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Lookup Tables for Estimation of the Probability of Geological Success (PoS) for Segments with Conventional Oil and Gas


Subsurface interpretations during petroleum exploration result in a prospect inventory, in which each prospect is characterized by prospective resources for the success case and the probability of geological success (PoS). Estimation of prospective resources for the success case is a relatively straightforward task, even though it has many caveats (Rose, 2001). However, estimation of the geological PoS is a more challenging endeavour. In many exploration organizations, estimation of the geological PoS is a highly subjective process which depends on explorers' education, knowledge, specialisation, previous experiences, motivation, salesmanship and other biases. Geological PoS values are often estimated inconsistently across the organization, which leads to a biased prospect inventory, sub-optimal drilling decisions and unpredictable long-term exploration performance. Sasol Petroleum International developed and implemented lookup tables for estimation of geological PoS for segments with conventional oil and gas. In this approach, each of the evaluated six geological risk factors (structure, reservoir facies, reservoir deliverability, seal, mature source rocks, and migration) is assigned a probability value using specified criteria based on available data and models. Positive, negative and neutral evidence for each risk factor are considered. Data-based evidence is derived from wells and geophysical data. Model-based evidence is derived from structural interpretations, maps of Gross Depositional Environment, source rock maturity maps and other subsurface models. The probability of each risk factor is maximum (as high as 1.0) when both data and model point to positive evidence for that factor and is minimum (as low as 0.05) when both data and model point to negative evidence for that factor. The total geological PoS value for the segment is then calculated by multiplying probabilities of each of the six risk factors. Lookup tables provide an explicit basis for the assessment of geological PoS, reduce subjectivity in the assessment, remove biases, ensure that no relevant criteria have been neglected in the assessment process, boost internal company consistency between assessments by different exploration teams and enable efficient exploration portfolio management at the company level. In addition explorers gain management's confidence as they demonstrate that risk assessment is transparent, consistent and rigorous.