Fractal features of pore structure are studied by using analytic data of capillary pressure of core samples.The fitting relationship between the volume percentage(S)of pores with pore diameter smaller than a given value(r)and the pore diameter(r)is used to calculate the fractal dimension of pore structure of the low porosity and permeability reservoirs in the Triassic Yanchang Formation and Jurassic Yan'an Formation of MHM oilfield

Ordos basin.The capillary pressure curves of both formations have a scale interval

in which pore structure shows fractal features.The macropore and micropore intervals of same core sample show multi-fractal features.For the 10th member of Yan'an Formation(Yan 10 reservoir)

the fractal dimension of pore structure in macropore interval is larger than that in micropore interval

while for the 6th member of Yanchang Formation(Chang 6 reservoir)

the distribution of fractal dimension is just the opposite.In macropore interval

the fractal dimension of Yan 10 reservoirs is smaller than that in Chang 6 reservoirs

indicating that the pore structure of the latter is more complex than that of the former.Fractal dimension of pore structure

in conjunction with the other pore characteristic parameters

can well reflect the complexity of microscopic pore structure.It provides a convenient and effective method for study on the microscopic heterogeneity of reservoir pore structure

and a solid basis for further reservoir evaluation.