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.
Highlights
In recent years, as a result of the huge resource potential of shale gas, organic rich shales have received renewed attention [1]
Numerous interparticle pores between authigenic microcrystalline quartz are identified, which are well preserved and connected (Figure 4d; Figure 6d,e). These interparticle pores and cleavage-sheet intraparticle pores within clay minerals in both samples YWT-75 and YWT-49 are usually filled by amorphous organic matter, probably bitumen, in which abundant organic matter pores developed (Figure 5b,c; Figure 6d–f)
total organic carbon (TOC) is positively correlated with micropore surface areas (R2 = 0.76) and micropore volume (R2 = 0.73) of siliceous shales (Figure 11c,d), suggesting that siliceous shales with higher TOC content tend to form more micropores, which is consistent with the results of field emission scanning electron microscopy (FE-scanning electron microscopy (SEM)) observation (Figure 6d–f)
Summary
As a result of the huge resource potential of shale gas, organic rich shales have received renewed attention [1]. The intense heterogeneity of shale pore structure results in distinct shale gas storage mechanisms and distribution characteristics [5,6,7], while pore structure is influenced by rock type, mineral composition (mineral types, sizes, and percentage), organic maturity and total organic carbon content [5,8,9]. Their results show that TOC plays an important role in the brittleness, pore structure and sorption capacity of shale reservoirs, potentially controlling the enrichment and productivity of shale gas. The influence of different mineral composition on pore structure, pore volume and surface area distribution of shale remains unclear. Scant attention has been paid to the influence of different mineral composition on pore structure, pore volume and surface area distribution of shale. With samples from over-mature Niutitang Formation in south China, the objectives of this work are to (1) clarify mineral composition of different lithofacies shales; (2) illustrate the pore structure characteristics of different lithofacies shales; (3) elucidate the influence of different mineral composition on pore development of over-mature shales
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