Geochemistry and sedimentology of the Lower Silurian Longmaxi mudstone in southwestern China: Implications for depositional controls on organic matter accumulation

The Lower Silurian Longmaxi Formation in the south of Sichuan is a key player in the exploration and development of shale gas in China. Due to a highly complex topographic area, electromagnetic methods (EM) become important exploration means in this area. Many studies have been conducted on the shale mineral composition and electrical properties of shale, however, the correlation between sedimentary environments and the electrical properties of shale remain poorly understood. The electrical properties and sedimentary environment of the organic-rich shale of the Longmaxi Formation have been studied by means of X-ray diffraction, organic geochemistry, scanning electron microscopy and complex resistivity measurements. The discovered high quartz content of the Longmaxi Formation shale results in low resistivity. Deep-water shelf biogenic quartz contributes lower resistivity more than that of shallow-water terrigenous quartz. The deep-water anoxic and organic sedimentary environment led to major enrichment of pyrite, leading to a high polarization effect in shale. We present the correlation between the lithofacies types and electrical properties of Longmaxi Formation. The mixed siliceous shale lithofacies is the most favorable among the three lithofacies, which is characterized by high total organic carbon (TOC) content, high brittleness mineral content, high polarization and low resistivity ("three high and one low"). This feature is an effective identification of shale gas reservoirs by electromagnetic prospecting. Our study can provide constraints on electrical parameters of rocks for electromagnetic "sweet spot" exploration of shale gas, and so this has important geological significance to shale gas exploration and development.

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“…Volcanic activity is another reason for the difference in redox environment and biological productivity between the Wufeng Formation-the lower section of the Longmaxi Formation 1st member and the upper section of the Longmaxi Formation 1st member. Influenced by the collision between the Yangtze Plate and the Cathaysian Plate, during the deposition of the Wufeng Formation-the lower section of the Longmaxi Formation 1st member, volcanic activity was frequent and intense, leading to dramatic climate changes, improving the reducibility of the lower water [37,38,[76][77][78][79][80]. After the ash materials formed by volcanic eruption subsiding into surface water, minerals [68] and Zhang et al [30].…”

Section: Volcanic Activity Analysis and Its Effect On The Enrichment mentioning

confidence: 99%

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in the Upper Yangtze Area: Taking Jiaoye-1 Well in the Jiaoshiba Block as an Example

Liu

¹

,

Jiang

²

,

Zhang

³

et al. 2019

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Organic matter is the material basis of shale hydrocarbon generation. The current organic matter content in shale is controlled by the original sedimentary organic matter abundance. Therefore, the study of the enrichment mechanism of sedimentary organic matter in shale has become an important issue to be solved. The Upper Yangtze area is the important exploration and exploitation area of marine shale gas in China. The shale of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Yangtze area is the research object. Choosing redox indicator and biological productivity indicator, the study explores the enrichment mechanism of sedimentary organic matter from two aspects, sealing of water and volcanic activity. The results show that excess siliceous mineral in the shale of the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area is bioorigin. Excess siliceous mineral can be used as one of the indicators of biological productivity. On the one hand, layer phenomenon occurred since the strong water sealing during the sedimentary period of Wufeng and the lower section of the Longmaxi Formation, which results in the high content of oxygen in surface water. On the other hand, the active volcanic activity brought volcanic ash which was beneficial to biological reproduction. Both of these factors led to higher biological productivity during this period. At the same time, the strong sealing of water made the lower layer of the water more reductive, and the active volcanic activity caused climate change, enhancing the reduction of the lower layer of the water, which made the rich organic matter deposited from the surface water well preserved. In the sedimentary period of the upper section of the Longmaxi Formation 1st member in the Upper Yangtze area, on the one hand, due to the weakened sealing of water, the oxygen content of the upper water decreased. On the other hand, the volcanic activity weakened until it stopped, and the source of volcanic ash rich in nutrient elements decreased. These two aspects led to lower biological productivity during this period. At the same time, the weaker water sealing could lead to a decrease in the reduction of the lower layer of the water, and the gradual cessation of volcanic activity no longer affected the climate, causing the destruction of sedimentary organic matter by oxidation.

show abstract

“…Volcanic activity is another reason for the difference in redox environment and biological productivity between the Wufeng Formation-the lower section of the Longmaxi Formation 1st member and the upper section of the Longmaxi Formation 1st member. Influenced by the collision between the Yangtze Plate and the Cathaysian Plate, during the deposition of the Wufeng Formation-the lower section of the Longmaxi Formation 1st member, volcanic activity was frequent and intense, leading to dramatic climate changes, improving the reducibility of the lower water [37,38,[76][77][78][79][80]. After the ash materials formed by volcanic eruption subsiding into surface water, minerals [68] and Zhang et al [30].…”

Section: Volcanic Activity Analysis and Its Effect On The Enrichment mentioning

confidence: 99%

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in Upper Yangtze Area-Taking Jiaoye-1 Well in Jiaoshiba Block as an Example

Liu¹,

Jiang²,

Zhang³

et al. 2019

Geological Society of America Abstracts With Programs

0

View full text Add to dashboard Cite

Organic matter is the material basis of shale hydrocarbon generation. The current organic matter content in shale is controlled by the original sedimentary organic matter abundance. Therefore, the study of the enrichment mechanism of sedimentary organic matter in shale has become an important issue to be solved. The Upper Yangtze area is the important exploration and exploitation area of marine shale gas in China. The shale of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Yangtze area is the research object. Choosing redox indicator and biological productivity indicator, the study explores the enrichment mechanism of sedimentary organic matter from two aspects, sealing of water and volcanic activity. The results show that excess siliceous mineral in the shale of the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area is bioorigin. Excess siliceous mineral can be used as one of the indicators of biological productivity. On the one hand, layer phenomenon occurred since the strong water sealing during the sedimentary period of Wufeng and the lower section of the Longmaxi Formation, which results in the high content of oxygen in surface water. On the other hand, the active volcanic activity brought volcanic ash which was beneficial to biological reproduction. Both of these factors led to higher biological productivity during this period. At the same time, the strong sealing of water made the lower layer of the water more reductive, and the active volcanic activity caused climate change, enhancing the reduction of the lower layer of the water, which made the rich organic matter deposited from the surface water well preserved. In the sedimentary period of the upper section of the Longmaxi Formation 1st member in the Upper Yangtze area, on the one hand, due to the weakened sealing of water, the oxygen content of the upper water decreased. On the other hand, the volcanic activity weakened until it stopped, and the source of volcanic ash rich in nutrient elements decreased. These two aspects led to lower biological productivity during this period. At the same time, the weaker water sealing could lead to a decrease in the reduction of the lower layer of the water, and the gradual cessation of volcanic activity no longer affected the climate, causing the destruction of sedimentary organic matter by oxidation.

show abstract

Geochemistry and sedimentology of the Lower Silurian Longmaxi mudstone in southwestern China: Implications for depositional controls on organic matter accumulation

Cited by 111 publications

References 114 publications

Relationship Between Electrical Properties and Sedimentary Environment of the Longmaxi Formation Shale, Southern Sichuan

Relationship Between Electrical Properties and Sedimentary Environment of the Longmaxi Formation Shale, Southern Sichuan

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in the Upper Yangtze Area: Taking Jiaoye-1 Well in the Jiaoshiba Block as an Example

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in Upper Yangtze Area-Taking Jiaoye-1 Well in Jiaoshiba Block as an Example

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