Abstract
Organic matter is the material basis for shales to generate hydrocarbon, as well as the main reservoir space and seepage channel for shale gas. When the thermal evolution degree is consistent, the organic carbon content in present shales is subject to the abundance of primitive sedimentary organic matter. Deep geofluids significantly influence the sedimentary organic matter’s enrichment, but the mechanism remains unclear. This paper is aimed at determining how hydrothermal and volcanic activities affected the enrichment of sedimentary organic matter by studying lower Cambrian shales in the lower Yangtze region and upper Ordovician-lower Silurian shales. Oxidation-reduction and biological productivity are used as indicators in the study. The result shows that hydrothermal or volcanic activities affected the enrichment of sedimentary organic matter by influencing climate changes and the nutrients’ sources on the waterbody’s surface and reducing water at the bottom. In the lower Cambrian shales of the Wangyinpu Formation in the lower Yangtze region, hydrothermal origin caused excess silicon. During the sedimentary period of the lower and middle-upper Wangyinpu Formation, vigorous hydrothermal activities increased the biological productivity on the waterbody’s surface and intensified the reducibility at the bottom of the waterbody, which enabled the rich sedimentary organic matter to be well preserved. During the sedimentary period of the lower upper Ordovician Wufeng Formation and the lower Silurian Longmaxi Formation in the upper Yangtze region, frequent volcanic activities caused high biological productivity on the waterbody surface and strong reducibility at the bottom of the waterbody. As a result, the abundant organic matter deposited from the water surface can be well preserved. During the sedimentary period of the upper Longmaxi Formation, volcanic activities died down gradually then disappeared, causing the biological productivity on the water surface to decrease. Besides, the small amount of organic matter deposited from the water surface was destroyed due to oxidation.
Highlights
Since 2000, progresses made in exploration concepts and technology have made shale gas prospection considerable success in North America [1,2,3]
Based on statistics like a detailed description of rock cores, element analysis, TOC content analysis, and data calculation, the paper analyzes the mechanism of the enrichment of the sedimentary organic matter in the Yangtze region during the Late Ordovician-Early Silurian
The fluids affected the enrichment of the sedimentary organic matter via controlling the nutrition on the waterbody’s surface, the reducibility of the waterbody’s bottom, and climate change
Summary
Since 2000, progresses made in exploration concepts and technology have made shale gas prospection considerable success in North America [1,2,3]. Organic matter is the material basis for shales to generate hydrocarbon and the main reservoir space and seepage channel for shale gas. As the organic carbon’s content in present shales is subject to the abundance of primitive sedimentary organic matter, it is imperative to ascertain the main factors that control the abundance [8]. The sediment and enrichment of unconventional petroleum are closely related to sudden geological environment changes. They result from the sedimentary coupling of global or regional structure and multiple geological events such as sea-level rise and fall, volcanic activities, abrupt climate changes, oxygen deficit in waterbodies, and biological extinction and prosperity. Qiu et al (2020) [9] underlined the fluid flow’s influence in the deep crust on the sedimentary organic matter’s enrichment
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