The Characteristics Of Reservoir Sand Body Development And Advantageous Region Prediction In Chang 8 Formation In North Of Ji Yuan Area

Based on the painstaking observation of field section and core, combined with the well logging data, the experimental data and the mathematical statistics methods, this paper analyzed the characteristics of reservoir sand body development of the Chang 8 Formation in North of JiYuan Area, Ordos Basin, including the sedimentary characteristics, the characteristics of sand body distribution, the types of sand body and the characteristics of sand in sequence stratigraphic framework. Studied by making full use of the Cathodoluminescence, X-ray diffraction, Scanning Electron Microscopy and core data, analyzed the diagenetic minerals, diagenetic environment, diagenetic stage and diagenetic evolution sequence of the Chang 8 Formation. Combined with the feature of sand body development and the characteristics of the diagenetic lithofacies, we predicted the favorable areas. The mainly conclusions are as followings:A shallow-water delta depositional system was developed in the Chang 8 Formation in North of Ji Yuan Area, the sandstone types are lithic feldspar sandstone and feldspar lithic sandstone, the particle size is mainly mid-fine sandstone, and the way of carrying was given priority to brisk. The compositional maturity and textural maturity of mineral are medium-poor, and the composition of authigenic clay minerals mainly are kaolinite and chlorite. The composition of cement mainly is calcium, which present as iron calcite, then is the siliceous cement; the porosity and permeability are low, attributed to a low porosity-permeability reservoir.Both the cumulative thickness and the thickness of single sand body, as well as the corresponding characteristics curve shape of well logging of the sand were reflected the differences and changes of the Chang 8 reservoir sand body. The sand body of largest thickness of Chang 82 and Chang 81 are 37.9m and 34.9m, respectively. The minimum thickness is 1.1m, concentrated in 2~4m. The average thickness is getting smaller gradually, become 11.20 m, 10.43 m successively. The GR logging shows that the Chang 81 and the Chang 82 most present bell shape and box shape, the shapes of bell and box in Chang 82 are more developed than Chang 81, and the shapes of funnel and finger in Chang 81 are better. The plane distribution of Chang 8 reservoir sand body shows an banded distribution extend from southeast to northwest, the Chang 82 are mainly channel sand body which is thick and narrow, the Chang 81 reservoir group are mainly channel sand body, mouth bar sand body and sheet sand body, which are relatively thin and wide.The study found that the sedimentary environment of Chang 8 mainly is delta front deposition. Chang 8 reservoir sand body types mainly are distributary channel sand body, mouth bar sand body, sand sheet and their combination. Underwater distributary channel and mouth bar caused sand mainly are medium-fine grained lithic arkose, poor sorted, have a high lithic content and a strong compaction, and it is property is ordinary. Sand sheets are mainly fine-grained feldspar sandstone, well separated, and the content of plastic lithic is low. It is a high quality reservoir in the region.The study suggests that Chang 82 Oil Formation is located in the upper of SQ2, attributed to the highs system tract sedimentary in initial expansion period of the lake basin, belonging to regressive system tract. The Chang 81 is located in the lower part of SQ3, attributed to the sedimentary of lacustrine transgressive system tract and poor low-stand system tract, belonging to transgressive system tract. Chang 8 reservoir have different cycles of different sand body combination, be divided into three types. We recognized three kinds of sand body architectures, including the stacked channel type, isolated channel type, sheeted channel type.The digenesis of Chang 8 reservoir is complex. The study shows that digenesis is compaction, cementation, dissolution and rupturing. The dissolution and rupturing played constructive roles which improve the property of reservoir, while the compaction and cementation destroyed the property. The cementation of chlorite films has double effects which not only reduced the porosity but also restrained the growth of the authigenic mineral, but limited in the protection of primary pore.Based on the analysis of diagenetic types, intensities, minerals and combined with the sedimentary background, the diagenetic facies in study area should be divided into compaction diagenetic facies, chlorite films diagenetic facies, feldspar dissolution diagenetic facies and carbonate cementation diagenetic facies. The compaction diagenetic lithofacies and carbonate cementation facies are destructive diagenetic facies which is harmful to reservoir development; the feldspar dissolution diagenetic lithofacies and chlorite films diagenetic facies are beneficial to reservoir development, be called constructive diagenetic facies.According to the prediction of favorable area, the areas of Yan50 well, Yan67 well are the favorable reservoir areas in Chang 82 Formation, the forecasting area is 110km2. The favorable reservoir area in Chang 81 reservoir are the areas of Yan37 well—Feng11 well, the predicting area covers 220km2.