Geochemical Characteristics and Significance of the Cambrian Qiongzhusi Formation in the Northern margin of the Sichuan Basin: A case study of the Chengjiaba Section in Guangyuan

Key words: 
• Sichuan Basin /
• Chengjiaba /
• Qiongzhusi Formation /
• sedimentary environment /
• geochemistry of element

Abstract: [Objective] The shale gas of the Cambrian Qiongzhusi Formation in the Sichuan Basin has good prospects for exploration and development. It is of great significance to reveal the characteristics of its sedimentary environment for the evaluation of shale gas resources in the Qiongzhusi Formation. [Methods] Taking the shale series of Qiongzhusi Formation in Chengjiaba section of northern Sichuan as the research object, based on detailed field geological survey, the whole rock X-ray diffraction and geochemical analysis were carried out, and the characteristic elements and ratio parameters were optimized to reveal the sedimentary paleoenvironment and conditions of organic-rich shale in Qiongzhusi Formation in northern Sichuan. [Results] (1) The first and second members of Qiongzhusi Formation are mainly gray-black or black mudstone and silty mudstone, with siliceous rock facies and siliceous shale facies, with horizontal bedding, which is a deep-water shelf facies. (2) The shale TOC content of Qiongzhusi Formation is relatively high, ranging from 0.22 % to 4.34 %, with an average of 2.68 %. (3) The geochemical characteristics of the elements show that the climate was warm and humid during the deposition of the black shale of the Qiongzhusi Formation, the paleoproductivity and deposition rate were high, and the water body was anoxic and highly retained. [Conclusions] It is considered that the formation of organic-rich shale in Qiongzhusi Formation is controlled by redox conditions, paleoproductivity, paleoclimate and deposition rate.