Mexico's President-elect Manuel Andres Lopez Obrador has said he wants to reduce the country's crude exports and increase domestic production of gasoline and diesel despite a persistent lack of investment in refinery expansion and modernizing.

Pemex's domestic refining network has underperformed again this year, increasing the need for imported fuel. Mexico has bought 1.19 million barrels per day (bpd) of US fuel this year, a 12 percent increase versus the 1.06 million bpd imported in 2017, according to the US Energy Information Administration (EIA).

Pemex's lightest crude grades, Isthmus and Olmeca, were traditionally offered and sold on a spot basis, not through long-term supply contracts, so the pause should not affect specific customers, according to a trader from a company that made purchases last year.

In recent years, Olmeca and Isthmus crudes were mostly exported to customers in Asia, according to the trader and Reuters trade flows data.

"Those crude are for our refineries," said the Pemex source who was not authorized to speak publicly about the matter.

Exports of Isthmus crude declined to an average of 53,000 bpd this year versus 86,000 bpd in 2017, while no barrels of Olmeca have been exported during 2018 compared with 19,000 bpd sold last year, according to Pemex figures.

Conversely, exports of Maya heavy crude, Mexico's flagship crude grade, has averaged 1.157 million bpd this year versus 1.07 million bpd in 2017. Most Maya crude exports ended up in the United States.

Pemex's crude output declined to 1.84 million bpd last month, including 1.07 million bpd of heavy grades. Its refineries worked in July at about 40 percent of their joint capacity of 1.6 million bpd, down from 57 percent of capacity for all of 2017.