to the argillaceous shale, the pore structure characteristics are mainly influenced by the content of illite (Niu et al 2021).…”