Sedimentary Facies Research Of Chang3 In Yanwu Area,Ordos Basin

The Yanwu area is located in the southwest of Ordos Basin,and its Yanchang formation is the main production layer of low permeability sandstone reservoir.The sand body of Chang 3 oil formation in this area is generally developed and rich in oil and gas resources,however the degree of single well production is low,the oil and gas production decreases rapidly,and the water content is high,especially in the process of production and construction,all kinds of problems are often encountered.In order to meet the requirements of oil field exploration and development,the sedimentary geology of Chang 3 formation in Yanwu area is studied in detail,so it is necessary to carry out systematic research on sedimentary facies.The main contents of this study are the identification of sedimentary facies,the classification of sedimentary microfacies and the analysis of sedimentary facies,the establelishment of sedimentary model,and the exploration of the controlling factors and distribution characteristics of sand bodies.The following understandings were achieved:1.The stratigraphic division and correlation of Chang 3 formation in the study area are carried out.Taking full advantage of the regional marker layer,combined with the correlation of offset wells and the control principle of equal thickness and cycle of formation,the Chang 3 oil formation in the study area is divided into three sub-layers of Chang 3₁,Chang 3₂ and Chang 3₃,in order to achieve the purpose of fine description of sedimentary facies,the above three sublayers are subdivided into 9 layers:Chang 3₁¹,Chang 3₁₂,Chang 3₁³,Chang 3₂¹,Chang 3₂²,Chang 3₂³,Chang 3₃¹,Chang 3₃²,and Chang 3₃³.Seven skeleton sections were establelished in the direction of source direction and vertical source.The stratigraphic distribution was uniform and the erosion at the top of Chang31 was serious.2.The braided river delta system.has determined in the study area.Based on core observation,logging data and experimental analysis data,the sedimentary environment characteristics of the study area are analyzed from four aspects:petrological markers,sedimentary structural markers,paleobiological markers,logging facies markers,etc.The sedimentary facies system in the study area is determined to be braided river delta.The subfacies types are delta plain and delta front.The main microfacies types are distributary channel,interdistributary depression,natural dike,underwater distributary channel,distributary bay and underwater natural embankment.The sand bodies of distributary channel and underwater distributary channel are well developed.3.The sedimentary facies model of braided river deltas and lakes in this area is establelished.According to the sedimentary characteristics and tectonic evolution background of the studied area,the sedimentary facies characteristics of Chang 3reservoir formation in Yanwu area are analyzed by using the point-line-plane research idea.Based on the analysis of vertical and lateral characteristics of sedimentary facies,the sedimentary facies model is establelished through the study of single well facies,combined well facies and plane facies.4.It is concluded that the distribution of sand body is mainly controlled by sedimentary microfacies.The sand body is distributed in the South West north east direction.The size of sand body becomes smaller from Chang 3₃ to Chang3₁ periods.The sand body is best developed in the Chang 3₃ period,the thickness of sand body is up to 27m,the maximum thickness of the sand body of the Chang 3₂ deposition period is up to 23m;the sand body of the Chang 3₁ deposition period is relatively thin and the largest sand body thickness is 19m.