MECHANISM ANALYSIS OF VOLCANIC ACTIVITY ON ORGANIC MATTER ENRICHMENT IN SHALE: TAKING JSBY1 WELL IN UPPER YANGTZE AREA OF SOUTH CHINA AS AN EXAMPLE

Organic matter is not only the material basis of shale hydrocarbon generation but also the main reservoir space and seepage channel of shale gas. The present total organic carbon (TOC) content of shale is controlled by the abundance of original sedimentary organic matter, so it is an important problem to study the geological factors and the mechanism of organic matter enrichment. The upper Yangtze area is the main area of marine shale gas exploration and development in southern China. The shale of Upper Ordovician Wufeng Formation - Lower Silurian Longmaxi formation in well JSBY1 is taken as the research object. The thickness and frequency of porphyry development in shale are used to indicate the intensity of volcanic activity, and oxidation-reduction index (U / Th) and biological productivity index (biological productivity index Cu, biological productivity index Mo, biological productivity index Ni) are selected to study the influence of volcanic activity on the enrichment of organic matter in shale. The results show that in the sedimentary period of the lower part of Wufeng Formation and Longmaxi formation, the more active volcanic activity brings volcanic ash which is beneficial to biological reproduction, resulting in higher biological productivity. The volcanic material is from neutral or acid magmatic rock in the deep crust. At the same time, the more active volcanic activity causes climate change, enhances the reducibility of the lower layer of the water body, which makes rich organic matter settled on the surface of the water body well preserved, so that the TOC content in the lower part of the first member of the Wufeng Formation Longmaxi formation is higher and the gas-bearing property is better. During the sedimentary period of the upper part of the first member of Longmaxi formation, volcanic activity weakened or stopped, and the source of volcanic ash rich in nutrient elements decreased, which led to low biological productivity. At the same time, gradually stopped volcanic activity no longer had an impact on the climate, which caused the organic matter in the sediments to be destroyed by oxygen, making the TOC content in the upper part of the first member of the Longmaxi formation low and the gas-bearing property poor.