It's depreciation. When a company buys an asset they write off the cost in proportion to its value decline. The depreciation rate is dependent on longetivity. I've heard real estate is depreciated on a 50 year schedule. Conversely, the Obama stimulus bill allowed companies to depreciate large truck purchases in 1 or 2 years. I don't know mining tax accounting at all, but there's an issue of hitting a gusher and placing, say, a $4,000,000,000 asset on the books in advance of realizing any gain (from sale or operations) but have only invested $400,000. In theory, that $4,000,000,000 could be taxed even though the well has not produced enough cash to even pay the tax bill. This, of course, would create an environment where the largest producers could feed off the hard work of small operators as they're required to fire sale just to pay the taxes. So we allow these small strip well operators to write off everything now and pay a much higher tax the following years as the oil is pumped out but the intangible expense deduction is already used up (or carried forward if expenses are in excess of gains in the first year).
Maybe nightmare39xx can come in here and explain this better.