Geochemical characteristics, origin, and mechanism of differential accumulation of natural gas in the carboniferous kelameili gas field in Junggar basin, China

Abstract Since the discovery of the Kelameili gas field, the exploration of volcanic gas reservoirs in the Junggar Basin has attracted considerable attention. However, owing to the limitations involved in studying the characteristics of the origin and distribution of the volcanic gas reservoirs in this area, the study of gas reservoirs in the South fault terrace area of the Kelameili gas field as a typical peripheral area is greatly restricted, and the exploration process is relatively slow. Therefore, it is necessary to analyse and compare the differences in the geochemical characteristics of the natural gas between the Kelameili gas field and the fault terrace area of the south belt by analysing the natural gas geochemistry, hydrocarbon generation evolution, and structural evolution. In addition, geochemical experiments involving the natural gas components, carbon isotopes, light hydrocarbons, hydrocarbon generation, and evolutionary history of the source rocks and the interpretation of seismic profiles reflecting the structural evolution, help clarifying the genetic type of the volcanic gas reservoirs and the source of the natural gas in this area. The results indicate the following. (1) The natural gas in the study area has undergone thermal genesis and is generated by kerogen cracking. It is a typical humic gas with high maturity. (2) The origin of the natural gas is the Carboniferous source rock in the depression, which is significantly different from the natural gas from the typical Permian and Jurassic. (3) The primary reason for the difference in natural gas accumulation is that the Carboniferous source rock entered the high-maturity stage in the Early Jurassic. The formation time of the trap is earlier than that of the high-maturity stage of the source rock evolution, which can capture multi-stage source rock products. (4) Based on the hydrocarbon source conditions that the hydrocarbon supply horizon of the Kelameili gas field has a high maturity of organic matter, and insufficient hydrocarbon supply, it is considered that traps capturing the multi-stage source rock products represent a larger scale of natural gas accumulation. The results of this study can deepen the understanding of volcanic gas reservoirs in the Junggar Basin and guidethe exploration and development of global volcanic gas reservoirs and the surrounding large-scale gas fields.