Relationship Between Overpressure And Hydrocarbon Accumulation Of Upper Member Of Lower Ganchaigou Formation In The Western Qaidam Basin

The upper member of Lower Ganchaigou formation is the most significant source rock in the western Qaidam Basin,which has made an important contribution to the Plaeogene-Neogene petroleum system.Overpressure is widely distributed in the upper member of Lower Ganchaigou formation in the western Qaidam Basin.Hence,accurate analysis of its mechanism and effect on hydrocarbon accumulation is critical to future oil and gas exploration and development.Based on the observed data,this thesis calculated the current pressure coefficient by equivalent depth method,analyzed the mechanism of overpressure by comprehensive study of logging curve characteristics,and restored the evolution of formation pressure by basin simulation,thus clarified the distribution characteristic,genesis,evolution of overpressure and its impact on hydrocarbon accumulation.The results show that:（1）Overpressure has developed in study area since2300m,in which the pressure coefficient in upper member of Lower Ganchaigou formation is mainly 1.2～1.7,with higher pressure coefficient in some area,such as Huangguamao and Yingzhong,is more than 1.9;（2）The pressure coefficient in the western Qadiam Basin gradually increases from the edge of basin to the center,and evolves form atmospheric pressure system to overpressure system and strong overpressure system;（3）The mechanism of overpressure in western Qadaim Basin is complex,includes fluid expansion,tectonic compression and diagenesis;（4）The different structural zone present various overpressure mechanism,for example,the overpressure of Huangguanmao structure is dominated by fluid expansion and disequilibrium,while that of Yingzhong area is dominated by multi-factor of tectonic compression and diagenesis;（5）According to the characteristics and mechanism of overpressure,overpressure evolution can be divided into 3 stages,namely,early slow increase stage influenced by disequilibrium（E₃²-N₁）,comprehensive rapid increase stage dominated by multiple factors（N₂¹-N₂³）and late adjustment stage controlled by hydrocarbon generation（N₂³-Present）;（6）The overpressure difference between upper member of Lower Ganchaigou formation and upper Ganchaigou formation in study area ranges from5MPa to 15MPa,which controls the expulsion and migration of oil and gad and is favorable for the formation of reservoirs,while the fluid potential is controlled by the pressure field,whose transition zone from high value to low value also indicates the favorable area for hydrocarbon accumulation;（7）Under the background of overpressure,the formation model of oil and gas reservoir can be separated into 2 types:medium-deep overpressure reservoirs,shallow atmospheric pressure reservoir,and the reservoirs in western Qaidam basin are all distributed radially along the overpressure center.