Let's say the cost to purify crude oil when it is $140 per barrel
Cost to purify example $10
Even when crude oil drops to $50 per barrel,
the Cost is still $10 per barrel.
I do not know the price to purify the oil for usage from crude oil status. It's just an example to show that cost never change, its the buying/selling price of the commodity Oil that is flunctuating. Your tariff calculation is bit off in this aspect.
Actually, its not quite that simple ... , this is what I understand.
Crude oil is refined into different products - the output slate consists of gasoline, diesel, kerosene (jet fuel) and lastly, fuel oil. Actually, the term "fuel oil" is quite misleading - this is actually the "tza" or residue. The higher value products are the abovementioned transport fuels.
So, the better, more complex refineries will have smaller percentage of fuel oil in their output slate. Hence they are more profitable.
This fuel oil - can be used in many different ways. Since its the cheap residue, some companies further refine it into other products. Others use it as fuel for ships (called bunker). And others, they use it to fuel their power stations.
So, the cost of fuel oil actually fluctuates according to the demand for ship fuel, or from other refineries. It doesn't quite move in line with the cost of crude oil.
You could have a situation where the cost of crude oil goes up but the market price of fuel oil goes down - in which case the refinery is screwed.
For the refinery, refining margin = (Mkt price of gasoline) + (mkt price of diesel) + (mkt price of kerosene) + (mkt price of fuel oil) - (crude oil)
Anyway, I suspect that SP took an indirect hedge on the price of crude oil as a proxy for fuel oil and the bet went against them.
Some bros who work in commodity futures or trading might have better insight here.
:p