Mechanism Of Textural Heterogeneity Of The Yanchang Formation In Angou Section,Ordos Basin

The exploration and development of unconventional oil and gas resources has been upsurge in the world,and the structure heterogeneity has become a hot issue.Research on the formation mechanism of structural heterogeneity will help us to understand the formation and distribution of restraining barrier and interbeds more clearly.Therefore,the Angou section of Yanchang Formation in Ordos Basin was selected as the research object.On the basis of field profile investigation,configuration interface identification and division,the cast thin section identification,fluorescence thin section identification,cathodoluminescence,carbon and oxygen isotopes,saturated hydrocarbon chromatography-mass spectrometry analysis and other experimental means were adopted.The diagenetic evolution process,oil charging period,diagenetic mineral distribution characteristics and controlling factors in the profile were studied,and the formation mechanism of structural heterogeneity was comprehensively discussed.The main achievements are as follows:(1)There are two superimposed five-level configuration interface in the geological section,and several six-level configuration interface because of multiple sand bank aggradation in the channel sandstone.Channel sandstone and sand bank showing positive rhythm.Five-level configuration interface and six-level configuration interface nearby sandstone are formed restraining barrier and interbeds.The inner part of channel sandsandstone or sand bank is oil layer.For the convenience of research,the sandstone located at the upper,lower and inside of the five-level configuration interface and six-level configuration interface are defined as the upper sandstone of configuration interface,the lower sandstone of configuration interface and inner channel sandstone.(2)The upper sandstone of configuration interface has a large grain size,and the tight intercalation is formed due to the cementation of calcite and laumontite.The lower sandstone of configuration interface,with small grain size,is densification due to compaction and calcite cementation.The inner channel sandstone is the oil layer,and its reservior quality is mainly controlled by the content of zeolite.When the content of zeolite is less than 3%,the compaction resistance of the sandstone is weak,and the reservoir quality is poor.When the content of zeolite is about 5%,zeolite can provide more dissolution pores for the reservoir and has ability of compaction resistance,and the reservoir quality is better.A large area of zeolite cement still blocks the pores,and the reservoir face ratio decreases with the increase of zeolite content.(3)The cementation in the reservoir is dominated by zeolite and calcite.Zeolite formation is controlled by origin of sediment and is form fromed early diagenetic stage.Calcite has a light carbon isotope(<-7‰),which is influenced by the hydrocarbon expulsion of source rocks,and is of organic origin.The reservoir experienced one stage(two act)of hydrocarbon charging.The diagenetic evolution sequence is as follows: compaction-laumontite-dissolution-Siliceous cement-first act of hydrocarbon charging(Hy1)-calcite cementation-dissolution-Siliceous cement-second act of hydrocarbon charging(Hy2).(4)All the crude oil in the Angou geological section came from the Chang 7 source rock.Organic geochemistry and fluorescence observations were used to characterize the distribution characteristics of crude oil in the geological section.It was found that the oil and gas in the second act(Hy2)were mainly distributed in the upper and central area of the section.The oil and gas in the first act(Hy1)had been charged in the whole section,and now the oil and gas are mainly distributed in the two sides and the lower area of the section.(5)Calcite distribution is the result of sedimentation-compaction-laumontite cementation-first act oil and gas charging(Hy1).During oil and gas charging in the first act,the sandstone at the lower part of the sedimentary interface does not contain oil due to compaction,and the sandstone at the upper part of the sedimentary interface does not contain oil due to laumontite cementation or insufficient oil and gas charging.After oil and gas charging,the inhibition of oil and gas on diagenesis in the sandstone in the river channel precipitates a small amount of calcite.The upper sandstone and the lower sandstone of the sedimentary interface are aquifer,and a large amount of calcite precipitates to form a tight interlayer.The interlayer interconnects with the large area of the sedimentation interface and forms a network structure,which forms the structural heterogeneity.