Volumetric Uncertainty: Monte Carlo

One Number Hides the Risk

Every volumetric input is uncertain, so a single STOIIP is just one point in a wide range. Monte Carlo propagates the uncertainty: it draws each input from its distribution, computes STOIIP, and repeats thousands of times to build the distribution of the answer.

P90, P50, P10

From that distribution come the reserves percentiles. By convention Pxx is the volume with an xx percent chance of being exceeded, so P90 is the low, conservative case, P50 the median, and P10 the high case: P90 is the small number, P10 the large one. The distribution is right-skewed because STOIIP is a product of uncertain factors, so the mean sits above the median.

Report the Range

The honest answer is the range, not a point. The P90 guards against optimism, the P10 caps the upside, and the P10-to-P90 ratio measures how uncertain the volume is. Widening any input distribution fans the range out, which is exactly what the next tool, the tornado, helps you target.