Geometry And Kinematics Of Diapir And Its Implication In The Yinggehai Basin

There are many types of diapirs in the Yinggehai Basin located at the northwestern continental margin of the South China Sea,which has significant influence on the hydrocarbon generation,migration and accumulation etc.across the Yinggehai Basin.However,there is a lack of effective interpretation of the diapir structural characteristics due to the poor quality of seismic data and the uncertainty of structural interpretation.Therefore,this paper analyzed the geometry and structure of the Yinggehai Basin and the diapir structure based on 2D and 3D seismic data,to conduct analogue experiments based on geometry-kinematics-dynamics similarity from nature to modeling.Which will significantly help us to understand the geometry and kinematics of the diapirs in the Yinggehai Basin,and their implications of hydrocarbon accumulation and migration across the basin.The main points of this paper are shown as follows:(1)Based on the analogue experiments,it suggests that the Yinggehai Basin is characterized with basement boundary fault zones and high-angle distributed strike-slip faults,of which are with different deformation,e.g.,concentrated and distributed strain and diapirs.Furthermore,there are two-layer structures for the sedimentary sequences in the basin,consistent to the model results,the upper layer of Miocene with dish-shaped depression dominates without faults,in contrast to the underlying layer with distributed faults,indicating of distributed deformation across the Yinggehai Basin,rather than block-deformation controlled by basin-regional boundary faults.(2)Based on the experiments with distributed strike-slipping shear with a circular basement fault system,it suggests the diapir material will vertically and laterally uplift and pierce with progressive deformation,resulting in elliptical and helical asymmetric geometry in the diapir core.In particular,there are distinct changes of shear speed between the front and backyard of the diapir,i.e.,“high-speed in the front and low-speed in the back”.Thus,it accommodates transpression in the front and transtension in the back due to different stress-strain along the diapir.We argued that the distributed strike-slipping of Red River Fault system is an important controlling factor for the formation and evolution of diapir structure in the Yinggehai basin.(3)The interaction between syntectonic sedimentation and diapir uplift controls the diapir morphology and the deformation of surrounding rock.The early rapid synsedimention model accommodates a tapered “buried diapir” or “weak piercing diapir”,the early slow synsedimention model results into a cylindrical-flared “strong piercing diapir” or “collapsed diapir”,and the late accelerated synsedimention model results into a finger-shaped “piercing diapir”.It should be noted that the syntectonic sedimentary processes in the Dongfang,Changnan and Ledong area of the Yinggehai Basin are consistent with the above three analogue models,and it suggests that a similar diapiric tectonic evolution process occurred across the Yinggehai Basin.Therefore,we argued that the diversity of diapir in the Yinggehai Basin is controlled by the difference synsedimentary rate or strata thickness caused by regional tectonic evolution.(4)Based on the comparison between models and prototype,it suggests that the hydrocarbon-conduits system is composed of flower structure,diapiric structure and distributed structure in the Yinggehai Basin.Diapirs and their associated faults are the main migration pathway of shallow anticline or faulted anticline traps,and the truncation traps and lithologic pinch-out traps at diapir flank rely on the fractured drag zone which provides hydrocarbon-conduits,while distributed structure provides migration conditions for lithologic traps in slope area.In addition,the diapir asymmetric structure has an important influence on the hydrocarbon accumulation,in particular along the diapir long axis.It indicates that the compressive anticline at the front of diapir has better conditions for the hydrocarbon accumulation.