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.

3 comments:

  1. Thanks Zhiyong for opening this discussion up (or perhaps we should thank Richard Bishop ?). Migration and volume aspects are one of the most difficult things to explain to our exploration community clients when assigning charge risk. I think this is partly because it is the thing we least well understand ourselves and partly because it is a topic which readily generates confusion. I liked Richard Bishops talk but did anticipate one aspect of this confusion it might foster, ie, that people would take away this message: "Most if not all traps we discover are fill to spill" (correct for traps which are recorded and examined as discoveries) and therefore.. charge volume limitation is never a problem we need to consider when risking prospects. Clearly not true but I've already seen this conclusion drawn. The further a trap is from the kitchen and the more tortuous the migration/fill/spill pathway, the greater the risk of drilling a dry hole. The risk is reduced for a trap along way from the kitchen in LATERAL migration if the seals are high capacity and increased in the same circumstances for VERTICAL migration.

    I also think we need also need to be a little careful in using drilling results to ground truth our charge predictions, at least in circumstances where volume limitation may occur. To detect an underfilled trap we would need to drill it near the crest - otherwise it will just be recorded as a dry hole. Dry holes don't often get much follow up to see if there may be attic volumes present. Some companies prefer to drill in a position which would prove the P50 volume and, for a big trap, this could leave significant volume undetected - perhaps enough to have charged a smaller trap on the same fill/spill chain. Also, companies which are very driven by direct hydrocarbon indicators (DHI) from seismic, tend to selectively drill traps which (a) are shallow enough to give clear DHIs (b) show conformance of the DHI to the prognosed structural trap (ie. fill to spill). We don't sample the charge system in an unbiased way...

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  2. Really nice post thanks for posting that stuff. I like that kind of amazing and useful posts thanks for sharing.

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  3. This is really thought provoking and has increased my understanding of the filling of traps. Thanks

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