--> Implications of Source Overcharge for Prospect Assessment


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Implications of Source Overcharge for Prospect Assessment


A fundamental aspect of prospect evaluation is what limits the volume of trapped hydrocarbons. Is it trap volume, or the charge of hydrocarbon arriving at the trap? Traps filled to a leak point are full traps, although we rarely describe them as such. We commonly say ‘full to spill,’ but rarely do we hear ‘full to a leak point.’ Why not? A summary of literature from fault leakage, seeps, field studies, and theoretical source- yield calculations illustrates the implication that source overcharge (i.e., charge exceeding trap volume) occurs in basins that vary widely in age and tectonic setting. Perhaps, surprisingly, this is true for both oil and gas fields and for a wide range of source rock quality from rich to lean. The most obvious implication from source overcharge is that the volume of trapped hydrocarbons is limited by the absolute volume of the trap. Less obvious is the recognition that if both oil and free gas are available to a trap, gas will displace the oil. Thus, if there are no gas leaks, the trap will contain only gas. If there is preferential leakage of gas, then the trap may contain both a gas cap and an oil leg.

Furthermore, the occurrence of oils saturated with gas likely indicate selective leakage of free gas by the top seal. Hydrocarbon contacts (whether oil-water, gas-oil, or gas-water) are interpreted to define the leak or spill point. Thus, instead of using continuous statistical distributions to describe all trapping elements, some elements (such as area) are more appropriately described as discrete values and a full assessment may be a combination of discrete plus continuous statistical distributions. Overcharge may also lead to different interpretations of risk. Interpreting the trap volume, particularly with leak points, gives rise to the notion that risk evaluation might consider the number and quality of potential leak points.