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

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 bio-origin. Excess siliceous mineral can be used as one of the indicators of biological productivity. In the sedimentary period of Wufeng Formation-the lower section of Longmaxi Formation, on one hand, due to the strong sealing of water, layer phenomenon occurred, making the high oxygen content of 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 Longmaxi Formation 1^st member in the Upper Yangtze area, on one hand, due to the weakened sealing of the 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 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.