The developmental characteristics of natural fractures and their significance for reservoirs in the Cambrian Niutitang marine shale of the Sangzhi block, southern China

The great potential that southern China has for marine shale gas resources is well known, yet shale gas production is very small. Exploration and development of southern China has lagged because of the complicated structural history of this region. Thus, it is important to recognize the tectonic history of this region and to carefully describe the developmental characteristics of natural fractures associated with the tectonic evolution of southern China. In this paper, we discuss the developmental characteristics of natural fractures based on the description and statistical analysis of natural fractures in outcrops, cores and thin sections, and from several experimental tests. Fractures examined in outcrop appear to be principally fold-fault related, regional tectonic, bedding or weathering fractures. Fractures observed in core are mainly shear fractures, tension fractures, and shear-tension fractures. Most of these fractures are sealed by calcite. Micro fractures and pores are well developed. Analysis of the cross-cutting relation of micro fractures suggests that the study area has been subject to at least two tectonic movements, consistent with our investigation of core. Fracture development appears to have been strongly influenced by structural evolution. However, fracture density displays a weak negative correlation with organic matter content. Further, fracture density appears to correlate positively with quartz content. Vertical variations of mineralogy appear to have influenced fracture generation and, therefore, hydrocarbon migration within the Niutitang Shale.