prices will fall if demand decreases and the supply is constant. the supply curve will be vertical and demand curve will be downward sloping.
OPEC acts like a monopoly on crude oil. They can cut production and decrease the supply of oil, thus raising the price, but this does not necessarily increase revenue. As the price increases, the demand decreases. The percentage change in quantity demanded in response to a one percent change in price, while holding all other factors constant, is called price elasticity of demand. If the price elasticity of demand is high, then the demand will decrease significantly as the prices increase, and revenue may not increase.