EXPERIMENTAL STUDY ON FRACTAL CHARACTERISTICS OF TIGHT SANDSTONE RESERVOIR TAKING CHANG 6 OIL LAYER IN ORDOS AS AN EXAMPLE

Taking Chang 6 oil layer in Yanchang, Ordos as an example, this study experimentally evaluates the fractal characteristics of tight sandstone reservoir. The petrology and oil-gas geochemical characteristics of the tight sandstone are analyzed by using high-pressure mercury injection, low-frequency nuclear magnetic resonance and field emission scanning electron microscopy. The pore structure characterization and fractal characteristics calculation of the tight sandstone are carried out. The results show that the rock type of Chang 6 is mainly feldspathic sandstone, the volume fraction of debris is 84.0-95.0%, and the debris components are mainly feldspar and quartz. The reservoir porosity is between 1.2-15.9% and the permeability is between 0.01 x 10- 3-3.27x10-3µm2. There is a good positive correlation between reservoir porosity and permeability logarithm of Chang 6 reservoir group. The results of FHH model method show that the fractal dimension of Chang 61 is the largest, followed by Chang 62 and Chang 63. The fractal dimension (Dwop) of all organic pores in FE-SEM images is the largest, followed by the fractal dimension (Dop>50) of organic pores with pore size greater than 50m and the fractal dimension (D0p4-50) of organic pores with pore size of 4-50m. The comparison between small layers also shows the same characteristics as the fractal dimension of gas adsorption pores calculated by FHH model method.