Here’s one simple reason oil prices keep falling: OPEC has steadily been pumping more oil than it was this spring, putting more crude on a market already awash in the stuff and further depressing prices.

OPEC nominally still has big oil-production cuts in place, but cartel members such as Iran and Angola—both suffering from the recession—are looking for revenue in extra barrels of crude. Compliance among OPEC’s 11 quota-bound members was down to 68% in June, after reaching 80% earlier this year, according to the International Energy Agency.

That compliance rate is par for the course, historically. But it means that since April, OPEC increased production by about 330,000 barrels a day. And additional oil has hit the market from other sources, the IEA says. Since demand is still fairly week and inventories are still bulging, that extra crude is simply feeding oil bears.

Crude oil futures were down almost 2% to $59.31 in midday trading.

The extra oil is coming from lots of places, in addition to countries backsliding on quotas. Iraq, not subject to output limits, raised production from April to June by 130,000 barrels a day, and has plans to pump another 100,000 to 250,000 barrels in coming months. Natural-gas liquids are also on the rise, the IEA says—NGL production in OPEC states will increase 850,000 barrels a day between now and 2010.

Even non-OPEC countries are pumping more these days. Non-OPEC supply is now expected to rise by almost 200,000 barrels a day this year—compared with a previous forecast of a 100,000 barrel decline, and could increase by another 400,000 barrels next year.

Anybody looking for an explanation for oil’s biggest weekly drop since January might just want to take a gander at fundamentals, as quaint an idea as that may seem when it comes to oil prices.