Transformation of Oil Pools into Gas Pools as Results of Multiple Tectonic Events in Upper Sinian (Upper Neoproterozoic), Deep Part of Sichuan Basin, China

A center in the present paper is referred to as an area or region which may include one or more hydrocarbon accumulations. A hydrocarbon generation center is referred to as an area containing high quality source rock which was subjected to thermal maturation. A gas generation center is an area in which an oil pool or accumulation was present, and oil was thermally cracked to generate gas. A gas accumulation center is referred to as an area in which natural gas generated from cracked oil accumulated. A gas preservation center is referred to as an area or region where the present natural gas pool/pools is/are located. As one of the oldest petroleum reservoir rocks in the world, the upper Sinian Dengying Formation (Upper Proterozoic) in the Sichuan basin was deeply buried, and its paleo-oil pools (gas generation centers) underwent complex transformation into paleo-gas pools (gas accumulation centers) and the present gas pools (gas preservation centers) as a result of multiphase tectonic activities. The paleo oil pools (gas generation centers) were the main hydrocarbon sources of the paleo gas pools (gas accumulation centers), which were in turn the main sources of hydrocarbons for today's (remaining) gas pools (gas preservation centers). The key factor in the oil accumulation was the presence of rich hydrocarbon source rocks (hydrocarbon generation centers) in the Early Cambrian strata and a good seal development. Being controlled by the early tectonics and sedimentary development of the basin, the hydrocarbon generation centers appeared to have been stationary in space, while in time the other three centers (gas generation centers, gas accumulation centers and gas preservation centers) migrated as result of tectonic events in the basin. Therefore, the time-spatial relationships between these “three centers” (gas generation centers, gas accumulation centers and gas preservation centers) decides the final distribution of natural gas in the Sichuan basin. Relationship between generation, accumulation and preservation of hydrocarbons in the marine carbonates buried deeper than 4500 m in the Sichuan basin, can be separated into: (1) an accumulation mode with the “three centers” being superimposed; (2) an accumulation mode with “the preservation center” disintegrated; (3) an accumulation mode with the “three centers” migrated for a short distance; (4) a destruction mode with the preservation center lost. The natural gas exploration of the upper Sinian carbonate rocks in the Sichuan basin can be most successful where the “three centers” overlap, such as at the front area of the Micang Mountains, which could be the most promising area for the future gas exploration.