Development characteristics and dominant factors of fractures and their significance for shale reservoirs: A case study from Є1b2 in the Cen'gong block, southern China

Abstract

The Bianmachong Formation above the Niutitang Formation in the Lower Cambrian of South China, which was mainly deposited in a marine and transitional environment, has highly brittle characteristics and is an important shale layer for shale gas exploration, but the current level of exploration is low. This paper discusses in depth the development characteristics and dominant factors of fractures and their significance for the shale reservoir in Є1b2 (middle member of the Bianmachong Formation of lower Cambrian) of the Cen'gong block in Guizhou Province based on the observations of outcrops, cores, polished thin sections and scanning electron microscope chips, as well as statistics on the fracture characteristic parameters and a test data analysis of corresponding samples. The results show that overlaps of organic-rich shale and tight sandstone vertically have created favorable generation conditions for shale gas and tight sandstone gas in Є1b2 shale. Є1b2 shale has similar sedimentary environment, mineral composition and organic type as American marine shale but exhibits differences in terms of the abundance of organic matter and degree of thermal evolution. The TOC content of Є1b2 shale is mainly 1–2% and over-maturation. Multiple types and periods of fractures are abundant in Є1b2 shale. They are mainly structural fractures, such as high dip-angle shear, bedding, tension-shear, interlayer and slip fractures. The micro fractures are mainly interlayer fractures, grain boundary fractures and organic matter contraction fractures. Structure is one of dominant factors for the fractures development, but others also have a strong correlativity, such as the lithology, abundance of organic matter, formation thickness, mechanical properties and mineral content. Under the same structural background, the content of brittle minerals such as feldspar, pyrite and quartz is higher. The brittleness is higher in shale, and more fractures are generated, which contributes to reservoir heterogeneity. The TOC content of Є1b2 shale has positive correlations with the content of quartz and the total gas content, which is also positively correlated with fracture development in the cores, and the source of organic matter is siliceous organisms. For over-maturation and organic-rich shale, a high TOC content and a high silica content are beneficial for micro pores and fractures to be preserved. In addition, the fracture development is positively correlated with the total and free gas contents in Є1b2 shale. Micro pores and fractures exist in internal minerals, such as calcite, quartz, and pyrite, as well as in their intervals with fracture surfaces, which reduce the capacity of fractures to resist tension and shear. The fractures are prone to rupture again. In addition, filled fractures make a greater contribution to the reservoir space of Є1b2 shale through dissolution.