Sequence Stratigraphical Architecture And Sand Distribution Of Chang7-6 Member,Yanchang Fm.in Zhenbei Area

Chang6-7 Member,Yanchang Formation is one of the main targets on oil and gas exploration in Zhenbei area.Recently,the key constraint on lithologic reservoir exploration in the area is the lack of systematic study of sequence stratigraphy and sand body distribution Based on the theory of sequence stratigraphy and sedimentology,the paper aims to study the sequence architecture of Chang6-7 Member,Yanchang Formation,and study the temporal and space evolution of the sedimentary systems,by using seismic,drilling/logging data and the cores comprehensively.Using an backstripping method to restore the deposited terrain,and the palaeogeomorphology map of studied area is drawn by short-term base level unit.And then predicted the better reservoir zones considering with oil/gas migration and congregation.According to the high-resolution sequence stratigraphy theory,2 middle-term base-level cycle and 9 short term base-level cycles are identified in Chang7-6 Member.Reflecting a process that the sediments stepping seaward,and experiencing temporary lake transgression and developing aggradation-retrogradation stacking pattern.In the sequence framework,we identifyed 3 kinds of sedimentary facies:braided delta,turbidite fans and lacustrine.The palaeogeomorphology map of studied area is drawn by short-term base level unit.The study finds that the Zhenbei area locates near the depositional slope break,ChangC75 is the maximum transgression.From ChangC75 to ChangC72,the base level decreases,the slope break zone gradually advance to the lake,and the angle of the break zone begins to decrease;from ChangC72 to ChangC71,the base level begins to rise,the slope break and the sand body gradually retreat to the shore.Base level cycle change leads to geomorphic change,the origin and distribution of the sand body changes regularlyAfter making clear of the sedimentary facies types and the palaeogeomorphology,we studied the distribution of sedimentary facies by short-term base-level unit,and summarized the sedimentary characteristics.In the period of C7,sediments prograde into lake gradually,the main reservoir sandbodys are braided-delta.plain,while east of the study area mainly delta front and gravity sediments.Delta front extends to the line H76-M24-Z74-Z373-Z146.In the period of C6,the delta progrades slowly into the lake,as the slope of the terrain decreases,the sedimentary facies change.Channel sandbodies develop widely over the slope,and coal bed is also abundant.Converted by tide,mouth bar is rare.By combining with the structure map,distribution of sedimentary facies,sandbody thickness,the play of oil/gas trap,and fault guidance system,the paper proposes that the potential layer is fromChang7 to the bottom of Chang6 member,Yanchang Formation,and the lithologic hydrocarbon reservoir should be focused on east of the line from Zhenyuan to Huanxian.