Sedimentary Characteristics And Three-Dimensional Geological Modeling Of Sandy Braided River

Ordos basin is a large multi-cycle superimposed basin,in which several large gas fields have been discovered,such as Sulige gas field,Mizhi gas field,Yulin gas field and Zizhou gas field.At present,abundant natural gas resources have been found in the Lower Shihezi Formation,of which the He8 member is the main stratum for the exploration of natural gas resources in the Upper Paleozoic.However,in recent years,oil and gas exploration has begun to expand to the southwest,and the Longdong area has become the next reserve replacement area.This paper takes the He8 member of Chengtan3 area in Longdong area of Ordos basin as the research object,takes the basic theories and methods related to sedimentology as the guidance,and makes full use of the drilling core data and test analysis data on the basis of many previous research results to conduct in-depth research on the provenance direction,sedimentary facies development characteristics,sedimentary microfacies types and sand body distribution law of the study area.Chengtan3 covers an area of about 1200 km~2.The complex geological conditions,vague understanding of sedimentary subfacies,and unclear understanding of the geometric form,scale,and spatial distribution of sand bodies can seriously affect the development effectiveness of oil and gas fields,especially when developing horizontal wells,due to unclear understanding of underground conditions,greatly increasing the risk of oil and gas field development.Therefore,it is urgent to carry out the fine description of gas reservoir.After determining the types of sedimentary subfacies and sedimentary microfacies through core facies,log response characteristics and log interpretation analysis,establish and optimize the 3D geological model to describe the heterogeneity of the reservoir,and provide a reliable basis for the next well location deployment in the study area.When establishing sedimentary facies models,we select three mainstream facies modeling methods,namely,object-based method,sequential indication method,and multi-point geostatistics method.When establishing the sedimentary facies model,in view of the difficulty in obtaining the 3D training image under the existing conditions in the study area,we adopted the sequential two-dimensional conditional simulation combined with the direct sampling method to establish the sedimentary facies model.This method can simulate the three-dimensional geological model by using the two-dimensional training images obtained from the digital plane facies diagram and the cross-well section facies diagram.This paper mainly achieved the following three results:1.The He8 member of Chengtan3 in Longdong area is a braided river delta plain subfacies,which develops three sedimentary microfacies:distributary channel,channel bar and interdistributary channel.The sand body of the lower 2 layers of He8 is the best developed,and the channel bar are the most developed.The gas reservoirs(effective reservoirs)are all developed on the core beach sand body.The channel bar is a favorable microfacies type.Most of the distributary channel sand bodies develop poor gas reservoirs,and no gas reservoirs are developed between the distributary channels.2.Quantitative analysis of gas reservoir distribution in the channel bar.According to the hydrodynamic conditions and sedimentary mechanism,the channel bar is divided into:the bar head,the bar tail,the bar main body and the bar wing.Through statistical analysis of the location of gas reservoir development,it is found that 63%of the gas reservoirs in the channel bar are developed in the bar main body,followed by the bar tail,and the smallest proportion of gas reservoirs is developed in the bar head and the bar wing.It is comprehensively concluded that the types of sedimentary microfacies,the thickness and particle size of sand bodies are the main controlling factors for effective reservoir development.3.The sedimentary facies model is established by the Object-based method,Sequential indication method and Multi-point geostatistics method based on two-dimensional training images respectively,and the advantages,disadvantages and applicability of the three methods are compared from multiple aspects.The results show that the Object-based method cannot reasonably depict the contact relationship between micro-facies vertically,and the coincidence rate of well data is low;Although the Sequential indication method can meet the distribution of well data,the established model does not well restore the geometric characteristics of the simulated object,and the facies boundaries are scattered.The Multi-point geostatistics method based on two-dimensional training image can not only meet the well data,but also well display the geometric characteristics and contact relationship of the simulation object.The modeling results are consistent with the actual geological conditions of the study area.It shows that this method is feasible in the 3D geological simulation of large-scale fluvial facies tight sandstone reservoir,and also provides a new idea and scheme for geologists to model in similar geological environment.