Reservoirs in block 4 of Tahe oilfield are mainly distributed in Ordovician weathered crust and appear as irregular chicken-wire pattern.Solution cavities and fractures are the main types of reservoir space.The height of most solution caviteis is less than 2m

and the formation intervals with average fracture width of over(0.1 mm) account for 58% of the total.Areal distribution of reservoirs is related with the development of fossil karst.Fossil karst channels appear as beaded strong seismic reflection

and magnitude differences exist between deep and shallow laterolog anomalies.Monte Carlo method is used to calculate reserves of fractured-vuggy reservoirs.One way is to achieve the probability distribution of porosity

net thickness and oil saturation based on interpretation of log data.Another one is to achieve reserves distribution of each well through assuming that the values of porosity

net thickness and oil saturation are accurate and the area of reservoir body accords with certain probability distribution.Tahe oilfield is a massive oil pool with bottom water

being communicated with deep fossil karst water channel system.Rational development of fractured-vuggy carbonate reservoirs should take fracture-vug units as bases

cyclic waterflooding and lateral drilling as the principal measures

and "one well one scheme" as the guiding principle.