Division of shale sequences and prediction of the favorable shale gas intervals: an example of the Lower Cambrian of Yangtze Region in Xiuwu Basin

Abstract It is a common method to use sequence stratigraphic theory to identify favourable intervals in hydrocarbon exploration. The Lower Cambrian shale of Well Jiangye-1 in Yangtze Region in Xiuwu Basin was chosen as the research object. The content of excess silicon of siliceous minerals in shale was calculated quantitatively, and the concentration distribution of Al, Fe, Mn showed that the excess silicon is of hydrothermally origin and the shale deposited in an environment with hydrothermal activity. Using U/Th values in the study, combined with lithology and logging data, in order to divide sequences of the Lower Cambrian shale in Yangtze Region in Xiuwu Basin. The result shows that the shale of the Lower Cambrian shale is recognized as 1 2nd sequence (TST-RST, TST = Transgressive systems tract; RST = Regressive systems tract) and then further subdivided into 5 3rd sequences (SQ1-SQ5). During the deposition of SQ2 and SQ3, hydrothermal activity was active, and their excess silicon content was generally above 20%-30%. Rising sea level and active hydrothermal activity were beneficial for the enrichment of siliceous minerals and organic matter. Based on the comparison of the reservoir parameters, it tells that SQ2 and SQ3 have relatively higher content of TOC, higher content of brittle minerals (such as siliceous minerals, carbonate minerals and so on), larger effective porosity and higher content of gas, which make it as the most favourable intervals of the Lower Cambrian in Xiuwu Basin.

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