Siliceous Origin of the Cambrian Qiongzhusi Formation Shale in the Middle Part of the Upper Yangtze Platform: Significance of Organic Matter Enrichment

Abstract

The early Cambrian Qiongzhusi Formation shale is rich in organic matter and is a high-quality marine Source rock. However, the origin of Qiongzhusi Formation siliceous rocks is unknown, and the role of siliceous rocks in the process of organic matter enrichment or preservation is also lacking. This study combines thin section, scanning electron microscopy, SEM/EDS, major and trace element analysis, and N 2 adsorption experiments to analyze and evaluate the shale of the Qiongzhusi Formation in the central region of the Sichuan Basin. The quartz types in the shale of the Qiongzhusi Formation are divided into four types, namely, bioclastic siliceous rocks, terrestrial detrital quartz, siliceous microcrystalline quartz particles, and microcrystalline quartz aggregates; at the same time, according to petrographic and geochemical parameters, the content of authigenic quartz in Qiongzhusi Formation shale decreases from top to bottom, and terrigenous detrital quartz tends to increase, and biogenic silicon accounts for the majority of authigenic quartz components; autogenous quartz has a positive impact on the pore structure of shale, providing sufficient pore space for the development of organic pores and protecting the internal pore network by forming intergranular pores as rigid frameworks. At the same time, it plays a crucial role in the enrichment and preservation of organic matter.

Characteristics and Controlling Factors of Pores in Different Shale Lithofacies Reservoirs of Lower Cambrian Qiongzhusi Formation, Southwestern Sichuan Basin, China

“…After saturation in ethylene glycol vapor (at around 40 • C for 7 h), the glass slide was identified by a Rigaku-Ultima-IV X-ray diffractometer, with Cu-Kα radiation, as a function of the 2θ angle (8 to 80 • , with 0.02 • step, and 5 s time per step). The clay mineral composition and content of shale samples were calculated based on the Chinese oil and gas industry standard SY/T 5163-2018 [38].…”