Ultra-deep petroleum accumulation systems in Fergana Basin

In the central graben zone of the Fergana Basin, overpressures were widespread during Cenozoic, which might be related to tectonic compression environment, disequilibrium compaction caused by quick sedimentation and continuous hydrocarbon generation. The ultra-high pressure environment had important effects on deep oil and gas generation, migration and accumulation. The main reservoirs were buried over 5 000 m deep in Neogene, and mainly gave birth to liquid hydrocarbons. The high-porosity and high-permeability sandstone reservoirs in deep formations came from fluvial sandstones under ultra-high pressure, and were featured by low coring recovery rate and high sand production. The low-porosity and high-permeability reservoirs were related to hydraulic fractures caused by high pressure. In the carbonate reservoirs in Paleogene, H₂S was in high content, mainly generating from thermochemical sulfate reduction (TSR), which obviously improved the physical property of gypsum carbonate reservoirs. The formation pressure coefficient was slightly lower than that of Neogene, which could be explained by heavy hydrocarbon gas loss and reservoir space increase. The main petroleum systems in the central graben zone can be divided into the upper E-E₃N system and the lower JK-E_1-2 system. High-production reservoirs of the upper E-E₃N petroleum system are controlled by channel sandstones and overpressure, while those of the lower JK-E_1-2 petroleum system are mainly affected by the gypsum-containing carbonate reservoirs of lagoon facies. The two systems are totally different in pressure and fluid properties, hence are suitable for slicing production.