Nanoscale pore characteristics of the Lower Cambrian Niutitang Formation Shale: A case study from Well Yuke #1 in the Southeast of Chongqing, China

Abstract

The Lower Cambrian Niutitang Formation Shale is one of the most important shales being studied for unconventional development in China. In this work, we focused on 21 core samples of Niutitang Shale from Well Yuke #1 in the southeast of Chongqing, to better understand their vertical reservoir characteristics and pore evolution. Using complementary approaches of X-ray diffraction, N2 and CO2 adsorption, petrology microscope, and field emission-scanning electron microscopy, we conducted a series of analyses for pore volume, pore-size distribution, surface area, fractal characterization, organic geochemistry, petrology, and mineralogy. Results indicate that most micropores (<2nm) are associated with grains of organic matter. Meanwhile, the meso-macropores (2–50nm to >50nm) are composed of organic pores and inorganic pores. Meso-macropore volume per unit of total organic carbon (TOC) content dramatically decreases with an increase of maturity and extent of diagenesis. Meanwhile, the TOC-normalized micropore volume also rapidly declines after maturity (Ro) values were higher than 3.13%. The surface fractal dimension D1 for relative N2 pressure P/P0>0.5 with capillary condensation and surface fractal dimension D2 for P/P0<0.5 with mono- and multi-layer adsorption, derived from N2 sorption isotherms, can be used to indicate the pore characteristics. D1 is controlled by the percentages of micropore volume. D2 seems to be affected by the clay mineral contents and thermal maturity.