Tectonic Evolution And Its Control On Hydrocarbon Accumulation In Xijiang Main Sag,Zhu ? Depression

Guided by the structural analysis and hydrocarbon accumulation theories,based on drilling,seismic,geological and logging data,this paper has finished a systematic study on sag structure,structural characteristics,tectonic evolution and hydrocarbon accumulation in Xijiang main sag.Research results have been achieved as follows:(1)Xijiang main sag consists of Xijiang 32 and 33 sub-sag,which controlled by the NEE main fault,forming the half-graben structure with the north faulted and the south overlapped.Xijiang 33 sub-sag is the main part of Xijiang main sag;it's a large,deep burial sub-sag with very thick Wenchang Formation and relatively thick Enping Formation,which is called "Thick Wenchang,Thin Enping" type sag.Xijiang 32 sub-sag is a smaller,medium burial sub-sag with thiner Wenchang Formation and Enping Formation,which is called inherited type sag.Two sub-sags are relatively independent during the period of Wenchang Formation,and are connected as a whole sag in the period of Enping Formation,with a larger sag area.(2)It is a "sag and uplift alternate" basement structural framework in Xijiang main sag and its periphery area.There are two sets of fault systems in this area.One is the deep layer fault system which is composed of early active faults,and the other is shallow layer fault system that consists of a little late active faults,and lack of long-term active fault.Distribution of mainly faults trend NEE,nearly EW and NWW,early faults distribute as NEE and nearly EW,and late faults for NWW and nearly EW.Research area could be divided into three structural belts,including the northern steep slope zone,the central fault belt,and the southern ramp zone.(3)Tectonic evolution of research area could be divided into three stages,which are chasmic stage from middle Eocene to early Oligocene,depression stage from late Oligocene to middle Miocene,and neotectonics stage since late Miocene.Chasmic stage is mainly tectonic subsidence with a high tectonic subsidence rate,and it's divided into two sub-stages which including chasmic I and II stage.Chasmic I stage,which is the mainly chasmic stage,developped half-grabens that deposited very thick Wenchang Formation controlled by the NEE faults,and sufferred upliftting and denudation in last stage;chasmic II stage,which is the secondly chasmic stage,inheritted the framework of early sags but had a more extensive scale that deposited thick Enping Formation with the depocencer moved westward,and sufferred slightly upliftting and denudation in last stage.Depression stage and neotectonics stage are mainly load subsidence with a low tectonic subsidence rate.The tectonic movement is relatively quiet,and fault is not active in depression stage.Influenced by Dongsha movement,research area developed a small amount of NWW and nearly EW faults in neotectonics stage.(4)Cap rocks and migration conditions are the main controlling factors of hydrocarbon accumulation in Xijiang main sag.Cap rocks condition is poor in the northern steep slope zone,and it's difficult for hydrocarbon vertical migration in the central fault belt,which are the main reasons for the drilling defeat.Because of developping some late active faults,good reservoir-cap combination,and locating in the main pointting area of hydrocarbon migration,the southern ramp zone is the favorable area of hydrocarbon migration-accumulation.(5)There are closely relationships between tectonic evolution and hydrocarbon accumulation.The faulted depression stage controls the formation and distribution of Wenchang Formation source rock and Enping Formation source rock.By influencing deposition,tectonic evolution indirectly controls many sets of reservoir-cap combination.The formation of structural traps is controlled by tectonic movement.Fault and unconformity surface are good channels for hydrocarbon migration-accumulation.Weak late tectonic movement is beneficial to hydrocarbon preservation.(6)The southern ramp zone is favorable hydrocarbon accumulation zone of Xijiang main sag,and Xijiang 34-1 structure is its favorable exploration target which has superior accumulation conditions.