Fracture Prediction of Deltaic Depositional Systems: A Case in C6₃ Oil-layer of Triassic Yanchang Formation of the Ordos Basin, NW China

The study area located in the southern of the Ordos Basin, has developed the delta front slumping turbidity fan, is relatively gradual in north and uplift in South. The average porosity in C6₃ oil-layer of Triassic Yanchang... [ view full abstract ]

The study area located in the southern of the Ordos Basin, has developed the delta front slumping turbidity fan, is relatively gradual in north and uplift in South. The average porosity in C6₃ oil-layer of Triassic Yanchang Formation is 10.8% with an average air permeability 0.34mD, which is a typical low porosity-ultra-low permeability hydrocarbon reservoirs. Field geological survey, coring and well logging data show that the natural fractures is highly developed in the area, which affect the oil exploitation and water injection.
In this research, we analyzed the characteristics of fracture and tectonic stress by using the Discrete Fracture Network(DFN) method. Based on logging curves of 385 wells, we built the correlation between conventional logging curve and imaging logging curve, and then, we amplified the response characteristic of fractures in conventional curves by using the fractal theory. Via equation“ Intensity（depth）=（（cumulative（depth）+w/2）-（cumulative（depth）-w/2））/w” (w indicates the length of the statistic window), we can get the fracture density log of all wells. Considering that the fracture density log is a continuous variable, we unscaling it with arithmetic mean method, and made the variation function analysis. Then, we took the attribute body of regional tectonic stress distribution that computed by finite element numerical simulation method as a restriction, and established the three-dimensional geological model of fracture density using sequential Gaussian simulation algorithm. Based on the statistical analysis results of fracture morphology parameters and the data of fracture geometry and azimuth, we stochastically simulate the fractures using the DFN fracture network modeling method, under the constraint of 3D geological model of crack density. After upscaling the fracture model to the uniform three-dimensional mesh, we obtained porosity and fracture permeability model of the fractures in the study, which provides a basis for reservoir numerical simulation.
Simulation results show that fractures distribute at directions NNW and NNE in the study area, and the NNW fractures formed at Yanshan geological period played are dominate. The fractures in north are more mature than that in south. In addition, the sedimentary microfacies which has different brittleness index of rock affects the development of the fracture. The fractures in the sandstone and calcareous silty sandstone which have high brittleness index are more developed, compared with the mudstone.
The simulation results in this study have more importance and significance for the flow mechanism of oil and water, which is instructional for the oil exploitation and water injection.