Abstract
This study uses logging data, mineral component content, total organic carbon (TOC) content, and microscopic characteristics of the organic-rich shales in the Wufeng and Longmaxi Formations, as well as data reported by other researchers, to demonstrate that upwelling has played an important role in the organic matter enrichment. The results show that (1) the organic-rich shales of Well N211 in the Upper Yangtze region are located in the Wufeng Formation and the lower Longmaxi Formation, with a burial depth between 2308–2357 m. (2) The organic-rich shales are enriched in biogenic silica. (3) Based on paleogeographic location and characteristics of organisms, this study determines that upwelling occurred during the deposition of the organic-rich shales in the Wufeng and Longmaxi Formations, promoting the enrichment of organic matter in the shales. (4) The upwelling intensity gradually increased from the sedimentary period of the organic-rich shales in the mid-lower Wufeng Formation to the sedimentary period of the Guanyinqiao Member, and then decreased gradually from the sedimentary period of the Guanyinqiao Member to the sedimentary period of the organic-rich shales in the Longmaxi Formation, and leads to the different enrichment of organic matter in the vertical direction. The different developments of upwelling led to the coexistence of both high and low TOC contents in the Guanyinqiao Member along the vertical direction.
Highlights
With the vigorous development of shale gas production, the formation mechanism and pattern of organic-rich shales have become topics of considerable interest in current research [1,2]
It is generally believed that the organic-rich shales in the Wufeng and Longmaxi Formations formed under the sedimentary background of an anoxic environment and high productivity [2,3,4,5,6,7,8], and that the semi-closed environment or rapid transgression was the main reason for the anoxic environment [2,3,4,5]
By combining the results from this work with the data reported by other researchers, our study preliminarily discusses the development conditions of upwelling during the sedimentary period of the organic-rich shales in the Wufeng and Longmaxi Formations of the study area and suggests that upwelling played an important role in the formation of the organic-rich shales in the Wufeng and Longmaxi Formations
Summary
With the vigorous development of shale gas production, the formation mechanism and pattern of organic-rich shales have become topics of considerable interest in current research [1,2]. Lu et al [9] and Wu et al [10] suggested that volcanic ash provided abundant nutrients for marine organisms and played a positive role in the enhancement of marine surface primary productivity, providing a material basis for the organic-rich shales in the Wufeng and Longmaxi Formations. The upwelling-induced high primary productivity played an important role in the formation of organic-rich shales during the geological periods. Studies on the contribution of upwelling to the organic-rich shales in the Wufeng and Longmaxi Formations remain scarce. By combining the results from this work with the data reported by other researchers, our study preliminarily discusses the development conditions of upwelling during the sedimentary period of the organic-rich shales in the Wufeng and Longmaxi Formations of the study area and suggests that upwelling played an important role in the formation of the organic-rich shales in the Wufeng and Longmaxi Formations
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