NEW YORK — Exxon Mobil has been working for years on a fleet of enormous new oil and gas projects in places such as Abu Dhabi, Russia, Papua New Guinea, and the Gulf of Mexico designed to turn around what has been a consistent and alarming slide in oil and gas production.

A record eight of these mega-projects came on-line last year, Exxon said Monday – just as oil prices were falling by more than half.

“Investors would be more than happy to see production decline but oil prices higher,” said Fadel Gheit, an analyst at Oppenheimer & Co.

No such luck this year. These enormous projects were conceived and started years ago, as prices were rising. Now they are starting to produce oil and gas at a time when the price of global crude has been eviscerated by rising supplies and weak growth in demand.

On Monday, Exxon posted a 21 percent decline in both revenue and profit for the fourth quarter because of lower oil prices. The company said it earned $6.57 billion in the quarter, the lowest since the first quarter of 2010, on revenue of $87.28 billion. Last year, Exxon earned $8.35 billion on revenue of $110.86 billion.

Exxon also said Monday that oil and gas production slipped 4 percent compared with a year earlier, to 4.05 million barrels per day. It was the 13th decline in the last 14 quarters. Four years ago, Exxon was producing nearly 1 million more barrels of oil and gas per day than it is now.

Chevron, which is in the midst of its own boom of new mega-projects, posted similar results Friday. It managed to eke out a small gain in production, but posted a 30 percent decline in profit.

Exxon and Chevron will reduce spending on big capital projects this year, but that will only affect projects far in the future.