The Global Oil System: The Relationship Between Oil Generation, Loss, Half-Life, and the World Crude Oil Resource: Discussion

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Miller (1992) proposed a new approach to assessment of the world's ultimate recoverable oil resources. Miller's approach generates an estimate of reservoired conventional oil by combining an assumed exponential decay from seepage and cracking of oil with estimates of global steady-state oil generation and expulsion into large reservoirs. Although recognizing that his approach is conceptually innovative, it is important to point out serious difficulties with the underlying assumptions and data that diminish the utility of the method and the estimate that he reported. The comments focus on what is considered to be three principal problem areas: (1) treatment of uncertainty, (2) underlying assumptions, and (3) insensitivity to temporal and spatial heterogeneity in the geology of petroleum basins. It is agreed that none of the presently used methods of resource assessment provide highly reliable and accurate results. However, this inaccuracy stems not so much from poor methodologies as from the limits on the quality and completeness of the data available to the assessors. In general, the more and better the data available, the more certain are the assessments. Most assessment methods currently in use require tremendous amounts of data. Moreover, they use the experience and knowledge about regional geology gained and refined by generations of petroleum explorationists and researchers. It is the authors' contention that oil and gas assessment is best constrained by use of specific geologic knowledge of regions, basins, plays, and fields within the limits of current geological knowledge and models. This type of region-specific geologic knowledge and experience is not used in Miller's approach. Instead, his method relies on derivative variables expressed as world-wide averages within a basic material balance equation modeling exponential decay: system size (i.e., world oil resources) = (half-life [of oil resources] [times] system filling rate)/0.693.

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