--> --> Pore Characterization and Shale Facies Analysis of the Ordovician-Silurian Succession of Northern Guizhou, South China: The Relationship Between Pore Distribution and Shale Facies

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Pore Characterization and Shale Facies Analysis of the Ordovician-Silurian Succession of Northern Guizhou, South China: The Relationship Between Pore Distribution and Shale Facies

Abstract

The Ordovician-Silurian Wufeng-Longmaxi Formations in the Upper Yangtze Basin have been considered as the most promising shale play in China. It is important to describe in detail the variability of shale facies and their relationship to pore types. Eight distinct facies were identified on the basis of their characteristic physical, biological, and chemical attributes, namely bioturbated shale, black siliceous shale, fossiliferous mudstone; black shale; black dolomitic siltstone; gray muddy siltstone; dark gray muddy siltstone; and dark gray silty shale. Ion-milled samples from all facies were examined with SEM for identification of pore types. Four major pore types appear dominant, interparticle pores, intraparticle pores, phyllosilicate framework pores, and organic matter (OM) pores. OM pores can be further sub-divided into “foam” type pores and “bubble” type pores on the basis of pore diameter. Interparticle pores between detrital grains and phyllosilicate framework pores are widespread in bioturbated shales, and these may be partially filled by organic matter with “foam” type pores. Pores in black siliceous shales are dominated by interparticle pores supported by authigenic silica. Most of these are filled with kerogen/bitumen that is host to “foam” and “bubble” type OM pores. Although interparticle pores supported by diagenetic silica or carbonate are the most important pore type, as the proportion of detrital grains and clay minerals increases phyllosilicate framework pores can make a significant contribution to overall porosity. Intraparticle pores within calcite or dolomite grains are common in fossiliferous mudstones and black dolomitic siltstones, but seem isolated and not contributing to productive porosity. In silty shales and muddy siltstones, interparticle pores between detrital grains and phyllosilicate framework pores are well developed within silty bands or clay-rich layers. Initial interparticle pores within silty beds or laminae, however, were subsequently cemented by calcite or destroyed by compaction. In short, interparticle pores between detrital grains and phyllosilicate framework pores would be mainly destroyed by mechanical compaction during shallow to intermediate burial, whereas interparticle pores between authigenic silica and organic pores would be largely survived during burial processes. Black siliceous shale that is considered as the most promising unit of well-developed pore systems.