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Assessments of undiscovered, yet-to-find, oil and gas are usually based on geological estimates of basin richness, untested basin segments and play intervals, as well as possible field size distributions in these targets. Recent advances in understanding diagenetic processes controlling the sub-surface evolution of porosity and permeability with increasing temperature and pressure have resulted in new models for hydrocarbon migration and entrapment efficiency, sometimes referred to as the 'Golden Zone'. These new models predict that entrapment efficiency for conventional oil and gas resources are strongly controlled by reservoir temperature. The impact of these diagenetic control, which are supported by global/basin discovery histories, have yet to be fully incorporated into commonly used yet-to-find assessment methodology. Because the optimal entrapment efficiency is confined to a fairly restricted 60C interval (60 to 120 C maximum burial temperature), this limits the number of large traps at the basin scale. Given the nominal observed global probability distribution of geothermal gradients in sedimentary basins (P10, P50, P90 = 40, 30, 20 C/km respectively), the optimal depth interval for petroleum entrapment is on average only 2 km thick. Furthermore, tectonic inversion of sedimentary basins, which is common for onshore basins, as well as other geological settings, can adversely affect the preservation of optimal traps and plays. Although the impact of this concept is still under investigation, empirical results suggest that assessment methods for yet-to-find hydrocarbons, which do not include these geological controls, could be too optimistic by a factor between 2 and 4. The 'Golden Zone' model, therefore, encourages a very selective, sub-surface geological approach, for assessment of the undiscovered potential, optimal basin segments, and play types. While it is important to broadly capture the geological opportunities present within sedimentary basins, it is also vital that assessment methods accurately estimate the likely findings of the exploration effort.
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