The Sinian-Lower Paleozoic (also called the lower association) in Sichuan (四川) basin has undergone geologic evolution for several hundred million years. The subsidence history of the Sinian-Lower Paleozoic can be divided into four stages: the stable subsidence during Cambrian and Silurian; the uplift and denudation during Devonian and Carboniferous; the subsidence (main process) during Permian to Late Cretaceous; and the rapid uplift and denudation since Late Cretaceous. The later two stages could be regarded as critical factors for the development of oil and gas in the lower as- sociation. The evolution of energy field such as temperature, pressure, and hydrocarbon phase in the lower association during the deep burial and uplift in the third stage might be induced as follows: (1) super-high pressure was developed during oil-cracking, previous super-high pressure was sustained, or changed as normal pressure during late uplift; (2) temperature increased with deep burial during per- sistent subsidence and decreased during uplift in late stage; (3) as a response to the change of the en- ergy field, hydrocarbon phase experienced a series of changes such as organic material (solid), oil (liq- uid), oil-cracking gas (gaseous) + bitumen (solid) + abnormal high pressure, gas cap gas with super-high pressure (gaseous) + bitumen (solid) + water soluble gas (liquid), and gas in pool (gaseous) + water soluble gas (liquid) + bitumen (solid). The restoration of hydrocarbon phase evolution is of important value for the exploration of natural gas in the Sinian-Lower Paleozoic in Sichuan basin.
