Sequence Stratigraphic Characteristic of the Nantun Formation in the Western Area of Beier Depression,Hailaer Basin

Seismic data,logging,cores,connecting with the characteristic of structure evolution are used to establish the sequence stratigraphic framework of the Nantun Formation in the western area of Beier depression,Hailaer basin.The Nantun Formation in the study area finally can be divided into one supersequence,four three-order sequences and eleven systems tracts.There are four facies including fan delta,braid river delta,sublacustrine fan and lake,and eight subfacies can be identified in the sequence stratigraphic framework of the Nantun Formation.During the depositing period of Nantun Formation,the study area experienced the tectonic evolution process which experienced from initial subsidence state to a rapid subsidence stage,then to a subsidence decrease stage,and to a shrink stage finally,which respectively corresponded with four three-order sequences.In the depositing period of sequence SQ1,this area was a separated rifted basin,fan delta mainly formed in the northwest and northeast of the area,and braid river delta mainly formed in the southeast of the area;at this time,the area of lake basin is also small.During the depositing period of sequence SQ2,the area of lake basin expanded rapidly with the lake level rising and finally formed a unified lake basin;the sedimentary framework is similar to the depositing period of sequence SQ1,but the difference is that in the southwest of this area,a provenance appeared and then fan delta sedimentary began.In the depositing period of sequence SQ3,the lake-basin area reached to the largest scale;the area scale of all fan(braid river) deltas from corresponding respective provenance direction obviously reduced and deep water sediments including sublacustrine fan and fluxoturbidite formed relatively more.In the depositing period of sequence SQ4,the lake basin area scale decrease rapidly;meanwhile,the sediment from the southeastern provenance supplied not enough to the basin,leading to the braid river delta in the southeast of centre almost disappear at that time.On the basis of comprehensive research on the reservoir-cap combination distribution and the conditions of the migration and accumulation of oil and gas,it shows that most favorable reservoir-cap combination formed in the lowstand system tracts and the highstand system tracts of sequence SQ2 and SQ3 of the Nantun Formation.These fault development areas are prone to form block fault traps and fault anticline traps because of having preferable reservoir rock and being near the hydrocarbon source rock.The slope and ramp can form unconformity-lithologic traps by the unconformity as the barrier bed and the stratigraphic overlap traps by all kinds of sand bodies as the reservoirs.