Diagenesis And Diagenetic Facies Of Triassic Yanchang Formation Chang 6 Reservoir In Pingqiao Area,Northern Shaanxi

Nowadays,oil and gas resources have become an indispensable factor for the development of the country and society.The proportion of tight oil in the current oil and gas exploration and development is increasing.Ordos Basin is the second largest sedimentary basin in China and also an important petroliferous basin,Chang 6 reservoir group of Triassic Yanchang Formation is one of the main oil-bearing reservoirs in Ansai area,and it has the characteristics of low permeability,ultra-low permeability and low oil saturation.The study of diagenesis and diagenetic facies of Chang 6 reservoir group is of great significance to understand the characteristics and formation mechanism of tight sandstone reservoir,and to predict the reservoir effectively,and to provide the basic geological basis for the efficient development of the oilfield in the future.In this paper,based on the basic theories of sedimentary petrology,petroleum geology,logging geology and sequence stratigraphy,Using the observation of outcrop and core in the field,by means of thin section of rock casting,X-ray diffraction,scanning electron microscope,porosity and permeability analysis,cathodolumine-scence,grain size image analysis,grain size analysis,etc.Combined with drilling and logging data,the development characteristics,sandstone reservoir characteristics,diagenesis,diagenetic facies and control factors of Chang 6 reservoir group of Triassic Yanchang Formation in the study area are studied,and the following main knowledge is obtained:According to the characteristics of lithology and logging curve,the Chang 6 reservoir group can be divided into four sub layers:Chang 6₁,Chang 6₂,Chang 6₃ and Chang 6₄,The main oil producing layers Chang 6₁ and Chang 6₂ are further divided into two small layers,and the stratigraphic division and multi well stratigraphic correlation are carried out.On this basis,the sedimentary characteristics of Chang 6 reservoir in the study area are studied.It is considered that Chang 6 is a set of delta front subfacies.And draw the corresponding sedimentary microfacies profile map and sand body distribution map.The sand body distribution direction is generally NE-SW.The Chang 61 reservoir in the study area is mainly fine-grained feldspathic sandstone or lithic feldspathic sandstone.The maturity of rock composition is low.The interstitial materials are mainly chlorite,calcite and laumontite,followed by ferruginous calcite,siliceous,feldspar and a small amount of other minerals.The rock is generally rounded,well sorted and medium structural maturity.The types of cementation are mainly pore-film type and film-pore type,and the contact relationship is mainly line contact.The average porosity of the reservoir is9.07%,and the average permeability is 0.44×10-3um2,which is a low porosity,low permeability and ultra-low permeability reservoir.The pore level of rock is mainly small pore,pore types are mainly intergranular pore,feldspar dissolved pore and turbidite dissolved pore.The study area Chang 6 mainly experienced diagenesis such as compaction,cementation,metasomatism and dissolution.Compaction and cementation can destroy the pores,while dissolution can improve them.The reservoir belongs to stage A in middle diagenesis.The sequence of diagenesis evolution is as follows:Mechanical compaction-formation of chlorite clay film-siliceous cementation-calcite cementation-formation of illite mixed layer-chlorite filling intergranular pores-laumontite cementation-feldspar,turbidite dissolution-ferrocalcite filling。The diagenetic facies of Chang 6 reservoir in the study area can be qualitatively divided into six types:compact compact facies,carbonate cementation facies,chlorite film facies,turbidite cementation facies,dissolution facies and intergranular pore dissolution pore facies.According to the apparent compaction rate,apparent cementation rate and apparent dissolution rate,on the basis of Q-cluster analysis,the reservoir diagenetic facies can be quantitatively divided into four types:strong compaction weak dissolution phase（A）,strong cementation weak dissolution phase（B）,medium dissolution phase（C）and strong dissolution phase（D）.Through the analysis of single well diagenetic facies,the logging characteristics of four types of diagenetic facies are statistically analyzed,and the corresponding relationship between diagenetic facies and logging characteristics is established.According to the results of oil testing,the diagenetic facies of all wells in the study area are divided and the diagenetic facies plan is drawn.Based on the statistics of sedimentary microfacies,sand body thickness and petrological characteristics of different diagenetic facies reservoirs in the study area,it is found that diagenetic facies is mainly controlled by sedimentary microfacies,sand body thickness and provenance.Based on the statistics of sedimentary microfacies,sand body thickness and petrological characteristics of different diagenetic reservoirs in the study area,it is found that diagenetic facies is mainly controlled by diagenesis,sedimentary microfacies,sand body thickness and provenance.