Abstract
The upper Ordovician-lower Silurian shale has always been the main target of marine shale gas exploration in southern China. However, the shale gas content varies greatly across different regions. The organic matter content is one of the most important factors in determining gas content; therefore, determining the enrichment mechanisms of organic matter is an important problem that needs to be solved urgently. In this paper, upper Ordovician-lower Silurian shale samples from the X-1 and Y-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 analysis. Based on the core sample description, well logging data analysis, mineral and elemental composition analysis, silicon isotope 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 results of elemental analysis and silicon isotope analysis are used to determine the origin of excess silicon. Finally, we used U/Th to determine the characteristics of the redox environment and the relationship between excess barium and TOC content to judge paleoproductivity and further studied the mechanism underlying sedimentary organic matter enrichment in the study area. 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 paleoproductivity and a strongly reducing lower water that is conducive to the preservation of sedimentary organic matter. Thus, for the upper Ordovician-lower Silurian shale in the upper Yangtze region, exploration should be conducted in the center of the blocks with high TOC contents and strongly reducing water body. However, 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 from the crust to improve paleoproductivity and enrich sedimentary organic matter. Therefore, for the upper Ordovician-lower Silurian shale in the lower Yangtze region, exploration should be conducted in the blocks near the junction of the two plates where hydrothermal activity was active.
Highlights
Shale gas is an important unconventional gas resource that is a current research focus in the petroleum geology field [1, 2]
Since the Wei-201 well obtained industrial gas in the upper Ordovician Wufeng-lower Silurian Longmaxi Formation marine shale in 2010, China began to develop shale gas vigorously and initially obtained commercial development in some shale gas blocks, including the Changning, Zhaotong, Fushun-Yongchuan, Fuling, and Dingshan blocks [5,6,7]
Zhao et al [17] categorized different lithofacies types by combining field surveys and drilling core descriptions of the Wufeng-Longmaxi Formation shale obtained at four different wells in the Sichuan Basin with observations of rock thin section. They discovered that Wufeng-Longmaxi shale contains a variety of elements, such as Si, Ca, Al, U, Fe, and Mn, as well as siliceous minerals that are closely related to organic matter enrichment
Summary
Shale gas is an important unconventional gas resource that is a current research focus in the petroleum geology field [1, 2]. Wang et al [16] used redox-sensitive elements in trace elements as important proxies to determine the redox environment of the ancient ocean and to further study the mechanism underlying organic matter enrichment They found that the organicrich shale present at the bottom of the Longmaxi Formation was deposited in an anoxic environment and that organic C content is positively correlated with V/Cr and Ni/Co ratios. Zhao et al [17] categorized different lithofacies types by combining field surveys and drilling core descriptions of the Wufeng-Longmaxi Formation shale obtained at four different wells in the Sichuan Basin with observations of rock thin section They discovered that Wufeng-Longmaxi shale contains a variety of elements, such as Si, Ca, Al, U, Fe, and Mn, as well as siliceous minerals that are closely related to organic matter enrichment. The difference in the organic matter enrichment between the upper and lower Yangtze regions during the late Ordovician-early Silurian period is analyzed
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