The Characteristics and Main Controlling Factors for the Formation of Micropores in Shale from the Niutitang Formation, Wenshuicun Section, Southwest China

Danlong Li,Dong Wu,Rui Xue,Meiyan Fu,Yun Huang

doi:10.3390/en14237858

Abstract

The characteristics of shale micro-pore development and its main influencing factors have important theoretical guiding significance for shale gas exploration and resource evaluation. In order to clarify the micro-pore development characteristics of lower Cambrian shale and the main controlling factors of micro-pore development, we used the lower Cambrian Niutitang formation shale, in the Wenshuicun section of the Guizhou Province in southwest China. The micro-pore development characteristics of the shale in the region were studied by argon ion profile field emission scanning electron microscopy and a low-temperature liquid nitrogen adsorption and desorption experimental system. The relationship between micro-pore and kerogen maceral composition, total organic carbon (TOC) content and different mineral content was analyzed in combination with mineral and geochemical characteristics. Inorganic pores (clay mineral pores, dissolution pores and pyrite intergranular pores) and micro-fractures (clay mineral shrinkage crack, tectonic fractures and overpressure fractures) were the main type of pore developed in the shale of the Niutitang formation in the Wenshuicun section, and no organic pores had developed. The pore size of shale is usually 2–50 nm, accounting for 58.33% of shale pores, e.g. mesopores. Clay mineral content has an obvious positive correlation with macropore volume and average pore diameter, and an obvious negative correlation with micropore volume. In addition, the content of feldspar in brittle minerals has a strong negative correlation with macropore volume and average pore diameter, and a strong positive correlation with micropore volume and BET-specific surface area. TOC content and the content of different kerogen macerals have no obvious correlation with the development of shale micropores in this region. It is concluded that inorganic mineral composition is the main controlling factor of micro-pore development within lower Cambrian shale, and organic matter abundance and maceral content have little influence on the micro-pore development. This study provides a case study for the characteristics of micropores in lower Cambrian shale in China.

Highlights

IntroductionMicro-nano pores, especially organic pores, are important reservoir spaces for shale gas
According to the experimental results, it was found that inorganic pores are commonly developed in the black shale samples of the Wenshuicun region, including clay mineral shrinkage fractures and dissolution pores, and pyrite intercrystalline pores
Large amounts of organic matter could be observed to be filled with pyrite or developed in association with clay minerals, but organic pores were hardly developed (Figure 2)

Summary

Introduction

Micro-nano pores, especially organic pores, are important reservoir spaces for shale gas. In the process of the pyrolysis of residual oil and bitumen into gas, abundant organic matter pores are produced [7]. The pore development of shale is an important consideration when evaluating the gas-bearing property of shale reservoirs. Previous studies have shown that shale reservoirs mainly consist of micron-nano scale pores [8]. Organic pores are an important reservoir space for oil and gas in marine shale with high-over maturity [9]. Shale pore development is mainly affected by thermal maturity, organic matter abundance, kerogen maceral and mineral content, as well as other factors [10,11,12,13,14,15]. In the polar development stage of organic pores, there is a positive correlation between TOC and pore volume of different pore sizes and total pore volume [13,14]

Methods

Results

Conclusion