Pore structure characteristics of the Lower Cambrian black shale in the Cengong block, southeastern Guizhou area

Based on the mineralogy, organic geochemistry, nitrogen adsorption, physical properties and FE-SEM observation, the pore structure characteristics of the Niutitang and Bianmachong marine shales of the Lower Cambrian in the southeastern Guizhou area were systematically analyzed. The dominant pore types of the organic-lean shale with a high clay mineral content in the Niutitang and Bianmachong formations are plate-like and slit-like inter particle pores with average pore sizes commonly greater than 5 nm. However, the organic-rich shale in the Niutitang Formation mainly contains slit-like and ink-bottle-like pores with average pore sizes less than 3-4 nm, and the specific surface area is 2-3 times of that of the organic-lean shale. In addition, the total pore volume and specific surface area have a positive correlation, and there is also a correlation between clay mineral content and average pore size, but the correlations between total pore volume/specific surface area and clay mineral content/average pore size are negative. Under favorable preservation conditions, the shale has a relatively high development level of organic matter pores, greater values of porosity, pore size and peak diameter with a positive correlation between porosity and permeability, which are represented as “high porosity and low permeability”. Under unfavorable preservation conditions, the reservoir parameters have lower values except the permeability, which leads to the overdevelopment of fractures and has the feature of “low porosity and high permeability”. Besides, the TOC content has a significant control on pore structure and generally has a positive correlation with total pore volume, specific surface area and porosity and a negative correlation with average pore size. Nevertheless, in the intervals with an excessive TOC content, the decrease and increase of pore size and ductility with increasing TOC coupled with compaction and/or unfavorable preservation will result in the atrophy, collapse and close of narrow pores and throats which leads to the negative correlations between TOC and reservoir parameters.