MECHANISMS OF ORGANIC MATTER ENRICHMENT AND SOURCE OF SILICEOUS MINERALS IN UPPER ORDOVICIAN-LOWER SILURIAN SHALES FROM THE UPPER AND LOWER YANGTZE REGION OF SOUTHERN CHINA

The organic matter content is one of the most important factors in determining gas content and the content of siliceous minerals affects the effect of hydraulic fracturing. Thus, how to determine the enrichment mechanisms of organic matter and source of siliceous minerals are important problems that need to be solved urgently. In this study, Upper Ordovician-Lower Silurian shale samples from the Ning-203 and Jiangye-1 wells that are located in the southern Sichuan area of the Upper Yangtze region and the northwestern Jiangxi area of the Lower Yangtze region, respectively, are selected for research. Based on the core sample description, well logging data analysis, mineral and elemental composition analysis, and TOC (total organic carbon) content analysis, the Upper Ordovician-Lower Silurian shale is studied to quantitatively calculate its content of excess silicon. Subsequently, the Al-Fe-Mn ternary diagram is used to determine the origin of excess silicon. Finally, the enrichment mechanism of sedimentary organic matter is analyzed from the aspects of the water body redox environment and paleo-productivity. The results show that the excess silicon from the Upper Ordovician-Lower Silurian shale in the Upper Yangtze area is derived from biogenesis. The sedimentary water body is divided into an oxygen-rich upper water layer that has higher paleo-productivity and a strongly reducing lower water that is conducive to the preservation of sedimentary organic matter. While, the excess silicon in the Upper Ordovician-Lower Silurian shale of the Lower Yangtze area originates from hydrothermal activity that can enhance the reducibility of the bottom water and carry nutrients to improve paleo-productivity and enrich sedimentary organic matter.