“Aren’t some oil fields essentially storage too?”
Yes and no. There are a number of old oil fields which serve in some capacity as storage for the US Strategic Petroleum Reserve. These are mostly just used as storage and not much oil now is pumped into them. The amount is quite considerable at over 700 million barrels, making them in aggregate the largest oil storage in the world, and the amount of oil is bigger than any of the US’s remaining oil fields save a select few. Their key is the ability to drawdown very rapidly these amounts for domestic usage.
Generally, it is infeasible to reinject oil back into oil fields as too much would be lost in the process unless specialized provisions are made such as in the SPR fields.
Natural gas is much more common to be reinjected back into production gas fields to use as storage as there are definite cycles when usage is so great that insufficient supply would otherwise be available for heating and power generation.
I was thinking more of Saudi Arabia’s reservoirs of yet unpumped crude that are supposed able to easily pump oil at a higher rate than they are currently plumbed for.
All of the US Strategic Petroleum Reserve are salt dome caverns. They are man-made caverns in salt domes. There are multiple caverns at each of the four sites.
The connections to each cavern are made above ground at a wellhead. The facilities are then connected to multiple pipelines and loading docks.
http://energy.gov/fe/services/petroleum-reserves/strategic-petroleum-reserve/spr-storage-sites
