Study On Structural Characteristics,Evolution,and Hydrocarbon Accumulation In Yahushua Sag,Hailar Basin

The Hailer Basin is one of the vital continental sedimentary basins located in Northeast China with rich oil and gas resources.The study area（Balanus Sag）is located at the western edge of the hilar Basin,sandwiched between the Manzhouli Uplift and the Hanwula Salient,and is a crucial area for the study of the Central Asian Orogenic Belt,the Mongolian-Okhotsk tectonic system and the Pacific Rim tectonic system.The predecessors have carried out the various study in this area,but the research on the tectonic characteristics of the sag mainly focus on structural geometry.The research on tectonic kinematics and dynamics is relatively weak,and the understanding of the tectonic evolution process of Bayanhushu Sag is not uniform,which restricts the further exploration of oil and gas.Therefore,this paper selects the Bayanhushu Sag as the research object to carefully analyze its tectonic deformation process and kinetic evolution;and to discuss the hydrocarbon accumulation conditions of the sag.It presents a new theoretical understanding of the scientific research in this region and also lays a solid theoretical foundation for the following deep analysis and exploration deployment.This study is guided by the theories and research methods of structural geology,seismic stratigraphy,basin dynamics,and stratigraphic stratigraphy.Through techniques such as detailed tectonic interpretation,tectonic geometry analysis,denudation volume recovery,prototype basin recovery,structural physical simulation experiments,and balanced cross-section recovery,the results of previous studies are synthesized and tectonic geometry-tectonic kinematics-tectonic dynamics are closely integrated.Under the premise of establishing a geometric model of the basin and initially delineating the key tectonic event nodes,forward modeling of the region was carried out by structural physical simulation experiments in both planes and sections for the boundary fault spreading characteristics and seismic profile characteristics,and then the tectonic evolution process of typical profiles was verified by inversion using balanced cross-section recovery techniques.After the mutual verification of forwarding analysis and inversion,the construction history and transformation process of the sag can be reproduced more intuitively.It further reveals the complex evolutionary pattern and basin-forming dynamic mechanism of the Bayanhushu Sag.Through seismotectonic interpretation and geometric analysis of the Bayanhushu Sag,three key unconformities were identified from the bottom to up:T₅,T_22,and T₀₄corresponding to the superfaces of the Budate Group（basement）,the Nantun Formation,and the Yimin Formation,respectively.The recovery of the prototype basin shows that the sedimentary centers of the sag have the characteristics of continuous migration from the western depression-controlling fault to the central depressional area,and the number of sedimentary centers at each evolution stage shows a fluctuating pattern.Through structural physical simulation experiments and comprehensive kinetic analysis,it is found that the plane distribution of faults in the Hailar Basin is due to the development of pre-existing fabric formed by early large-scale strike-slip,and the selective activity of faults in the later extensional process,which gradually evolved into the depression-controlling faults of each sag.The evolution of Bayanhushu Sag has been controlled by the Adunchulu Fault for a long time,and the spreading of the fault on the plane shows a growth pattern of pre-existing en echelon faults→“soft connection”→“hard connection”.The development of the faults in the profile is a top-down inherited evolution,during which the attitude of the fault surface has undergone several changes,finally forming the tectonic pattern of the slope-flat fault.The gentle anticline（intra-sag uplift）developed above the flat is a product of the extensional geodynamic background,not the result of tectonic inversion as traditionally thought,but a typical normal fault-related folds.Through the balanced cross-section recovery,the evolution of the sag is divided into four stages:the initial extension fracture stage,the syn-rift stage,the rift-depression transform stage,and the post-rift stage.Structural inversion and denudation occurred at the end of the syn-rift and rift-depression transform stages（corresponding to the end of the Nantun and the Yimin Formations,respectively）.The two regional angular unconformity surfaces,T₂₂ and T₀₄,were formed at that time.Combined with the experimental simulation results,it shows that the compression force during the tectonic inversion period mainly acted in the SE direction.A comprehensive analysis of the regional geodynamic background is speculated to be related to the subduction of the Paleo-Pacific Plate under the Eurasian Plate and the intermittent compression caused by the transmission of the stress of the arc-continent collision.Through the analysis of hydrocarbon accumulation conditions,the sag tectonic framework,tectonic evolution process,tectonic construction,and modification characteristics,two hydrocarbon accumulation models were finally established in the steep slope zone and gentle slope zone of the sag.