Study On Sedimentary Facies Of The First Member Of Palaeogene Shahejie Formation In The Third Structure Region Of Nanpu Sag

The third structure region of Nanpu Sag is located in the southern part of the Nanpu Sag within the Huanghua Depression.The Palaeogene Shahejie Formation is the main source rock.However,there are still different views on the types and distribution of the sedimentary facies.Based on core data,and comprehensively applied logging data,seismic data,as well as test data,this paper researches on the source direction,the sedimentary facies types and their distribution features,as well as the sedimentary model.This study of the Shahejie Formation of the Paleogene in the third structure region of Nanpu Sag has mainly achieved the following results:First,the sedimentary source of the study area is identified.According to the distribution of the percentage of sandstone,the light mineral maturity coefficient,the ZTR index of heavy minerals,the continuity of RMS amplitude and the particle size sorting coefficient,it is suggested that the source direction is from southwest to northeast.The main source area is the Shaleitian Heave in the southwest of the third structure region.Second,the rock facies markers of the study area reflect the underwater depositonal environment with strong tractive current.It is shown that:the rock is mainly composed of feldspar lithic sandstone and lithic feldspar sandstone with low structural maturity;the probability curve of particle size is normally low slope two-segments and three-segments,scour surfaces and layering structure is common to be seen.On the logging facies markers,bell-shaped and box-shaped curves are common;funnel-shaped as well as finger-shaped are also visible.Additionally,filling seismic facies and foreset seismic facies develop.Third,it is revealed that the major depositional facies are the braided river delta front subfacies and the slip turbidite fan microfacies.Along the vertical direction,it develops the braided river delta front depositional environment which is dominated by the sand body of underwater distributary channel,and the mudstone of the underwater diversion bay overlaps with the channel sand body.Thin layers of silty sheet sand body often interacts vertically with the mudstone of underwater interdistributary bay.Fourth,the facies distribution of joint well section profiles are analyzed.It is suggested that along the source direction,the sedimentary facies gradually changes from underwater distributary channel near the source to the mouth bar-distal bar-blanket sand microfacies,the lacustrine mudstone increasing toward the northern.Along the vertical direction of the source,the middle part is the microfacies dominated by underwater distributary channel and gradually thinning towards both sides.The facies transition characteristics are not significant.Finally,a braided fluvial delta-shallow lacustrine/turbidite sedimentary system which developed on the background of the gentle slope belt of continental rift basin is established.It is considered that from the period of SQEs1~x to SQEs1~s,the relative lake level has decreased,and the corresponding lobes of the subfacies of the braided river delta front have been increasing by time with the overall sand thickness increasing.Meanwhile,the main body of the underwater distributary channel widened.The massive sand bodies of river mouth bar are separately distributed at the channel end.The distal bar and the sheet sand bar develop along the outer edge of the channel while slump turbidite deposition occurs at the far end.Under the control of such sedimentary mode,the underwater distributary channel and the slump turbidite in the lower place in the north are classified as a type I favorable sand body which should be set as the future exploration target.