Gross Rock Volume (GRV) constitutes the largest uncertainty for probabilistic volume assessments of undrilled prospects. It is therefore of the greatest importance to understand this uncertainty well and to capture it realistically in probabilistic prospect volume assessments.

The parameters that contribute to the overall GRV uncertainty include time-depth conversion, reservoir thickness, trap compartmentalization and uncertainty of hydrocarbon (HC) column length. Of these, it is this last factor (column length uncertainty) that generally has by far the largest effect on the total GRV uncertainty range. When discovered HC volumes fall outside the predicted range of prospect volumes, in many cases it is because of errors in the column length prediction: the total uncertainty range tends to be not fully captured, and the mode of uncertainty distribution is too often a “hoped-for” (possible) value instead of a geologically sound most likely estimate.