The Organization of the Petroleum Exporting Countries pumped 27.88 million barrels per day (bpd) last month, up 70,000 bpd from November, according to the survey that tracks a wide array of shipping, flows and production data. Output is down more than 1 million bpd from the same month a year ago.

The boost comes ahead of further OPEC+ cuts in 2024 and Angola's exit from OPEC, which are set to lower January output and market share. OPEC's market share has already been falling due to output restraint and the departure of some members.

In December, the biggest increases of 60,000 bpd came from Iraq and Angola, which both boosted exports, the survey found. Nigeria also shipped more crude abroad without, as yet, beginning oil products output at its new Dangote refinery.

Angola's increase was seen by two sources in the survey as a one-off and likely not sustainable into January, while Iraq still has a sizeable amount of production offline because of the ongoing halt in its northern crude exports via Turkey.

Among those showing lower output, Saudi Arabia trimmed production slightly below 9 million bpd, the survey found, as the top exporter extended a voluntary 1 million bpd output cut to provide extra support for the market.

OPEC's output is still undershooting the targeted amount by almost 600,000 bpd, largely because Angola and Nigeria lack the capacity to pump at current targeted levels.

Angola's exit from the group and a new Nigerian quota for 2024 should bring actual output closer to the target level.

The Reuters survey, which aims to track supply to the market, is based on shipping data provided by external sources, Refinitiv Eikon flows data, information from companies that track flows such as Petro-Logistics and Kpler and information provided by sources at oil companies, OPEC and consultants.