| The oil and gas reservoir in Ordos basin have the typical characteristics of low permeability,low pressure,low abundance,and the anomaly of formation pressure widely developed.Evolution of formation pressure plays an important role in oil and gas reservoir,and has a very close relationship of accumulation and preservation of oil and gas.With application of petroleum geology,sedimentary,pool forming dynamics,physics and chemistry theory,based on quantity drilling data,analysis of test and logging,focused on oil reservoir of the Yanchang Formation in Ordos Basin,this paper have studied the distribution characteristics of abnormal low pressure,restore ancient pressure of reservoir,quantitative analysis of controlling factors of abnormal low pressure,formation pressure evolution and the accumulation effect and other aspects of the Yanchang Formation and Yan'an Formation of Mesozoic in Ordos Basin.Coupled with the evolution characteristics of the reconstruction of Yanchang Formation pressure and abnormal formation pressure characterization,will help further understanding the dynamics process of hydrocarbon generation,migration,accumulation of oil and gas,to reveal the mechanism of the Mesozoic strata in low pressure formation and accumulation mechanism.The main achievements are as follows.(1)The fluid pressure is obviously lower than the static pressure in reservoir of the Mesozoic in Ordos Basin.904 measured data of formation pressure of Mesozoic in Ordos Basin show that formation pressure range of 6 ~ 22 MPa,pressure coefficients are mainly from 0.60 to 0.90,with an average of 0.76.44.6% of the the pressure coefficients are less than 0.75,38.9% are from 0.75 to 0.90.The average formation pressure coefficients of Mesozoic in Ordos Basin are mainly ultra-low-pressure and abnormal low pressure,normal pressure and overpressure is rare,and its distribution is changes little in the vertical,are mainly from 0.70 to 0.77,The average formation pressure coefficients of Jurassic is slightly higher,the average is 0.84.From bottom to up of Yanchang formation,the pressure coefficient is similar.The average formation pressure coefficients of Chang 8,Chang 6 and Chang 4+5 are 0.75,0.73 and 0.74.(2)The formation pressure and pressure coefficients distribution pattern is high in the west and lower in the east of the Yanchang Formation and Yan'an Formation of Mesozoic in Ordos Basin.Shows the characteristics of the abnormal low pressure,ultra low phase distribution.The pressure coefficients of Jiyuan,northern Shaanxi,Huaqing and eastern Gansu are mainly in the 0.55 ~ 1.13,0.46 ~ 1.17,0.52 ~ 0.99 and 0.44 ~ 1.16,and the average pressure coefficients of Jiyuan,northern Shaanxi,Huaqing and eastern Gansu are 0.86,0.76,0.76 and 0.71.The formation pressure and pressure coefficients of eastern Gansu,Jiyuan and Huaqing are higher than northern Shaanxi.The formation pressure and pressure coefficients decreased from bottom to up in the vertical,Jiyuan and eastern Gansu are obvious.The formation pressure of Jurassic is mainly ultra-low-pressure and abnormal low pressure,the distribution pattern is high in the west and lower in the east in the overall background of abnormal low pressure,and inheritance the distribution pattern of of Yanchang Formation.(3)Based on the study of low pressure distribution characteristics of the Mesozoic reservoir in Ordos Basin,the formation mechanism of abnormal low pressure are quantitative analyzed from several aspects,such as tectonic uplift,stratum temperature decreased and dissolution pore increasing effect.The reservoir pressure of the Mesozoic experienced two major evolutionary stages: the pressure rising and achieved maximum pressure stage from the Late Triassic to the Early Cretaceous,the pressure decreasing and achieved abnormal low pressure stage from the end of Early Cretaceous to now.Since the late Early Cretaceous,pore rebound caused by tectonic uplift cause pressure decreases of up to 2.32~7.45 MPa of Chang 6 and Chang 8,reduce the ratio of 6.91%~26.65%.The pressure reduces amplitude is maximum in northern Shaanxi due to the maximum erosion thickness,reduce the ratio of Chang 6 is about 26.65% and Chang 8 is about 22.83%.The pressure reduces amplitude is minimum of Chang 6 and Chang 8 in Jiyuan,reduce the ratio of 7%,and reduce the ratio of Chang 6 and Chang 8 in Huaqing is 15%,and eastern Gansu is 10%.Tectonic uplift cause pressure decreases is the second factors that influence the pressure reduction.Stratum temperature decreased cause pressure decreases of up to 7.78~12.86 MPa of the Mesozoic reservoir in Ordos Basin,reduce the ratio of 23.18%~45.99%.The pressure reduces amplitude is maximum in northern Shaanxi due to the maximum temperature decreased,reduce the ratio of Chang 6 is 45.99%.and the reduce the ratio of Chang 6 and Chang 8 are about 34.44% and 32.42% in eastern Gansu.The pressure reduces amplitude is minimum in Jiyuan due to the minimum tectonic uplift.Stratum temperature decreas cause pressure decreases is the most important factor that influence the pressure reduction.Pressure decreases caused by erosion is 0.75~1.28 MPa,reduce the ratio of 2.23% ~ 4.90%,dissolution pore increasing effect has only little factors that influence the pressure reduction.Pore rebound caused by tectonic uplift and stratum temperature decreased common cause pressure decreases of up to 10.10~20.31 MPa,reduce the ratio of 30.10%~72.64%.Pore rebound caused by tectonic uplift and stratum temperature decreased are the main factors that influence the pressure reduction.The pressure reduces amplitude is maximum in northern Shaanxi,the pressure reduces of Chang 6 and Chang 8 are 20.31 MPa and 18.00 MPa,reduce the ratio of 72.64% and 55.16%.The pressure reduces of Chang 6 and Chang 8 is 11.90~13.68 MPa,reduce the ratio of is 39.27%~45.54%.The pressure reduces amplitude is minimum in Jiyuan,reduce the ratio is about 30%~35%.Add other effects make the formation pressure is reduced gradually,eventually formed the abnormal low pressure distribution pattern which the formation pressure distribution on 10~18 MPa.(4)Through the ancient pressure recovery of maximum buried depth period of the Mesozoic Yanchang Formation in Ordos Basin,this paper analyzes the evolution of formation pressure from abnormal high pressure to abnormal low pressure,based on the study of controlling factors of abnormal low pressure,such as tectonic uplift,temperature decrease,etc.The oil and gas generally migrates to low residual pressure area along the path of least resistance during the process of accumulation,nowdays,the oil pool that has been found is mainly distributed in the accumulation stage and areas that pressure coefficient is relatively low.According to the quantitative analysis of the pressure evolution of the Chang 6 formation,we find the average formation pressure of Jiyuan?northern Shaanxi?Huaqing?and eastern Gansu area decreases 16.01 MPa?18.10 MPa?15.62 MPa?and 14.13 MPa respectively from maximum buried depth period to present,the pressure of northern Shaanxi area decreases highest,and always declines higher than that of the rest three areas,which formed a certain pressure difference between northern Shaanxi area and other areas,leading to the second migration of oil.In conclusion,the evolution and formation of abnormal low pressure played an important role in large-scale accumulation of hundred million tons oil of Chang 6 formation in northern Shaanxi area.Developmental mechanism and evolution process of the low pressure control of formation fluid to a great extent the flow,low pressure pump reservoir is the mainly accumulation mechanism of Chang 6 in northern Shaanxi.Evolution of abnormal low pressure formed a million tons scale reservoir which related with abnormal low pressure of Chang 6 in northern Shaanxi.(5)On the basis of thermal evolution of hydrocarbon source rocks,hydrocarbon generation and expulsion,accumulation period and pressure evolution stage,put forward the reservoir has experienced 2 stages of pressure evolution of accumulation.The first stage is the hydrocarbon generation pressurization of abnormal high pressure driven accumulation phases: continuous subsidence basin,of large number hydrocarbon generation,pressure continued to increase of source rock,crack closure in abnormal high pressure breakthrough,continuous accumulation under the action of abnormal pressure,as the representative to reservoir of Jiyuan,Huaqing and eastern Gansu.The second stage is low pressure pump reservoir stage that the pressure difference increase between source and reservoir: basin uplift stage,hydrocarbon source rock to stop the hydrocarbon generation,because of the unbalanced pressure reduction between source and reservoir,the pressure which between the source and the reservoir further increased,low pressure in sandstone formation of a huge suction power to the hydrocarbon source rocks which have relative higher pressure.So the residual hydrocarbon source rocks in the generated hydrocarbon expulsion in the pump suction effect,Jurassic reservoir. |
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