It's sort of like being a kid at Halloween: You can gobble up all of your goodies now, but you'll have a stomachache later. Similarly, the oil companies can enjoy their commodity-inflated results for the third quarter, but what goes up ... well, you know.

The first of the Big Oil trick-or-treaters to report was ConocoPhillips (NYSE:COP), which told us on Wednesday that it had earned $5.18 billion in the third quarter, up 41% from the same quarter of 2007. Revenue climbed 52% for the same period to $70 billion, from $46.1 billion last year.

It probably won't surprise you to know that upstream (exploration and production) net income rose 47% to $3.93 billion, based largely on the increases in oil and gas prices. And there's where the potential tummyache lurks: We know that crude-oil prices have been cut in half from their July highs. Those cuts will be reflected in the coming quarters, and with volumes pretty well fixed, the exploration and production engines of the past few periods stand to be slowed substantially.

The company's daily production averaged 1.75 million barrels of oil equivalent (BOE) in the quarter, a decline from last year's 1.80 million. Part of the slippage reflected a hurricane-induced reduction of about 17,000 BOE per day.

The production figures include its Canadian Syncrude operations, but not its investment in Russia's Lukoil, which it reports separately. Conoco's investment in the Russian company yielded about $438 million in the most recent quarter, up about 13% from the same quarter in 2007.

On the downstream (refining and marketing) side, net income slid about 35%. The decline reflected several factors, including lower refining volumes -- thanks in part to hurricanes Gustav and Ike -- and a change in German tax legislation.

Conoco's third-quarter results will be followed by reports from the other Big Oil companies, including ExxonMobil (NYSE:XOM), Chevron (NYSE:CVX), BP (NYSE:BP), and Royal Dutch Shell (NYSE:RDS-A). However, the key for these companies is what lies ahead -- lower average commodity prices.

On that basis, energy-company buyers should be aware of the income downturn that likely lurks as average crude realizations fall.

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Fool contributor David Lee Smith doesn't have financial interests in any of the companies listed above. He does welcome your questions, comments, or criticisms. The Fool has a disclosure policy.