Implications of source overcharge for prospect assessment

Abstract

A fundamental aspect of prospect evaluation is whether the trap volume or the charge volume limits the volume of trapped hydrocarbons. Traps filled to a leak point are full traps, although I rarely describe them as such. I commonly say “full to spill” but rarely do I 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., the charge exceeding the trap volume) occurs in basins that vary widely in age and tectonic setting. Perhaps surprisingly, this is true for 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 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 a gas cap and an oil leg. Furthermore, the occurrence of oils saturated with gas likely indicates selective leakage of free gas. Hydrocarbon contacts (whether oil-water, gas-oil, or gas-water) are interpreted to define the leak or spill point or seal capacity. Thus, instead of using continuous statistical distributions to describe all elements of traps, 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, leads to the notion that risk evaluation might consider the number and quality of potential leak points.