Abstract:

At present, Archie’s formula with variable cementation exponent（m） and saturation exponent（n） is still applied in quantitative evaluation of saturation of low resistivity oil reservoirs as one of the main methods. However, the influencing factors of m and n in low resistivity oil reservoirs are varied with area due to the different causes of low resistivity oil reservoirs. Therefore, the study on influencing factors and variation laws of m and n in Archie’s formula in low resistivity oil reservoirs in different areas is necessary. Samples of low resistivity oil reservoirs are cored in Putaohua Formation of southern Golong area according to division standard of low resistivity oil reservoirs, and also have definite variation of lithology, physical property, electric property. Then, petrophysical experiment of one sample is designed. A plot of m and n with parameters of petrophysics, pore structure, excess clay conductivity is made from petrophysical experiment data. The analysis shows that the values of m and n in low resistivity oil reservoirs is less than those of m and n in conventional oil reservoirs, and m and n in low resistivity oil reservoirs increase as porosity and permeability composite index, median radius, mean radius, radius mean increase. The m of low resistivity oil reservoirs decreases as shale volume fraction, cation exchange capacity, the ratio of micropore volume and movable fluid volume increase, and increases as T₂ geometric mean value increases, while the n of low resistivity oil reservoirs increases as porosity, the ratio of porosity and shale volume fraction increase. The m has a good correlation with T₂ geometric mean value, the ratio of micropore volume and movable fluid volume in low resistivity oil reservoirs, while the n has an excellent correlation with porosity and permeability composite index, mean radius, maximum radius, radius mean. The result reveals that pore structure has a larger effect on m and n of low resistivity oil reservoirs, and also shale volume fraction has a larger effect on m of low resistivity oil reservoirs, and T₂ geometric mean value or the ratio of micropore volume and movable fluid volume can be used to determine the mof low resistivity oil reservoirs, and porosity and permeability composite index, average radius, maximum radius, radius mean can be used to determine the n of low resistivity oil reservoirs.

Key words: low resistivity oil reservoirs, cementation exponent, saturation exponent, petrophysical experiment, pore structure parameters