Abstract: In order to standardize overseas oil reserves evaluation,a method for classifying and categorizing overseas lithologic reservoir reserves,determining lithologic boundary,o il bearing area and effective thickness was proposed. The results show that:（1）Reserves classified as P generally require a development plan approved by the company or the government of the resources country. P1 reserves are calculated based on wells with commercial production or commercial test flow rates,and their area is usually determined by the drainage area of the production wells and the range limited by the known gas bottom or the known oil top and oil bottom. The reserves within half the distance from the P1 reserves boundary to lithologic boundary are P2 reserves,and the reserves within the range from P2 reserves boundary to lithologic boundary are P3 reserves. If there is no oil water contact within the trap or cannot be determined yet,P3 reserves should be determined based on the spill point of the trap. If there are significant changes in lithologies or reservoir physical properties,P2 reserves can be determined by extrapolating one development well spacing based on P1 reserves,and P3 reserves can be determined by extrapolating one development well spacing based on P2 reserves.（2）When the distance from a class P reserve well to the pinchout line of a permeable reservoir is no more than 3-4 times the development well spacing,for medium to high porosity and permeability reservoirs,the pinchout line can be directly determined as the lithologic boundary,while for low porosity and permeability reservoirs,the minimum effective thickness contour line that can meet the class P reserve standard is determined as the lithologic boundary. The oil and gas bearing area should be comprehensively delineated by lithology boundaries,oil（gas）water contact,tight layer sealing zones,etc,the wells within the area should meet the class P reserve standard. For oil and gas reservoirs with identified fluid interfaces,the fluid interfaces used to delineate oil and gas bearing area should be confirmed by drilling and coring data or testing data. For oil and gas reservoirs with unidentified fluid interfaces,the oil and gas bearing area should be determined by extrapolating the lowest confirmed bottom boundary of the oil and gas producing layer or the effective thickness value through testing.（3）The determination of the effective thickness of P1 reserves should have reliable formation testing data or sufficient logging data and have demonstrated its production capacity. The determination of the effective thickness of P2 reserves usually lacks conclusive testing data and has not confirmed its production capacity. Due to the uncertainty in rock physical interpretation,there is significant uncertainty in the effective thickness of P3 reserves.（4）It is recommended to use the volumetric method for evaluating lithologic reservoir reserves. The application example in TP oilfield in Ecuador has confirmed the effectiveness of the classification and categorization of lithologic reservoir reserves and reserves evaluation method.

Key words: overseas reserves evaluation, lithologic reservoirs, reserves classification and categorization method, SPE-PRMS, lithologic boundary, oil-bearing area, effective thickness, volumetric method, TP oilfield in Ecuador