Abstract

Shale gas exploration requires studies on the enrichment mechanism of sedimentary organic matter. The Lower Cambrian shale is taken as a study object to analyze the effect of organic matter on gas content using TOC content and porosity analyses, isothermal adsorption experiments, and FIB-HIM scanning electron microscopy observations. Then, we selected typical wells to determine the presence of excessive silica in the siliceous minerals by quantitative calculations. Besides, we analyzed the genesis of excessive siliceous minerals using elements including Al, Fe, and Mn, thus speculating the controlling factors of the redox environment and biological productivity. Results show that total organic carbon content controls the content of free and adsorbed gas, while shale gas mainly exists in organic pores and is developed in large numbers and with high roundness, showing the characteristics of “small pores inside big pores.” In the Lower Yangtze region during the Early Cambrian, the excessive siliceous minerals were of hydrothermal origin, and there were frequent hydrothermal activities due to its closeness to plate boundaries. These activities can intensify the reducibility of the waterbody’s bottom and improve the biological productivity on its surface, resulting in the enrichment of this matter. Most excessive silicon in this region is biogenic, while only a small part is of hydrothermal and biogenic mixed origin. The enclosed waterbody of the Upper Yangtze region was far from plate boundaries and close to the semiclosed “gulf,” resulting in its delamination. The waterbody’s surface was abundant with oxygen, thus increasing the biological productivity, while the high reducibility at the waterbody’s bottom was conducive to preserving sedimentary organic matter.

Highlights

  • Since 2010, shale gas exploration in Sichuan and its surrounding areas has achieved great success with exploration concepts and technologies

  • Exploring the enrichment pattern of sedimentary organic matter is becoming increasingly significant as the total organic carbon (TOC) content in shale is influenced by thermal evolution degrees and abundance of this matter

  • By studying strata of the Lower Cambrian Niutitang Formation (LCNF) in the upper reaches of the Yangtze region, southeast of Chongqing, they found that the formation was influenced by deep-sea hydrothermal fluids carried by oceanic currents [10]

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Summary

Introduction

Since 2010, shale gas exploration in Sichuan and its surrounding areas has achieved great success with exploration concepts and technologies. After studying the lower boundary of the Cambrian series in the upper reaches of the Yangtze region, Zhang et al concluded that the shales rich in organics in the shelf area are affected by seawater redox conditions. Zhang et al pointed out that shales were sediments of redox water conditions in the anoxic deepsea environments on passive continental margins. These shales were affected by ascending currents or hot water to a certain extent [12]. Recent technological advances and massive shale gas exploration have provided unprecedented data for studying the effect of organic matter on gas content as well as the enrichment pattern of organic matter sediments. The origin of excess silicon was discovered, and the enrichment pattern of organic matter sediments was dissected. e organic matter sediments in the lower and upper reaches of the Yangtze region during the Early Cambrian are compared in this paper

Geological Settings
Impacts of Organic Matter on Gas Content
Index Selection for Redox Conditions and Bioproductivity
Analysis on Sedimentary Characteristics of Well TX-1 in the Lower Cambrian
80 Saanich
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