Sequence Architecture And Depositional Evolution Of Wj Block,South Sumatra Basin In Rifting Period And Its Paleostructural Controlling Characteristics

As one of the most important petroliferous basins in Indonesia,the South Sumatra basin has abundant oil and gas resources.The South Sumatra basin is a continental faulted basin with complex tectonic activities in the rifting period.The sequence filling and sedimentary evolution of this are affected by the Syndepositional faulting.Determining the sequence stratigraphic framework and the sedimentary system types is the basis for the study of the hydrocarbon accumulation rules in basin.However,at present,the current lack of understanding on how the syndepositional structure of the WJ block affects sequence filling and sedimentary evolution features restricts the in-depth study of oil and gas exploration in this area.The sequence stratigraphy,sedimentary filling evolution of the WJ block in the south Sumatra basin during the rifting period were studied by analyzing the 3D seismic,well-logging,and drilling data.The control effect of syndepositional structure on sequence and sedimentation in this block was revealed,which has positive significance for the prediction of favorable traps in the study area.Under the guidance of the classic sequence stratigraphy theory,the depositional system types and the evolution characteristics are analyzed in the third-order sequence unit by building an isochronous sequence stratigraphic framework.The interior Paleogene of the WJ block can be classified into 2 secondary structural sequences and 3 tertiary sequences(SQ1-SQ3).The development of sequence stratigraphy in rifing period was influenced by palaeogeomorphology and syndepositional faults.By restructuring the palaeogeomorphology,WJ block can be divided into the western uplift zone,gentle slope zone,local subsidence sag zone and eastern depression zone.By comparing the well profiles,it can be found that the thickness of the NW-SE stratum is affected by the paleo-uplift and syndepositional fault in the west.The thickness of the stratum gradually increases from west to the east.In the NE-SW section,the sequence deposition thickness is basically unchanged.By observing the thin slice under the microscope,and studying the logging curves,seismic reflection characteristics and paleontological data,five sedimentary facies,including fan deltas,braid deltas,shore-shallow lacustrine,sublacustrine fans,and nearshore subaqueous fans were identified in the sequence grid.Subsequently,the sedimentary evolution of each sequence was studied.During the SQ1,alluvial fan and fan delta were deposited.During the SQ2 and SQ3,the fan delta and the braided delta deposited were developed in the steep slope zone and in the gentle slope zone,respectively.While in the cetral sag and in the eastern depression margin steep-fault zone of the Shore-shallow lake and sublacustrine fan was deposited,respectively.By studying the characteristics of the syndepositional fault in WJ block,the structural slope-break belts can be summarized as the following three types: the single fault-step fault half graben structure slope break zone,gentle slope reverse adjust fault half graben structure slope break belt,and single fault-reverse adjust fault-step graben structure slope break belt.The control effect of syndepositional fault characteristics on sequence filling and sedimentary systems is also analyzed.Meanwhile,combined with the source-reservoir-cap,four favorable traps are summarized:updip pinchout traps,sand-lens lithologic traps,structural-lithologic composite traps,stratigraphic unconformity traps.