Pore characterization and the controls of organic matter and quartz on pore structure: Case study of the Niutitang Formation of northern Guizhou Province, South China

Quartz and organic matter (OM) are two important components in marine shales that have significant effects on shale gas reservoir quality, particularly on pore structure. However, a limited number of in-depth studies exist on characterizing the OM and quartz. In this study, types of quartz and OM in the organic-rich marine Niutitang shales and their impacts on pore structure were investigated using X-ray diffraction analysis, scanning electron microscopy, nitrogen adsorption, and geochemical analyses. Three OM types (scattered OM, stripped OM, and interstitial OM), four types of OM pores (primary OM pores, convoluted OM pores, shrinkage OM pores, and thermogenic OM pores) and two quartz types (detrital quartz and authigenic quartz) were identified. The Lower Member (LM) and Upper Member (UM) of the Niutitang Formation have different types of OM and quartz, resulting in the differences of pore structure characteristics. Scattered OM and convoluted OM pores and extrabasinal detrital quartz are widespread in the UM, whereas interstitial OM and thermogenic OM pores and authigenic quartz are common in the LM. Quartz in the LM shales positively correlated with TOC, as well as TOC positively correlating with excess-Si, which may indicate that quartz may be mostly biogenic in origin. The biogenic authigenic quartz can act as a rigid framework, which can resist compaction and preserve the internal pore structure and provide enough space to be in-filled by OM. Abundant OM filled inter-particle pores formed by authigenic quartz is the principal matrix for OM pore development. There are positive correlations among TOC and quartz with pore structure parameters and shale with higher contents of TOC and quartz lead to preservation of some primary pores and development of OM pores, indicating that the pore structure in the Niutitang shale may be mainly controlled by OM and quartz. This study suggests that the shale in the LM would more likely contain better pore systems and provide favorable reservoir spaces for shale gas due to its high content and favorable types of OM and quartz.