Abstract: This study of mercury injection curves from cores of extra low permeability reservoirs in Y and Z blocks in Ansai oilfield shows that the reservoirs are of singular and multiple fractal natures using fractal geometric formula with two parameters. The linear relation assumes between the mean characteristic parameter, sorting coefficient, variation coefficient of pore structure by rectangular method and the fractal dimension by integrated linear regression, indicating that the fractal dimensions can be used to quantitatively describe the heterogeneity of reservoirs. Also, the microscopic analysis of pore structure with different fractal feature comes to the conclusion that the reservoir rocks with similar fractal nature have similar pore type, depositional environment and diagenetic process, and the fractal dimension will increase with pore throat radius (or total number of pore throats) decreasing. Therefore, it is proposed that application of fractal nature to classification of deposition-diagenesis facies is probably a feasible method.

Key words: fractal dimension, pore structure, low permeability reservoir, diagenesis, depositional environment