the lower we go in oil price.....
b)the bigger the difference between natural decline and observed decline

Natural decline in oil fields is defined as the decline taking place without any improvements. So that decline is there irrespective of oil price. If the oil price is lower and these improvements are scaled down the "observed decline" will go towards the natural decline rate, that is the difference is actually getting smaller

ya - sorry i was tired
i meant the bigger the overall decline rate and the smaller the difference. thanks