Developmental characteristics and controlling factors of natural fractures in the lower paleozoic marine shales of the upper Yangtze Platform, southern China

Abstract

This work elucidates the developmental characteristics and controlling factors of natural fractures, and their effects on the shale gas content in the lower Paleozoic marine shales of the upper Yangtze Platform, southern China. These objectives have been achieved by systematic observations and descriptions of fractures in outcrops and drilling cores, as well as experimental tests on samples from the corresponding fractured intervals. The types of natural macro-fractures observed in both the Niutitang and Wufeng-Longmaxi shales mainly include vertical tension fractures, high-angle shear fractures, bed-parallel slip fractures and bedding fractures, with bedding fractures accounting for the largest proportion. In addition, Field Emission-Scanning Electron Microscopy (FE-SEM) images show that interlayer, inter-particle, intra-particle and organic matter-associated fractures are the primary types of micro-fractures. Tectonism is one of the dominant factors in fracture development, but other factors also have a strong influence, such as TOC content, mineral composition and shale-bed thickness. Under similar tectonic settings, the development of fractures is mainly influenced by the TOC content. For the Wufeng-Longmaxi shale, fracture abundance is positively correlated with the TOC content. However, the TOC content in Niutitang shale displays a positive correlation with fracture density only when the TOC content is less than 6%; when the TOC content is greater than 6%, the correlation is shown to be negative. Shale with higher quartz or lower clay contents exhibits greater fracture development. Furthermore, the shale-bed thickness has a negative influence on the fracture development, which can be essentially attributed to the change in mineral composition of shale. Natural fractures provide reservoir spaces for free gas and are favorable for the desorption of adsorbed gas; therefore, the better shale fractures are developed, the greater the gas content will likely be.