2018 Volume 83 Issue 1 Pages 67-76
More than 30 years has passed since quantitative and probabilistic geological risk analysis was introduced to the oil and gas exploration industries, which is a method composed of two processes 1） to get Chance of Success （CoS） by multiplying grouped geological factors （trap, reservoir, source rock and so on） that are essential for hydrocarbon accumulation in the prospect and 2） to get probability density function （PDF） of resources by running Monte Carlo simulation.
However, it is difficult for some prospects to apply above steps simply due to their geological complexities, which include multi-reservoir, multi-segment and multi-scenario.
Multi-reservoir prospects have more than one target reservoir. In this case, CoS and PDF for resources should be calculated for each reservoir at first, then combined CoS （probability of at least one reservoir has hydrocarbon accumulation） and combined PDF （weighted summation of PDFs for all cases for single or multiple reservoir discoveries with relative probabilities）. Dependencies of geological risk factors and resources calculation parameters between target reservoirs have to be considered.
Multi-segment prospects are separated to more than one segment by faults or other geological obstructions, where combined CoS and PDF for resources can be calculated like as multi-reservoir prospects.
In multi-scenario prospects, there are more than one aspect （scenario） for any of geological factors required for hydrocarbon accumulation. For example, some 4-way dipping anticlinal traps becomes fault-dependent 3-way dipping traps when hydrocarbon-water contacts are deeper than specified depth. In such prospects, probability that the fault seal works can give relative probabilities for both scenarios and separated PDFs for both scenarios can be combined by using those relative probabilities.
Reasonable geological risk and uncertainty models can be constructed for most of prospects in oil and gas exploration businesses with careful combination of above three types of geological complexities.
