The characteristics of natural gas in Sichuan basin and its sources.

Sichuan basin is a large-scale superimposed basin, which is mainly a gas-bearing basin with subordinate oil. Multi-stage sedimentary processes developed many sets of different types of hydrocarbon source rocks in different areas. At present, five hydrocarbon source rocks, the Lower Cambrian, the Lower Silurian, the Lower Permian, the Upper Permian and the Upper Triassic, have entered into the high-matured phase, mainly generating gas. The complexity of multi-phase hydrocarbon generation and accumulation of mixed sources, and subsequent secondary alterations make the gas drier, carbon isotopes more complicated, and gas to source correlation more difficult. Based on an analysis of sedimentary evolution, we have analyzed the gas reservoir in different districts by probing into the developmental characteristics and distribution pattern of active hydrocarbon source rocks. We have primarily determined the main source rocks for different districts by the synthetic research of natural gas, non-hydrocarbon components, carbon isotopes, etc. The Carboniferous, Permian and Triassic natural gas in the eastern gas district of Sichuan basin are originated from the Silurian, the Upper Permian Longtan Formation and the Lower Permian hydrocarbon source rocks, respectively. The Sinian Dengying Formation, Permian and Triassic gas in the northern gas district are derived from the Lower Cambrian, the Upper Permian and Lower Permian hydrocarbon source rocks, respectively. In the west gas district, the Jurassic and Triassic Xujiahe Formation gas are from the coal hydrocarbon source rocks of Xujiahe Formation; the Lower Permian and Lower Triassic Jialingjiang Formation gas are generated from Permian source rocks. In the central oil district of Sichuan basin, the oil of Lower Jurassic Ziliujing Formation is generated from Jurassic source rocks; the shallow gas comes from continental source rocks of the Upper Triassic Xujiahe Formation; and the gas in deep reservoirs comes from Cambrian source rocks. Because the northeast Sichuan basin developed multilayered, good-quality hydrocarbon source rocks, it is the most favorable enrichment area for hydrocarbons, and for future prospecting.