The initial step in the evaluation of any petroleum resource is the identification of an appropriate geological population that can be delineated through subsurface study or basin analysis. A geological population represents a natural population and possesses a group of pools and/or prospects sharing common petroleum habitats. A natural population can be a single sedimentation model, structural style, type of trapping mechanism or geometry, tectonic cycle, stratigraphic sequence, or any combination of these criteria. Reasons for adopting these criteria in the definition of a geological model are the following: • The geological population will be defined clearly and its associated resource can readily be estimated. • Geologists can adopt known play data for future comparative geological studies. • Geological variables of a natural population can be described by probability distributions (e.g., the lognormal distribution). Statistical concepts such as the superpopulation concept can be applied to geological models so that, for specific plays, an estimate of undiscovered pool sizes can be made. Figure 2.1 illustrates various sedimentary environments (tidal flat, lagoon, beach, and patch reef) that can be used as geological models in resource evaluation. Each of these models has its own distinguishing characteristics of source, reservoir, trapping mechanism, burial and thermal history of source beds, and migration pathway. In resource evaluation, to ensure the integrity of statistical analysis, each of these should be treated as a separate, natural population. Therefore, the logical steps in describing a play are (1) identify a single sedimentation model and (2) examine subsequent geological processes. Geological processes such as faulting, erosion, folding, diagenesis, biodegradation, thermal history of source rocks, and migration history might provide a basis for further subdivisions of the model. In some cases, two or more populations might be considered mistakenly as a single population because of a lack of understanding of the subsurface geology. If the resulting mixed population were to have two or more modes in its distribution, this could have an impact on resource evaluation results.
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