Abstract: The M20 condensate gas field is a complex one with abnormal high pressure. Condensate gas is found in the bioclastic dolomite of Shahejie 1, the dolomitic conglomerate of Shahejie 2, the Mesozoic volcanic rock and the Proterozoic granite. There are three kinds of reservoir in the field, i.e. the porous in the Shahejie, porous and fractured in the Mesozoic and fractured plus porous in the Proterozoic. According to reservoir distribution, type of reservoir space and oil and gas distribution pattern, three principles for division of reserves calculation units are established and the relationship between Mesozoic reservoir and the unconformity, reservoir porosities and their changing trends determined. Then we use the Neutron density method and the Archie formula to calculate the porosity and hydrocarbon saturation for the Shahejie, Mesozoic and Proterozoic reservoirs respectively. For fractured reservoirs, the state discrimination experience formula, area ratio method and fracture water saturation empirical formula are adopted to calculate porosity and oil, and gas saturations. Only the geological reserves in pore space is calculated for the Shahejie and Mesozoic, and the geological reserves in both pores and fractures are calculated for the Proterozoic.