Friday, August 3, 2012

Probability of a Trap Not Filled to Capacity

We often estimate how much charge (hydrocarbons) is available to a trap, based on assumptions of source rock potential, maturity and migration losses, etc. The purpose is to see if the trap could be potentially not receiving enough charge relative to its capacity. Aside from huge uncertainties involved in such an exercise, I would argue here that it is relatively rare that a trap would be under charged.

Let's think about a basin that has a mature source rock providing hydrocarbons from the kitchen. Hydrocarbons may migratie up-dip in different directions. Along the fill and spill pathway, the volume may eventually run out, and the last trap is not filled to spill point. All traps further up dip will not receive any charge, and all traps down-dip from this trap are filled to spill point. Let's say there are 10 of these migration fairways and each has 10 traps, there can be only one trap along each fairway that is not filled to spill. Counting all possibilities of charge volumes, the probability of a trap receiving charge but not filled to spill is only one in 10 (10% probability for any trap being half full).
In a system with a good source rock, more of the traps are filled, while with a poor source fewer traps are filled. But in either case, most traps that receive charge are full.

The same is true in a vertically drained system, where traps are limited by seal capacity. Vertically through the stacked reservoirs along each fill-leak path, there can be only one trap that is not filled to seal capacity. All others are either not filled at all, or are filled to seal capacity.

Of course, nature is more complex and we may have a mixture of fill-spill, and fill leak scenarios in a typical basin. Trap geometry and seal capacity are also not constant through geological time. We may often have situations where trap capacity is not limited by structural closure, nor seal capacity, but by fault juxtaposition, etc. The above analysis should be valid in all possible scenarios, including the example mentioned in the earlier post (see below) where a trap can both leak and spill at the same time. Migration cannot continue unless the trap is filled to "capacity".

Richard Bishop, former president of AAPG, gave a presentation titled "Percent Trap Fill and Its Implications", in which he states: "Observations of hundreds fields in many different types of basins and source rocks shows that traps are full to either a leak point or spill point". I agree and perhaps there is a logical reason behind the observation. The implication here is that charge volume estimates may not be such a useful exercise. We should instead focus on the probability whether the trap can be charged, rather than whether it receives enough charge. Moreover, it may be also be useful to try to understand why if a trap is not filled to the structure closure, and its implications in finding accumulations in nearby traps.