The Longdong Areas Chang 6-8 Reservoir Group Deposition, Reservoir Characteristics And Fracturing Of Geological Research

The patterns of horizontal distribution and vertical overlapping, and shape and size of the reservoir sandbody are studied in the dissertation, on the basis of sedimentary system, sedimentary facies and sequence stratigraphic studies. The reservoirs are analyzed on the aspects of diagenesis, causes of pore formation, and their evolution. The author puts forward the controlling factors of the reservoirs, and forecasts and evaluates the favorable targets for oil exploration in the study area. The concept and method of geologic research for fracturing are also put forward, and deepening the understanding of the fracturing geology through the research.The major results and new findings are as follows.The system of fan delta, braided river delta, and meandering stream one is identified, of which the former two are mainly distributed in the western margin of the Ordos basin, and they are a system in which the facies belts are not completely developed. The fore ends of the fan and braided river delta often evolve into turbidite fan. The meandering stream delta, which has a large-scale sedimentary size, is mainly distributed in the northern, eastern, and SE regions of the study area. The common characteristics of the three delta is that the river mouth bar is not developed. The framework sandbody is underwater distributary channel.Demolishment and alteration of diagenesis for reservoirs of the Chang 6-8 are very obvious. The cementation and replacement are especially strong. Change of media conditions of water in the pores at different stages leads to intergranular pores filling up by the upper carbonate, siliceous, and authigenic clay minerals, consequently reduces the porosity and permeability of the reservoirs. Pores in reservoir Chang-8 are mainly secondary pores, of which the most are contracted intergranular pores and eroded micro-pores. The throat of these pores is narrow, of which the most ones are extra micro-ducts, thin-slice-shaped or bend slice-shaped throats and secondly are contracted ones.Heterogeneity of the reservoir porosity and permeability are studied mathematically by applying firstly the theory of fractal geometry. The accuracy of reservoir prediction has been further improved.According to the theory and method of sequence stratigraphy, the sequence stratigraphic frame of the Longdong (eastern Gansu) area is established through detailed analysis and studies of field outcrops and downhole data.The present area in which the exploratory wells are few and no important breakthrough has been analyzed on sedimentary facies, the favorable oil/gas-bearing facies belts are forecasted.The method of geologic research for fracturing is put forward in the paper. The author points out definitely that oil field may be found in the old areas in which prospecting for oil had been carried out in early stage through researches of the fracturing geology.A few point-of-views with regard to sedimentary features of oil reservoirs of the Chang 6-8 in the Longdong area, being put forward no the basis of studies of sedimentary facies, sedimentarysystem, reservoir characteristics, and sequence stratigraphy, changes the train of thoughts of oil exploration in the southern parts of the basin; that is, from exploring for oil in lacustrine facies to looking for hi-permeability belts along with the braided river deltas. Oil exploration in the regions of flanks of Xifeng, Maling, Chenghao, Huachi and Zhenyuan has been obtained great progress. Application of the method of fracturing geologic analysis to the old Ningxia, Jiyuan, and Baibao areas of Changqing oilfield has gained good results, and arose the leaders of Changqing oilfield to think much of the method.