Study On The Reservoir Formation Rule Of Volcanic Rock Of Carboniferous In The East Of Junggar Basin

Hydrocarbon exploration of volcanic rocks in eastern Junggar basin has gained rapid advance in recent years.Along with the tempo of exploration accelerating continuously,the problems existed in the exploration of volcanic reservoirs are prominent constantly.This reaserch project is set up in this background,aiming to promote the hydrocarbon exploration of the volcanic rocks in eastern Junggar basin.The reaserch shows that the basement of Junggar basin is the continental basement with double structure,which is the Precambrian crystalline basement and Palaeozoic folded basement. Carboniferous volcanic rocks are developed and formed on the Palaeozoic folded basement. The whole Carboniferous has frequent tectonic activities and the developed faults. Studies on temperature-pressure field nowadays show that the south area of eastern Junggar basin is low temperature and high pressure,the midlle area is normal temperature and pressure,the north area is high temperature and negative pressure. Carboniferous exists more than two temperature-pressure system at least.The research shows that oil-source is one of the main controlling factors of Carboniferous volcanic reservoirs in the area. Oil-source correlation suggests that Carboniferous has the oil-gas source both of Permain Pingdiquan Formation and Carboniferous itself as well. In Wucaiwan area, hydrocarbon generates from the hydrocarbon source rock of Dishuiquan Formation which has the best hydrocarbon generation potentail. In Jimusaer sag, hydrocarbon generates from the hydrocarbon source rock of Bashan Formation which has the preferable hydrocarbon generation potentail. In the whole Beisantai area, the top Carboniferous is dominated by the oil-source of Permain Pingdiquan Formation.The hydrocarbon source rock of Carboniferous matures early which matures and discharges hydrocarbon in late Indo-Chinese epoch, and mainly discharges gas in Yanshan epoch. The hydrocarbon source rock of Permain Pingdiquan Formation matures and discharges plenty hydrocarbon in Yanshan epoch,so Yanshan epoch is the main formation period of Carboniferous hydrocarbon reservoir .The transport system is the nessecery condition of the volcanic reservoirs formation. Well developed faults of Carboniferous and unconformity surfaces formed by long-term weathering denudation are the main transport system of the volcanic reservoirs.The features of volcanic rocks that quick lateral variation and intense heterogeneity determine lithologic-lithofacies and reservoir physical property is another important factor of volcanic reservoirs formation.In the area, andesite and volcanic breccia are the main rock type.The burst facies primarily made up by volcanic breccia is the most favorable facies belt.The secondary porosity is the main reservoir type of volcanic rocks.The top Carboniferous is suffered from long-term weathering denudation and eluviation by surface water.,so the secondary porosity is well developed in the top Carboniferous,especially burst facies is more easy to eluviation,which forms lithology and lithofacies belt of developed secondary porosity and favorable physical property.The discovery of seepage mud and geopetal structure of volcanic rocks in the south area of Beisantai and the exploration result of volcanic reservoirs in Bei-32 well area well explain this opinion and understanding.Trap is the primary condition of reservior formation,and structural trap is one trap type which has the highest exploration success rate at present. Whereas, the drilling wells that are deployed in view of stratigraphic trap and seismic anomaly bodies are all frustrated. The analysis suggest that seismic anomaly bodies are merely geologic body identified in seismic.The detailed analysis should be carried out to know whether the seismic anomaly bodies is able to form the trap. Without effective means to understand its true appearance, it can not be seen as the important exploration target to drill temporarily.Carboniferous can be divided and identified as reservoir formation dynamic system of allo-source(self-source partly) and normal pressure (high pressure partly) semi-closed system in the top, as well as self-source and normal pressure closed system in the inside story of Carboniferous.Carboniferous exists one reservior-seal assemblage that has top crust of weathering as reservoir and Permain Pingdiquan Formation as cover,and another reservior-seal assemblage that has lower Carboniferous as reservoir and stable mudstone upside the reservoir as the cover. Over 90% reserviors presently discovered are all located in the upper reservior-seal assemblage.Comprehensive researches and analysis suggest that Wucaiwan sag, round Beiantai arch,round Jimusaer sag are the most favorable exploration field ,with bright exploration prospect and great potential,and the exploration should be increased in these areas.