Friday, March 4, 2011

Is Timing of Hydrocarbon Generation Really Important ?

Haven't posted for a while now. Here I would like to challenge another good old petroleum system concept. We have been told that if a trap forms after oil generation is finished, it will not be able to receive charge, or it will receive only gas charge. It is one of the responsibilities of the basin modeler to demonstrate the timing of oil and gas generation, relative to timing of trap formation, perhaps using what is called a petroleum system event chart like this one.

Many of us now realize this is not necessarily true. Actually, if we believed this concept, we might have missed a lot of big important petroleum discoveries.   Below are some examples that contradict the theory:

1) Perhaps a good example is the Bohai Bay, where Phillips Petroleum made a big discovery in 1999 at only about 5000 ft in the Minghuazhen/Guantao formation ( check out the story here). What is interesting to me is that the reservoir was merely deposited about 5 million years ago. This would mean that the oil has to have been generated in the last couple of millions years if we allow the reservoir to be deep enough to have a seal. Right? Well, no, geo-history modeling shows that the Shahejie 3 source rock in the kitchen went through the oil window about 23 million years ago, currently at about 8 km deep and 300 °C, and the reservoir contains low maturity oil!      

2) In deep water Gulf of Mexico, the Jurassic source rock is currently 35,000 to 45,000 ft deep near many big fields and models show oil generation occurred about 15-10 million years ago and the source is currently in the "gas window" under these fields. The Miocene reservoirs are deposited about 9 million years ago, yet they contains very low maturity oil. 

3) The Foinaven and Schiehallion fields in the West Shetlands basin contain under saturated oil in the Paleocene reservoirs. Again, the basin model shows that oil generation happened late Cretaceous, prior to the reservoir deposition. A "Motel model" (oil had to migrate to a parking lot and wait for trap formation) was used to explain the apparent timing mismatch (Lamers and Carmichael, 1999). Interesting, isn't it. 

I can list more examples, but suffice to say, this seems to become a norm rather than exception. Perhaps we should rethink about how important timing of generation is ? I actually argue that these fields are probably still receiving charge today.

Perhaps the companies which discovered the fields in these examples did not listen to their basin modelers ? How could they have predicted oil in the reservoirs based on the so called petroleum system event chart ? I think at least in the deep water of Gulf of Mexico, certain large oil company may have listened to their modelers and missed most of the action that lead to the discovery of many multi-billion barrel fields there.    


4 comments:

  1. Interesting and thought provoking article, as usual, Dr Beta Factor. One nit to pick: the apparent fact that the SR is more mature than the reservoired oil does not mean that timing of generation is unimportant, does it? More accurately, it means that the relationship between timing of generation and timing of charge is more complex than often assumed. And that timing of generation has to be coupled with an understanding of migration, and especially of migration losses, in order to properly predict charge.
    Maybe you should develop an events chart tool for the industry that includes migration lag...

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  2. I might add that if we see all these examples of a long lag between timing of generation and timing of charge, we may need to shift our attention to the opposite problem of generation happening too late. The fact that you have active generation in the fetch area today does not guarantee that sufficient quantities have been expelled and migrated to the trap.

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  3. hello beta factor,
    can you please give me some references from publications. Thanks

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  4. Capillary forces, yes good point - more often than not when we do not have a supper source rock to push the HCs into traps further up dip or up section. We drill dry holes.

    Amda, I am afraid I am not aware this is published, but some folks in the industry do try to take this into account in their petroleum system assessment.

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