It may have the same effect as a subsidy
If it looks like a duck....
If the government agreed to send oil companies a check for $10 for every barrel of oil produced, we'd all agree that that's a subsidy, right?
If the government instead says, "We'll credit your tax bill $10 for every barrel of oil you produce, reducing the amount on the tax check you send us", it's THE SAME DAMNED THING.
Oh and calling tax write-offs that oil companies take over employee benefits and such a "subsidy", when every other type of company can use those same write-offs is being disingenuous.
No one is calling those tax write-offs available to all businesses subsidies. The subsidies are the tax write-offs available ONLY to oil production companies. One example is the ability to write off the "declining value" of oil wells.
So, if you're an oil company, you spend $20 billion looking for oil reserves, and deduct those expenses. Then, you find a reserve, worth say, $100 billion. Then, you spend $20 billion getting that oil out of the ground, and deduct those expenses, and then you sell the oil for $100 billion. This is all the normal way a business would run. For example, someone might spend $20 million researching a new product, $20 million making the products, and then sell the products for $100 million, making $60 million in profits they are taxed on.
But on top of the normal deductions for ACTUAL COSTS, the oil companies ALSO deduct the "declining value of the wells". You know, since the oil in the ground was worth $100 billion, as they pump the oil out of the ground and the "value" of the oil in the ground declines, THEY DEDUCT THE DECLINING VALUE OF THE WELLS TOO!
And that's a subsidy. It's a tax deduction no normal business gets.