| Longdong area is located in the southwest of Ordos Basin,and its present structure is a gentle west dipping monocline,which is terrigenous clastic fluvial lacustrine deposit.The Chang 7 reservoir of Upper Triassic Yanchang Formation in Longdong area belongs to tight sandstone reservoir.The development degree of fractures greatly affects its oil and gas distribution and single well productivity.In this paper,the outcrop profile,core,cast thin section and imaging logging data are used to quantitatively characterize the fracture types and distribution characteristics of Chang 7 oil formation reservoir,identify the main controlling factors of fracture development,analyze the fracture formation stages,and quantitatively predict and comprehensively evaluate the fracture distribution law combined with rock mechanics test and numerical simulation.Based on the analysis of outcrop area,drilling core,cast thin section and imaging logging data,it is found that fractures are generally developed in Longdong area.In macro fractures,tensile high angle structural fractures are mainly developed,while in micro fractures,transgranular fractures are mainly developed.The overall development degree of fractures is high.Four groups of fractures are mainly developed in Longdong area,among which the NE trending fractures are the most developed.The average porosity is 0.2%,and the average permeability is 107.8×10-3μm2.The development of macro and micro fractures greatly improves the pore connectivity and overall performance of the reservoir,which is of great significance to the reservoir space and permeability of tight reservoirs.Lithology,lithofacies,rock brittleness and rock mechanics layer are the main factors affecting the fracture development.At the same time,with the increase of brittle mineral content in rock,the strength of fracture development also increases.The rock mechanics layer has obvious control effect on the fracture,which makes it develop inside the rock and terminate at the bedding plane or lithologic interface.The distance between fractures is positively correlated with the thickness of the rock mechanics layer.According to the statistics of fracture density of different diagenetic facies,calcareous sandstone facies is the most developed fracture,followed by dissolution sandstone facies,and the fracture development degree is poor in sandstone facies and mudstone facies with rich soft complex foundation.The Chang 7 oil-bearing formation mainly experienced two tectonic movements,and the fracture formation period was Yanshanian tectonic period and Himalayan tectonic period.Through rock mechanics experiment,the physical parameters of rock are tested,and the brittleness of rock is evaluated.Among them,calcareous sandstone is more brittle.Based on the analysis of outcrop,core,thin section and logging fractures,a mechanical and geological model is established.Combined with petrophysical parameters,the paleotectonic stress field during the fracture formation period in Longdong area is simulated.Combined with the core natural fracture distribution law and rock fracture criterion,the distribution characteristics of structural fractures in tight reservoir of Chang 7 oil formation in Longdong area are quantitatively predicted.By comparing the fracture prediction results with the fracture development intensity observed by core fracture,the relative error is less than 30%,which indicates that the predicted reservoir fracture results have high reliability. |
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