| The Fushan Depression in the Beibuwan Basin is an important region for hydrocarbon exploration in northern part of the South China Sea because of its potentials for hydrocarbon-rich reserve.Because it is located at the junction of the India-Australian plate,the Eurasian plate and the Pacific plate,it has experienced strong tectonic activities and deformations since the Cenozoic,resulting in its structural differences in vertical and horizontal.Due to structural differences,distribution of hydrocarbon varies across the depression,with oil in the upper and gas in the lower,and with oil in the west and gas in the east.Given the complex tectonic evolution and various structures,cores,wells,seismic data and geochemical data were firstly used to established sequence stratigraphic framework,then analyzed the characteristics of structural and its formation mechanisms.After that,I discussed the responses of sedimentation to varying structures,and further explored the linkage between structures and hydrocarbon accumulation.Through this research,the following achievements and understandings have been reached:1.Through fine interpretation of 3D seismic,it is found that the structural architecture of the Fushan Depression presents the characteristics of two sets of structural systems in the upper and lower formations respectively,and a paleo-uplift between two sags which are situated in the east and west of the depression.The Lingao fault in the western margin of the Fushan Depression is a shovel-shaped detached fault formed under the action of strong extension,with strong sag-controlling.However,the trend of the Changliu fault in the eastern region is consistent with the extension direction of the sag,which produced a certain left-hand strike-slip component and was accompanied by the characteristics of translation and torsion,and the ability of sag-controlling was weak.The Dingan fault in southern slope of the depression is a high-angle normal fault formed by brittle rupture of rigid basement accompanied by multi-stage uplift during clockwise rotation of the Hainan uplift.The Meitai fault in the western part is a syndepositional fault originated from the second member of the Liushagang formation.It was strongly active and controled the sedimentation and filling of the western depression.There are two sets of fault systems in the Fushan Depression,which are basement-involved type and caprock-slip type respectively.The strike analysis of the fault system showed that the strike of the shallow fault system in the upper structural layer rotated clockwise to some extent compared with that of the deep fault system in the lower structural layer.In addition,due to the structural pattern and stress differences,the fault system in spatial can be divided into the western extensional fault system,the central structural transfer system and the eastern extensional strike-slip fault system.In addition,the Paleogene tectonic styles are different between the eastern and western parts of the depression: the western part is mainly affected by strong extension,and the eastern part is mainly affected by strike slip,and the Paleogene tectonic styles are mainly developed by torsion.Through the study of subsidence history and extensional amount,the characteristics of Paleogene tectonic activity evolution in the Fushan Depression are revealed: the subsidence process in the eastern and western regions is obviously different,and the subsidence center has the trend of moving from the west to the east;The extensional direction is mainly NWSE,and the extensional rate is much higher than the subsidence rate in each period of the depression.The process of extensional and subsidence has episodic evolution,but the episodic changes of extensional and subsidence are not synchronous.2.Based on sequence stratigraphy theory,seven 3rd sequence members were identified in the Paleogene Fushan Depression.The provenance,sedimentary filling and sedimentary system of each sequence strata are closely related to tectonic activities.(1)The thickness distribution of the sedimentary strata at different stages of tectonic evolution has different characteristics: the thickness center of the strata during the deposition period of the Changliu formation in the first stage of the rift was only developed in the Hetian sag in the western depression,and the strata in the east was very thin;During the deposition period of the Liushagang formation,with the enhancement of rifting,the thickness of the strata gradually increased,and the thickness of the strata increased most obviously in the eastern region.In the deposition period of the Weizhou formation in the late period of the rift,the rifting was weakened and the fault activity was attenuated.The control effect of the thickness distribution is weakened(2)Typical control sand modes related to structure:(1)Multi-level fault-step control sand mode,(2)Sand control mode of structural transition zone,(3)Sand control mode of surrounding slope break zone,strike-slip adjustment zone control Sand pattern The sedimentary filling process of Fushan Sag is closely related to tectonic activities in Paleogene.The sag started to rift and subsided during Changliu formation,and began to enter the initial expansion period.There was mainly highstand systems tract in the sag,and developed alluvial–fluvial facies.It deposited a set of inter-bedded strata with reddish brown,dark purplered sandy mudstone,shaly sand,and thick conglomerate.In Els3 sedimentary period,for the intensification of subsidence and extension,it resulted in intense fault activities within the depression.The lowstand system tract(LST),the lake expansion system tract(EST),highstand system tract(HST)were all developed in the depression,but different areas were with sequential and depositional filling characteristics.During Els2 period,the depression was of intense subsidence,entering the regional expansion period.The expansion range of the lake turned to the widest,with the development of LST,EST,and HST.Due to deeper water,subsidence rate was large than the sedimentation rate,a set of thick dark shaly source rocks were deposited and the scale of delta sandbodies become much smaller than the Els3 period.In HST,it lacked of sandbodies and the upper strata of slope suffer local truncation.Els1 corresponded to rifted period ?.During this stage,it was with the characteristics of weak fault activities,further the uplift of Hainan Island,lake shrinkage,and most of the upper strata of the south slope were eroded except the northern deep area;in the east,the range of LST was small,but the fan delta of HST was of a wide range.During the sedimentation period of the Weizhou formation,it was with the features of weak boundary fault.The depression entered the post-rifted stage;the depression gradually increased,and quickly deposited a set of fluvial conglomerate.3.The distribution characteristics of hydrocarbon resources have been studied through statistics of 250 boreholes within sequence stratigraphic framework.The eastern area is enriched in gas with gas-oil ratio over 20000,indicating that the source rocks in this area are in high-over maturation.The statistic results show that hydrocarbon resources of the Fushan Depression mainly distribute in SQEls3 HST,SQEls2 LST and SQEls1 HST.In the highstand system tract(HST)stratas,hydrocarbon resources are enriched in the central region,followed by the western region.In contrast,in the lowstand system tract(LST)stratas,hydrocarbon resources are enriched in the eastern region,followed by the western region.4.Based on the complex structural evolution process of the Fushan Depression in the Paleogene,and structural differences in the depression,the hydrocarbon accumulation model in the Fushan Depression was discussed.Studies have shown that the Fushan Depression had a good response relationship between structure-sedimentation and accumulation.The structural styles generated by different structural backgrounds control different types of sedimentary filling characteristics,and corresponding trap distribution patterns were formed in different structures-deposition sites.The deep and shallow fault systems developed in the Fushan Depression played an important role in controlling oil and gas accumulation.The deep faults were early and the activity were poor,which were conducive to the formation of various traps and the blocking of oil and gas reservoirs,and controled the formation and distribution of most oil and gas reservoirs in the Fushan Depression;the shallow faults were late and long-term Multi-period activity,the height of the oil column blocked by the fault is small,which were not conducive to the accumulation and preservation of oil and gas.Oil and gas reservoirs controlled by deep faults were located in the footwall of the fault.Oil and gas are mainly concentrated in the footwall traps controlled by deep reverse faults.Based on the above analysis,four main accumulation modes are proposed: the rolling anticline accumulation mode,the structural transition zone accumulation mode,the reverse normal fault accumulation mode,and the traction strike-slip structure accumulation mode.The exploration of hydrocarbon traps in the Fushan Depression should be conducted at different levels.The central area is recommended as the key region for exploration.A variety of different kinds of traps have developed in the central area of the Fushan Depression,including structural traps(horst traps and tilted traps)on the top,up-dip pinch-out traps on the flanks and sublacustrine fan traps in the deep lake.The western area can be regarded as another significant exploration region.The structural traps should be the main exploration targets in this region.The sublacustrine fan distributed in the Huangtong Sag can be identified as a highrisk exploration area requiring further research.In the eastern area,the structural traps associated with the deep antithetic fault system should be the main exploration targets in SQEls3,while up-dip pinch-out traps are the main exploration targets in SQEls1.In the eastern area,“self-generation,self-reservoir and self-coverage” lithologic reservoir should be the most significant exploration targets due to excellent source-reservoir-seal assemblage.To achieve good exploration results,it is of great importance to determine the distribution range of sand bodies of sublacustrine fans. |
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