Hon Indus Pakistan,
1.5-billion bbl OE reservoir may be below our expectations but even "IF" that much is actually found it is something to be happy about. At today's prices, 1.5-billion bbl oil in place is worth about $100-billion!
Additionally, you would not exhaust the reservoir in 4 or 5 years. Too much extraction too soon can seriously damage the oil bearing structure. Reservoir Engineering is a discipline which estimates the maximum output from a hydrocarbon reservoir without damaging its integrity.
One seldom exhausts a petroleum/gas reservoir at the maximum rate determined by the reservoir engineers. Arguably any investor would like to take out as much money as possible for the longest period possible. Since international petroleum prices fluctuate wildly; "Optimum" level also takes into account future price expectations.