Analysis Of Hydrocarbon Accumulation Conditions In Zhiluo Formation Of Middle Jurassic In Yanchang Exploration Area Of Ordos Basin

It is well known that the Ordos basin is rich in oil and gas resources,the Yanchang Exploration Area is located in the hinterland of the basin.With the deepening of petroleum exploration and development,gradually,the remaining exploitable oil and gas resources in the exploration area are exhausted and the longitudinal reserve formation is dificient.Therefore,it is urgent to find new exploration system to prolong the production of oil field.However,in recent years,the Zhiluo Formation of middle Jurassic has good oil and gas display in the exploration process of the research,some of the wells have obtained high-yield oil,which indicates that the Zhiluo Formation has certain exploration potential and is the most realistic replacement exploration Formation for longitudinal expansion at this stage.However,due to the low degree of exploration and research in Zhiluo Formation,the conditions of hydrocarbon accumulation and the regulation of oil and gas enrichment are not clear,which restricts the formulation and deployment of the next exploration target.The study is guided by petroleum geology,sedimentology,geochemistry and related disciplines,based on previous studies,data collection,testing and analysis,to conduct a study for basic elements of hydrocarbon accumulation in Zhiluo Formation in the study area,including source rocks,reservoirs,agglomerations and preservation conditions.Analysis of hydrocarbon accumulation pattern and main control factors,revealing the regulation of oil and gas enrichment,finally combined with the existing degree of exploration to predict favorable areas.The study mainly obtains the following conclusion :(1)By comparing the characteristics of crude oil and the parameters of oil source geochemistry,it is determined that the crude oil of Zhiluo Formation comes from the Chang7 source rock.the Chang7 source rock is widely distributed in the study area and with high organic matter abundance and good type,which is the excellent source rock.(2)Zhiluo Formation is based on river facies deposition,thick-layer sandstone developed and widely distributed,which is beneficial to reservoir development.The physical characteristics of the reservoir are medium-low porosity,low-medium permeability.The types of reservoir clastic rocks are mainly medium-fine-grained feldspar sandstone,and the reservoir space types are mainly intergranular pores and feldspar-soluble pores.(3)The direct cover of mudstone of Zhiluo Formation and the regional cover of mudstone of Anding Formation are the two main cover layers in the study area,which play an effective sealing role on the oil and gas in the lower part of the study area.(4)Excess pressure and buoyancy are the main driving forces of oil and gas migration;high angle fractures,connected sand bodies and non-integrated surfaces are the main channels of reservoir migration in Zhiluo Formation,especially the deep valley is of great significance for communicating oil and gas in upper and lower strata.(5)The type of reservoir fluid inclusions are rich in the study area,the uniform temperature combines with burial history-heat history of inclusions shows that the study area has experienced thermal events,the reservoir of Zhiluo Formation has experienced at least two stages of hydrocarbon fluid activity.(6)There are two main reservoir types in the Zhiluo Formation:lithologic reservoir and structural-lithologic reservoir,which are characterized in general by a reservoir-forming mode of lower-growth upper storage,far-source oil supply,ladder dredging,multi-stage filling and multi-factor control.High quality source rock,favorable sedimentary facies zone,local nasal uplift structure,sand mudstone thickness and good reservoir cover combination are important controlling factors for oil and gas reservoir Formation.(7)To determine the prediction criteria for favorable areas in the Zhiluo Formation in the study area:1)Sand body thickness:the upper member of the Zhiluo Formation>10m,the lower member of the Zhiluo Formation>50m;2)mudstone thickness>30m;3)development of high quality source rocks;4)location of nasal uplift;5)good reservoir-cap rock assemblage.According to the standard screening,four favorable areas were determined in the 4986-4 well area,the 3387 well area,the 40063 well area and the 45-179 well area.