Study On Reservoir Heterogeneity Of Yanchang 8Th Formation In Zhenbei Area

The braided delta-lake deposits developed in the Yanchang 8th formation in Zhenbei area of the Ordos Basin.Although with smooth tectonic features,the reservoir has strong heterogeneity under the combined influence of sedimentation as well as complicated and intense diagenesis.And the oil layer is difficult to explore and develop with the characteristics of poor reservoir properties and complex pore structure.Therefore,this paper tookl study of the heterogeneity of reservoirs at the study area in order to provide effective guidance for optimizing the effects of injection and production development of low-permeability reservoirs and improving the recoverable reserves.This paper took Chang 8₁ sand group as the research object.Using the methods of cycle contrast,grading control and lithology-thickness contrast,the stratigraphic layer was subdivided and correlated finely.Using rock signs and well-logging signs,and according to the research of microfacies in single well,profile and plane,the study of the characteristics of the sedimentary microfacies in each layer was carried out.Scanning and analysing the rock and mineral characteristics,diagenesis and pore-throat structure with the data of rock slices,electron microscopy and mercury pressure,the study of the characteristics of microscopic heterogeneous features of reservoir were made out.With the analysis of core data and logging interpretation data of 86 wells in the study area,macroscopic heterogeneous characteristics were traditionally analyzed through the distribution and variation of permeability and coefficients of variation within layer,interlayer and plane.Based on the various factors above which affect the heterogeneity of the reservoir,the heterogeneous composite index was calculated,so that the heterogeneous characteristics were further quantitatively described.The main achievements of the research show that the depositional system of study area was braided-delta front subfacies in which the microfacies of underwater distributary channel and interdistributary bay were widely developed.Rock characteristics in study area showed that the rock was in low composition maturity and moderate structural maturity.The clay minerals and carbonate minerals were occupied a larger proportion of the filler,and this susceptible mineral were effected by diagenesis which resulting in strong heterogeneity.According to the mezzanine characteristics,it was pointed out that the pore structure is complex and heterogeneous of the layer was clear.The regularity of the high frequency interlayer distribution in the layer is poor,which resulting in poor fluid seepage ability.As the distribution of interlayers is stable,the permeability changes significantly,which lead to the strong heterogeneity between layers.According to the characteristics of physical properties,coefficient of variation and sand body thickness,it was found out that the planar heterogeneity was enhanced from the center of underwater distributary channel to both sides and from the southwest to the northeast of the study area.Based on the above influence of factors on heterogeneity,we used the entropy method to calculate the heterogeneity comprehensive index to balance the limitations of each parameter.Finally dividing that into four levels to quantitative characterization of heterogeneity of layers and planar in each layers.