Accumulation Characteristics Of Chang 9 And Chang 10 Members In Longdong Area,Ordos Basin

Longdong area is located in the southwestern part of the Ordos Basin.In recent years,the Chang 9 and Chang 10 Members of Yanchang Formation have made new discoveries and breakthroughs in exploration,and have become important replacement strata for petroleum exploration.The core,logging,geochemical analysis and uniform temperature measurement of fluid inclusions are used to systematically carry out research on the accumulation characteristics of Chang 9 and Chang 10 Members in Longdong area,to identify the source of the oil,and analyze the conditions and processes of accumulation,determine the control factors of reservoir distribution,and establish a reservoir formation model to provide theoretical guidance for further guidance of petroleum exploration and deployment.It is indicated in the study that the physical properties of the crude oil of the Chang9 and Chang 10 reservoirs in the Longdong area are basically the same,and they are normal.There are two genetic types of crude oil.Type I is distributed throughout the study area and originates from the Chang 7 source rocks,type II is only distributed in Huachi area.and originates from Chang 9 source rocks.The Chang 9 source rocks in Longdong area are a set of effective source rocks.The abundance of organic matter is general.Types of organic matter are mainly II2 and III.They are in the mature-high mature evolution stage,mainly distributed in the east.The northeastern part of the district is the thickest,reaching 20 m,gradually thinning to the southwest,and it is less than 5m to the line of Huanxian-Zhenyuan-Changwu areas.The reservoirs are developed in dominant sand bodies such as underwater diverted channel sand bodies.The reservoirs have poor physical properties.It is indicated the reservoir are lithological reservoirs with low porosity and low permeability.At the end of Cretaceous,the distribution of sourcereservoir residual pressure can be divided into three categories.Type A: the highest value occurs at the bottom of Chang 7 Member,decreases down to Chang 10 Member,and the difference of Chang 7 and Chang 9 Members residual pressure is large.Type B: the highest value occurs in the middle or the lower part of Chang 7 Member,decreases to Chang 8 Member,and appears relatively high again in Chang 9 Member,the difference of Chang 7 and Chang 9 Members residual pressure is small.It occurs in Huachi area.Type C: the residual pressure of each Member is low and the difference between Members is small.It is distributed in the west of the study area where the source rocks of Chang 7are weakly developed.Multiple sets of sand bodies in Longdong area are stacked vertically,and high-angle or upright fractures are generally developed,providing channels for vertical migration of oil.The accumulation time of the Chang 9 and Chang10 reservoirs are the late Early Cretaceous and the Paleogene and Neogene since the tectonic uplift in the late Cretaceous.Reservoirs are controlled by source rocks,sedimentary facies and sand bodies distribution,residual pressure difference and fracture development.Two kinds of reservoir forming models are established,one is driven by residual pressure difference between source rocks and reservoirs(upper generation and lower storage): at the end of Early Cretaceous,the crude oil mainly came from the source rock of Chang 7.Driven by the residual pressure difference of source rocks and reservoirs,it flowed into Chang 9 and Chang 10 reservoirs through sand bodies superposition and microfracture,forming lithologic reservoirs.This model has been found in the whole study area.The other is forming by fracture adjustment(self generation and self storage):in the Paleogene and Neogene since the late Cretaceous structural uplift,source rocks developed in Chang 9 Member,and the oil was transported along the fractures.The lithologic reservoirs were formed in the favorable sand bodies of Chang 9 and Chang 10 reservoirs within the range of source rocks.And now it is mainly distributed in Huachi area where Chang 9 source rocks are developed.Based on the understanding of the main controlling factors of oil accumulation and the accumulation models,combined with the actual exploration situation,six favorable oil accumulation areas are predicted.