PANEL DISCUSSION: World Reserves of Oil and Gas

Panel Discussion 12 consisted of two parts. In the first part 3 papers concerned with methods of reserves estimation have been presented:

1. A. M. KHALIMOV and M. V. FEIGIN, 'The Principles of Classification and Oil Resources Estimation';

2. G. J. DeSORCY, 'Estimation Methods for Crude Recoverable Reserves of Oil and Gas';

3. C. BOIS, H. COUSTEAU, A. PERRODON and G. POMMIER, 'Méthodes d'Estimation des Réserves Ultimes'.

Paper 1. The lack of common criteria for oil resources classification together with the existing differences in terminology and definitions lead to uncorrelated results of oil resources evaluation in individual regions. Oil resources classifications developed in the USSR and in the USA are most widely adopted. Both are based on the same principles: the degree of geological assurance and economic expediency of production. The new classification of oil resources proposed is based on the following principles:

1. the degree of geological assurance of resources evaluation;

2. economic significance of resources;

3. the maximum percentage of field reserves produced and the highest degree of bowels exploitation;

4. economic expediency of current production of reserves (profitability).

'Discovered resources' include the following three groups:

1. 'recovered reserves' (cumulative production);

2. 'explored reserves';

3. 'reserves in unexplored area within discovered pools'. ‘Explored reserves’ are those explored to the extent sufficient for their production to commence.

Depending on the stage of industrial utilization, this subgroup is subdivided into ‘developed’ and ‘undeveloped’ reserves.

Appraisal of ‘undiscovered oil resources’ results from numerical evaluation of petroleum potential of different geological units. The report also deals with the basic terms and definitions applied to classify and evaluate oil resources.

Paper 2. Volumetric methods for estimating inplace reserves are useful during early life or for small reservoirs but the expected accuracy cannot be better than *lo% and frequently ranges to greater than *20%. Properly applied material balance calculations, particularly if incorporated into reservoir models, are a better estimation method, but, again, individual reserve estimates cannot be expected to be more accurate than *lo%. Reserve estimates based on production history are typically less accurate than the previously mentioned methods.

Recoverable reserve estimates approximately as accurate as in-place estimates can be obtained from detailed reservoir depletion projections and economic forecasts. However, when recovery estimates are based on qualitative assessments such as correlations, they may add 120% to the range of uncertainty.