Geological Conditions And Distribution Prediction Of Jurassic Tight Oil In Jiulongshan Area,North Qaidam Basin

Jurassic tight oil and gas reservoirs are widely developed in the northern margin of Qaidam Basin,which is a new field of oil and gas exploration in Qaidam Basin.However,due to the unclear geological conditions for the formation of tight oil in this area,the exploration of tight oil in the northern margin of Qaidam Basin is still in the early stage.In this study,we take the Jiulongshan area in the northern margin of Qaidam Basin as the research area,and the Dameigou Formation of Middle Jurassic as the key research interval.On the basis of collecting research data of predecessors,core observation,sample collection and experimental testing of the Middle Jurassic Dameigou Formation in the Jiulongshan area were carried out.The sedimentary environment,distribution and organic geochemical characteristics of hydrocarbon source rocks and pore structure and physical properties of clastic reservoirs were analyzed.The upper and lower physical limits,formation time,oil and gas filling mechanism and formation model of the tight reservoir were further studied.By using two-dimensional seismic data,this study predicted the distribution range of the Jurassic tight oil“sweet spot”and provided direction for the next oil and gas exploration in Jiulongshan area.The Middle Jurassic is composed of braided,braided delta and lacustrine deposits from bottom to top in the Jiulongshan area.The seventh member of Dameigou Formation has reached the maximum flooded surface.Middle Jurassic source rocks are mainly developed in the sixth and seventh members of Dameigou Formation and have high organic carbon value,II₁-III organic types and a maturity range from low maturity stage to mature stage.Porosities and permeabilities of upper and lower physical limits of the middle Jurassic tight reservoir are 10%and 1 mD and 3.65%and 0.05 mD,respectively.The middle Jurassic reservoir has been dense during the late Eocene and the oil and gas filled.The moving power for the oil and gas in the reservoir is the remaining source reservoir pressure,the vertical accumulation is the main source,forming lower source layered group.The distribution of tight oil in Jiulongshan area on the northern margin of Qaidam Basin is controlled by sedimentary facies,characteristics of source rocks,source-reservoir matching relationship and dynamic system of oil-gas accumulation.The main reason for the non-scale accumulation of tight reservoirs in the Jiulongshan area is that the thermal maturity of source rocks is low maturity and maturity,and the residual pressure of hydrocarbon expulsion from source rocks is small,which makes the power of oil-gas migration smaller,and limits the distribution of tight oil.After the analysis of reservoir forming conditions,we think that the area of CY1-L6-L2-GZ20 in the western Jiulongshan area covering an area of34 km²,can be served as current exploration targets.