When a commodity costs more to produce than the current market price, producers usually stop producing it.

When it comes to U.S. crude oil, that's not happening, according to an analysis of global oil production by Wood Mackenzie, a commodities market research firm.

The oil glut continues to weigh on crude prices, but the decision to stop pumping when prices fall is not as simple as it may seem.

For starters, the cost of producing a barrel of oil varies widely from one well to another, based on the initial cost of finding and developing the oilfield, along with such costs as borrowing, current operations and maintenance.

Some of the recent expansion of U.S. crude production, for example, has come from so-called "stripper" wells — once abandoned finds that are seeing new life thanks to advanced production techniques. Many of these are profitable even at current market prices.