@okie,
There are, as I understand it, two methods of calculating the depletion allowance for tax purposes. One is the "cost" method essentially akin to depreciation. A producer invests $100 in setting up an oil well. An engineer estimates that there are 100 gallons below the well that will be extracted over time. The depletion allowance is calculated as $100 cost divided by 100 gallons = $1 per gallon. If 5 gallons is extracted in a year, the depletion allowance is 5 times $1 = $5. That recurs until the $100 original cost is used as a tax deduction.
The second method, and perhaps the one Cyclo refers to, is called the "percentage depletion allowance" which does allow more than the original cost to be deducted. That could be called a subsidy.
It dates back to the mid-1920's when much of the drilling was being done by
"wildcatters" who as often as not came up with a dry hole. In an effort to get those guys to drill somewhere else, the tax code was altered to allow taxes to be reduced by revenues on wells where the original cost had already been used up. 1924.
That is the best explanation I can find and try to explain. I'll leave further discussion of depletion to others.