Research paperGeochemistry characteristics and significance of two petroleum systems near top overpressured surface in central Junggar Basin, NW China

- Two different petroleum systems near top overpressured surface in central Junggar Basin.
- The characteristics of the overpressure and hydrocarbon geochemistry in central Junggar Basin.
- Using geochemistry and basin modeling analysis reflect the reservoir formation process in Jurassic-Cretaceous overpressured formations.

Most hydrocarbons discovered in the central Junggar Basin are endowed in limited areas below and above top overpressured surface. By studying geochemical and distribution characteristics of hydrocarbons in reservoirs near top overpressured surface, it was found that the lacustrine dark mudstone in Middle-Lower Permian and coal measures in Jurassic are the major source rocks. Crude oils from these two sets of source rocks have apparently different geochemical indexes. Their maturity values are 0.7-1.0% (higher in Permian). The natural gas is mainly coal-derived wet gas from Jurassic formation, with general maturity of 0.9-1.0%. Hydrocarbons in tectonic units have different distribution features depending on sources. Crude oil above top overpressured surface in western Well Pen 1 Sag is from both Middle and Lower Permian, while crude oil in overpressure compartment is mainly from Middle Permian, and natural gas is from hybrid sources. Crude oil near top overpressured surface in western Changji Sag is mainly from Middle Permian, including some oil from hybrid sources; the natural gas is mainly coal-derived wet gas. Hydrocarbons in northern Dongdaohaizi Sag and eastern Changji Sag are mainly coal-derived oil and gas in overpressure compartment. Hydrocarbons in the central Junggar Basin underwent two stages of accumulation, and experienced two large-scale chargings at the later stage. To summarize geochemical and distribution characteristics of hydrocarbons in reservoirs near top overpressured surface in the central Junggar Basin can provide reference for understanding hydrocarbon accumulation regularity and predicting favorable exploration targets near top overpressured surface in this region and similar high overpressure and hydrocarbon generation basins.