opec lost 700bn , page-4

Not true. You can maximise earnings by adjusting production, especially if prices are close to marginal production costs and you're the price giver (ie OPEC).

Say production costs $50 and the price is $51, and your quantity is 1000 units. Your profit is 1000 x (51-50) = $1000

Now imagine you can cut production but that the demand curve stay the same. You go up the demand curve and achieve a higher price.

So you produce only 500 units but you achieve a price of $54, quite a conservative price increase I think for halving production.

Your profit is 500 x (54-50) = $2000: you've doubled your earnings.