My own experience is that marginal projects are marginal in every sense.  Except in the unique types of cases I mentioned above, of which there are few, marginal projects don't make an impact on even a single company's bottom line or overall production level, much less the gross production of an entire country (or the world for that matter).
Marginal projects don't much effect company profits but they do affect product prices. 1% of the oil supply determines 10% of the prices because it's the marginal 1% that sets the prices.
A good example is the oil shock of 1973 that reduced supply by 5% and increased prices by 100%. 1980 increased prices by 400% and was the result of an even smaller decrease in production.