Origin of authigenic quartz in organic-rich shales of the Niutitang Formation in the northern margin of Sichuan Basin, South China: Implications for pore network development

Abstract

Quartz has a significant effect on shale reservoir properties and plays an important role in shale gas development. However, quartz types, origins and the effect on pore network are still unclear. In this study, the approaches of X-ray diffraction, geochemical analyses, scanning electron microscopy (SEM) imaging, and low-temperature nitrogen sorption isotherm were jointly used to evaluate Niutitang Formation, which has gradually become a key target formation for shale gas exploration and development in south China. Shale samples from two wells (A and B) of Niutitang Formation in the northern margin of Sichuan Basin, South China, were used to identify the types, origins and effects on pore network development for quartz. The results show that shale lithofacies type for the Niutitang Formation shale is siliceous shale, and quartz mainly exists in four forms, including siliceous skeletal fragments, silt-size detrital quartz, microcrystalline quartz coexisting with clay minerals, and euhedral quartz aggregates consisting of numerous quartz crystals. According to Al–Fe–Mn ternary diagram, silica in shale samples from Well A and Well B are not of hydrothermal origin. Furthermore, a series of cross-plots based on geochemical proxies, including SiO2 vs. Zr, Siexcess vs. Al, and Ti/Al vs. Siexcess, suggest that the biogenic silica is a major silica source in both Well A and Well B, and the proportion of detrital silica content in Well B is higher than that in Well A. Both microcrystalline quartz and aggregates of euhedral quartz can jointly improve the connectivity of pore network by providing the space for organic matter-hosted pores. In addition, the euhedral quartz can also protect organic matter pore and internal pore network as a rigid framework by restraining the compaction effect.