Tectonic Evolution And Forming Mechanism Of The Southern Liaozhong Sag

Liaozhongnan Sag is located in the north of Liaodong Bay depression.It is an important oil and gas enrichment zone within the Liaodong Bay depression,which underwent multi tectonic evolution making the structure style various and the genetic mechanism is complex.Therefore,it is the basis and key for oil and gas exploration in southern Liaozhong Sag to make clear the tectonic features and evolution process.Based on the 3-D seismic data,drilling and so on,using method of calculating fault activity rate and balanced cross-section,the authors determine the structural evolution process,establish the evolution model of basin structure.On this basis,combined with the regional dynamic background,this paper discusses the influence of the pre-existing faults,the superposition of different stress ratios and the effect of strike-slip derivation action.The faults in southern Liaozhong Sag has the characteristics of multi-strike,multi-properties and multi-combination style,which have four groups of faults in different directions: NEE,NE,NW and EW.The faults are strike-slip and extensional.In addition,the faults with different striking cut each other,and the faults system is further complicated.On the plane,the faults combine as abut and horsetail,and combination style in cross-section is shown as "Y" shape,flower shape,flower-like shape and so on.Controlled by the strike slip and extension,the structural styles of southern Liaozhong Sag were varied.Based on the mechanics,it can be divided into three types: strike-slip structural style,extensional strike-slip structural style,extrusion structure.The development of faults in different strata is different in time and space.The NW-striking faults compose an extensional fault system developed in the deep strata(Mesozoic-Paleozoic,early Paleogene),and the continuity gradually deteriorated to the shallow strata.Later,it was cut and transformed by NNE-striking strike-slip faults,while the secondary faults in the EW and NE-striking gradually increased from the deep to the shallow strata.In different evolutionary stages,the activity intensity of faults in different strike is different.In the sedimentary period of Ek formation – Es1 member,NW-striking fault activity dominated,and later activity gradually weakened.In this period,the NEE-striking faults began to activate,cutting NW-striking fault,and became the main basin controlling fault in the sedimentary period of Es3 member,Es1 – Es2 member and Ed formation.The secondary faults mainly started to activate in the Es2 member.In terms of strike-slip strength,the strike-slip strength of Liaozhong No.1 fault is the strongest,and that of the Ed formation is the strongest in each active period,which is characterized by strong east and weak west regionally.The tectonic evolution of southern Liaozhong Sag can be divided into four stages: early Paleozoic(Ek-Es3),middle Paleozoic(Es2-Es1),late Paleozoic(Ed)and Neogene(N+Q).In the early Paleogene,NW-striking basement fault controlled the stratigraphic deposition,NNE-striking strike-slip faults activate weaker,and Ek-Es4 in the central strike-slip zone began to form.In the middle Paleogene period,the overall stratigraphic thickness of southern Liaozhong Sag was thin,the fault control was weak,and the depositional center was not obvious.In the late Paleogene period,strike-slip action was strong and extensional action was weak.Strike-slip fault cut and transformed early extensional normal fault.EW-striking secondary faults developed in large amount in this period and distributed in echelon along the main fault.The Neogene extensional and strike-slip activities were weak and the whole area was in the stage of depression.The temporal and spatial differences of the basin structure are caused by the differences of kinetics backgrounds in different stages,pre-existing faults,the superimposed ratio of extensional and strike-slip stresses,and the local stress field caused by strike-slip derivation.The NW and NNE-striking pre-existing fault systems cut each other forming conjugate transition zones,which provide extrusion and torsional stresses for southern Liaozhong Sag.The superposition ratio of extensional and strike-slip stresses controls the time-space differences in the structural development of southern Liaozhong Sag.Early Paleozoic period is dominated by extensional action;middle Paleozoic period is the structural transformation period of southern Liaozhong Sag.The extensional action is weakened and strike-slip action is enhanced,late period is dominated by strike-slip action,and Neogene period is weak in extensional and strike-slip action.The strike-slip derivation leads to the difference of local structure,and different local stress fields are derived from the bending part,the tail end and the side joints part of strike-slip faults,which results in the difference of local structure development in the depression.