Coal Facies and Its Effects on Pore Characteristics of the Late Permian Longtan Coal, Western Guizhou, China

Abstract

The coal facies and their effects on the coal pore characteristics of the Late Permian Longtan Formation, western Guizhou, were studied, using diverse experiments of macerals and minerals compositions, vitrinite reflectance, and pore characteristics. The results show that five coal facies exist in studied samples, and they are shallow-water covered forest peat swamp facies, moist forest peat swamp facies, dry forest peat swamp facies, low peat swamp facies, and wet land herbaceous peat swamp facies. Among them, the moist forest peat swamp facies accounts for the largest proportion, followed by the shallow-water covered forest peat swamp and dry forest peat swamp facies. The order of pore development degree is identical with the order of moist forest peat swamp facies coal > shallow-water covered forest peat swamp facies coal > dry forest peat swamp facies coal. In contrast to shallow-water covered forest peat swamp facies and dry forest peat swamp facies samples, moist forest peat swamp facies samples have higher porosity and larger pore size. Coal facies play an important role in controlling the pore structure of coal reservoir, which have more effect on pore size distribution than coal rank for the samples in this study.

Effect of Coal Rank and Coal Facies on Nanopore–Fracture Structure Heterogeneity in Middle-Rank Coal Reservoirs

“…The study shows that the development of seepage pores is controlled by the coal phase under similar coalification conditions. Lou et al 17 indicate that the shallow-water-covered forest peat swamp facies exhibit higher porosity and larger pore size, thereby emphasizing the coal facies’ control of a key role in the reservoir pore structure of coal, which, in this study, influences of pore size distribution more clearly than coal rank. Zhao et al 10 reported a bimodal pattern in the pore structure of the wetland forest swamp facies in the eastern Ordos basin featuring well-developed micropores and poorly developed macropores.…”

The Coal-Forming Environment at the End of the Late Permian and Its Control on Trace Elements: The Upper Xuanwei Formation in Eastern Yunnan, China

“…A high GI value indicates a wetter coal-forming environment and deeper water cover [44]. The TPI reflects the degree of preservation and degradation, and the higher the TPI value, the better the preservation degree of plant cells, which can be used to determine the type of coal-forming plant [14,45]. The GI and TPI indexes are modified based on Diessel [11] and Jiu et al [46] as follows: GI = (Total Vitrinite + Macrinite)/(Fusinite + Semifusinite + Inertodetrinite) TPI = (Telinite + Telocollinite + Fusinite + Semifusinite)/(Collodetrinite + Macrinite + Inertodetrinite + Vitrodetrinite)…”

“…However, the impact of the peatland environment on coal formation has not been considered. Coal facies contain different combinations of maceral components, and the study of coal facies mainly focuses on the sedimentary environment of coal formation, including coal-forming plants, redox conditions, water activity, pH value, and coal pore structure [10][11][12][13][14][15][16]. Minerals are the major hosts of the vast majority of elements present in coal, such as rare earth elements generally associated with clay minerals, while other trace elements are often associated with pyrite, clay minerals, and many other minerals [17,18].…”