Pore characterization of Lower Silurian shale gas reservoirs in the Middle Yangtze region, central China

Abstract

Organic-rich shales from Lower Silurian are widely distributed in the Middle Yangtze region, central China. However, the lack of fundamental data for shale gas reservoirs increases the difficulty of gas exploration. In this study, 34 core samples were collected to characterize the shale pore structure and conduct a preliminary evaluation of the shale gas reservoir. The TOC (total organic carbon) content of the successively-deposited black shales range from 1.6% to 5.9%, while the total porosity range from 0.5% to 4.2%. The positive correlation between TOC and porosity indicates that TOC is the key factor determining porosity. The major component of the mineral matrix is quartz (content of 21.4%–69.2%), followed by clay minerals (content of 16.7%–44.5%). Field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) results illustrate that organic matter, mixed with clay minerals, can form an organo-clay complex containing many nanopores. Furthermore, larger organic pores are developed in organo-clay complexes with higher clay content than in those with lower clay content.Correlational analyses between pore volume (or pore surface area) and TOC (or clay content) demonstrate that micropores are associated with organic matter, while mesopores and macropores are probably associated with clay minerals. Many of the clay-related nanopores are organic in nature and are developed in organo-clay complexes containing both organic matter and clay minerals. Overall, the TOC content controls development of nanopores in the shale pore structure, followed by clay content. The DFT-derived PSD indicates that the pore volume is comprised primarily of pores having widths larger than 10 nm, while the surface area is comprised primarily of micropores. When considering the gas in place model and mechanisms of shale gas storage, further shale gas exploration in central China should aim to the deep (>1000 m) and well preserved Longmaxi Shales.