Study On Delta-turbidity System And Lithologic Traps Prediction Of The Chang 7–Chang 8 In Huangling Area Of Ordos Basin

With the guidance of the theory of high resolution sequence stratigraphy sedimen-ology and petroleum geology,the paper aims to study the oil layers of yanchang formation Chang 7-Chang 8 in Ordos Basin Huangling Triassic on the basis of previo-us studies.The sequence stratigraphic framework of this area is estabilshed,Based on the observation and description of core and the analysis of drilling/logging data,the fourth-order cycle is divided systematically.The types,distribution and evolution of sedimentary facies are identified.Eventually,the favorable zones are predicted.By using the method of high-resolution sequence stratigraphy,combined with the existing research results and drilling/logging data,this article identifies three sequence boundaries which named SB1,SB2,SB3,and flooding surface.The long-term sequence,is divided into five mid-term cycles.The sequence stratigraphic framework of Huangling is built.By analyzing of backbone profile and making the isopach maps of different sequences,the stratigraphic distribution of each sequences of chang7-chang8 is analyzed.The study area develop two depocentres in the northeast and southwest,and the northeast is the main sediment source.Through the fine core observation and detailed characterization of color,lithology,structure and texture,and comprehensive analysis of logging data,every logging response characteristics of sedimentary microfacies is carried out.The study shows that three kinds of sedimentary system are developed in this area,including river delta,lacustrine and turbidite fan.The facies include delta front,shallow lake,semi-deep lake,middle fan and ten sedimentary microfacies.After analysing of single well and conneted wells facies,the spatial distribution and the development and evolution of characteristics of sedimentary facies are analysized within the sequence framework.Delta front and shallow lacustrine are developed in Chang 8,there are two underwater distributary channel in the northeast and the southwest.Because of the strong subsidence of the lake basin in Chang 7 period,semi-deep lake continues to expand its range and massive turbidite fan are concentrated in the northeast region.Then in Chang 7~1 period,delta front begen to constructe.The main facies types is changed from semi-deep lake and turbidite fan to delta front in northeast.The oil and gas in Huangling area accumulate in fews layers and the subtle reservoirs are mostly ithologic traps.Based on the analysis of sequence stratigraphy,sedimentary facies and hydrocarbon accumulation conditions,this paper the petroleum is accumulated in the source rock,which is reservoir sand bodies in contact with the source rocks.The thick Zhangjiatan shale deposited in Chang 7 is good oil source rock.Underwater distributary channel sand body and turbidite sand body is the main reservoir.It can point out that Chang 8~2,Chang 7~2,Chang 7~1 layer is advantageous hydrocarbon exploration.This paper make prediction about five favorable horizons for subtle reservoir exploration comprehensively and analysis the forming conditions.