Research On The Sedimentary Characteristics Of Neogene Shawan Formation In Chepaizi Area Of Junggar Basin

Chepaizi area belongs to the sub-structural unit of the uplift in the west of Junggar basin, and it was formed in early Hercynian, when the pangea began to merge and the basement rock series of Chepaizi area and Chepaizi Salient was forming. In the Shawan Formation sedimentary period, Chepaizi Salient was generally a southward inclineded slope, which had slow slope gradient, low water and no fault developed. The exploratory survey showed that the shallow layer of Shawan Formation in Chepaizi area had a satisfactory oil-bearing potential but was constrained by the lack of drilling fluid and well logging data. Based on the seismic data and the integrate studying of the cores and the logging data,analysis assay data, the sequence framework of this area, the source direction and the genesis and the distribution of the sand body were studied systematically. The result and conclusion are obtained as follow:(1) According to the sequence dividing principle, the sequence framework of Neogene Shawan Formation in Chepaizi area was established. The Neogene Shawan Formation can be divided into two forth-order sequences followed as the first members of Shawan Formation, the second members of Shawan Formation, and according to the strata correlation of the study area, first members of Shawan Formation can be divided into eight sand sets, of which the spatial distribution was analyzed.(2) With several methods, such as analysis of dip log,sandy conglomerate distribution,analysis of paleo-geomorphic,detritus component, the provenance and palaeocurrent direction of Neogene Shawan Formation at Chepaizi salient in Junggar basin are analysed. The results shows that, there were two sediment origins in this area, one from the north and northwest, and the other from the northeast. In the early sedimentation period, the north and northwest provenance controlled the sand body nearly south-north distribution, and there developed predominantly gentle slope fan delta, while in the late sedimentation period, the provenance direction shifted eastward and became mainly northeast, so the sand body showed southwest-northeast distribution, and there developed predominantly shallow braided river delta and shore-shallow lake. (3) During the sedimentation period of Shawan Formation, there developed four types of sedimentary facies as gentle slope fan delta, shallow braided river delta, alluvial fan, coastal shallow-lake. Gentle slope fan delta is a sedimentary type between fan delta and braided river delta, its sedimentary particles is slightly finer than the normal fan delta, and it develops more braided river on the plain. Shallow braided river delta is more or less different from normal braided river delta, the former one does not exist Gilbert delta mode foreset bed, topset bed and bottomset bed. The shallow braided river delta develops no foreset bed, has slow slope gradient, its main sand body is underwater distributary channel, the river mouth bar is not well developed, the vertical sedimentary sequence is discontinuous and the sheet sand is widely spreaded.