| The Songliao Basin is a famous Cretaceous continental basin in China,which not only contains rich oil and gas resources,but also has unique geological advantages in studying the characteristics and evolution of the terrestrial greenhouse climate environment during the Cretaceous period.In order to obtain the continuous stratigraphic data of the Cretaceous period,the International Continental Scientific Drilling Project(ICDP)carried out the drilling project of the Cretaceous Continental Scientific Drilling borehole SK1(South and North boreholes),SK2,and SK3 in the Songliao Basin.The continuous cores from Denglouku Formation to Quantou Formation drilled from borehole SK3,which provides a geological basis for exploring the hydrocarbon generation evolution model of paleogreenhouse climate source rocks in the middle Cretaceous,clarifying the basin evolution mechanism during the fault depression transition period,and evaluating the potential of oil and gas resources in Songliao Basin and its surrounding areas.This paper conducts a study on the stratigraphic sequence and reservoirs of the Denglouku Formation and Quantou Formation based on the core data of the borehole SK3.It is of great significance in the division of the upper and lower Cretaceous boundary in the Songliao Basin and the prediction of favorable reservoirs of the Denglouku Formation Quantou Formation in the Dehui Fault Depression.This paper defines the lithological sequence of Denglouku-Quantou Formation through detailed core description of the complete coring data of borehole SK3,and classifies the sedimentary facies of Denglouku-Quantou Formation based on relevant identification indicators such as core lithology,color,sedimentary structure,inclusions,and logging curve characteristics.The results of lithological sequence and sedimentary facies division indicate that the Denglouku-Quantou Formation of borehole SK3 mainly develops 10 types of clastic rocks with different grain sizes,and four main sedimentary facies(braided river,meandering river,delta,and lake,including 9 sedimentary subfacies and 23 sedimentary microfacies).According to the lithological sequence and sedimentary facies division results of borehole SK3,a thin intercalation of sedimentary tuff is developed in the upper part of the second member of the Quantou Formation,and the U-Pb dating analysis of clastic zircons is conducted.The weighted average age of four young zircons is 96.8 ± 2.9 Ma,belonging to the Middle Cenomanian Period of the Late Cretaceous.Based on previous studies on age constraints and sedimentation rates of the Denglouku and Quantou Formations,it can be inferred that the potential Upper and Lower Cretaceous boundary of the borehole SK3 may be located in the middle and lower part of the first member of Quantou Formation.The calculation results considering the dating error indicate that the boundary may also be located in the middle of the second member of Quantou Formation to the upper part of the fourth member of Denglouku Formation.The continuous sedimentary records of the Denglouku-Quantou Formation,which are widely developed throughout the basin,indicate that there are also geological records of continuous sedimentation of this boundary in the same horizon in other areas of the Songliao Basin.The lithological sequence of borehole SK3 reveals that clastic rock reservoirs are developed in the Denglouku Formation and Quantou Formation,with the following main characteristics:(1)The reservoir lithology of Denglouku Formation is mainly medium to coarse grained sandstone,while the reservoir lithology of Quantou Formation is mainly fine sandstone.(2)The reservoir mainly develops three types of reservoir space: primary pores,secondary pores,and fractures,which can be further divided into six sub pore types.(3)The average porosity and permeability of the reservoir are 7.09% and 2.479 m D respectively,with a moderate overall correlation deviation.(4)According to the difference in capillary pressure curve shape and pore throat size of high-pressure mercury injection,the reservoir of Denglouku Formation-Quantou Formation can be divided into three types: I,II,and III.The reservoir capacity is reflected in: I>II>III.Among them,intergranular pores and secondary pores are well developed in Type A reservoirs,some intergranular pores are retained by cementation in Type Ⅱ reservoirs,and carbonate cements are prone to dissolution.Type Ⅲ reservoir cements block most of the pores,indicating that the type and intensity of diagenesis are the main factors affecting the pore throat size and physical properties of the Denglouku-Quantou Formation reservoir.(5)According to the constant velocity mercury injection capillary pressure curve,the pore throat configuration relationship of the Denglouku-Quantou Formation reservoir is divided into pore throat equalization type and throat dominant type.Cements and clastic particle corrosion pores are commonly developed in reservoirs with equal pore throat,and their physical properties are relatively better;Primary and secondary pores of throat dominated reservoirs are mostly blocked by cement,and throat channels dominated by small cracks are relatively developed with poor physical properties.This indicates that the order of dissolution and cementation is the main factor affecting the pore throat configuration.(6)The core CT scan results show that the Ⅰ-type pore throat equalization type coarse sandstone reservoir based on the classification of high-pressure mercury injection and constant velocity mercury injection curves in borehole SK3 has extremely developed pores and good permeability;Type Ⅱ-throat dominated gravelly coarse sandstone reservoirs have poor pore throat space development,porosity,and permeability.This indicates that the combination of high-pressure mercury injection and constant velocity mercury injection curve features can better characterize the pore throat space of the reservoir,thereby making a more accurate determination of the reservoir’s reservoir and permeability.By using the lithological sequence and sedimentary facies division of borehole SK3,as well as the observation results of cast thin sections,the control factors of the Denglouku Formation Quantou Formation reservoir are analyzed,and the following understanding can be obtained:(1)The reservoir properties of the Denglouku-Quantou Formation are positively correlated with quartz content,negatively correlated with rock debris and interstitial material content,and poorly correlated with feldspar content.(2)DengloukuQuantou Formation reservoirs mainly improve reservoir performance through dissolution;Mechanical compaction,pressure solution,and cementation will destroy the porosity and permeability of the reservoir.(3)The clastic rock reservoirs in the Denglouku-Quantou Formation are mainly braided and meandering river channel sand bodies,The braided river channel core beach microfacies in the upper part of the second member of Denglouku Formation to the lower part of the third member of Denglouku Formation and the meandering river channel bed retention sedimentation in the middle and lower part of the first member of Quantou Formation develop residual intergranular pores and secondary pores with good physical properties,which are favorable facies zones for reservoir development. |
