(Corrects 4th paragraph to reflect Braskem's 75 percent (not 70 percent) stake in the consortium; 11th and 16th paragraphs to reflect price formula (not fixed price) in contract)
By Diego Oré
MEXICO CITY, Oct 17 (Reuters) - Mexico's incoming government will soon inherit a costly dilemma over an ethane supply contract between national oil company Pemex and a consortium led by a unit of Brazilian builder Odebrecht.
Under the contract's terms, Pemex has had to supply ethane well below current market prices.
A hydrocarbon that comes from natural gas, ethane is used to make ethylene, which in turn is used to make the common plastic polyethylene at the Braskem-Idesa plant near the Gulf coast port of Coatzacoalcos.
The plant is operated by the consortium in which Odebrecht's unit Braskem has a 75 percent stake and Mexico's Grupo Idesa holds the remainder.
Energy aides to President-elect Andres Manuel Lopez, who takes office Dec. 1, have said the contract is problematic, but have not yet said what the new government will do about it.
"The contract with Braskem is very damaging to Mexico's interests," Sen. Armando Guadiana, of Lopez Obrador's Morena party and heads the Senate energy commission, told Reuters last week. Pemex is fully owned by the government.
The Braskem-Idesa consortium told Reuters last week it has no plans to void the contract.
If President-elect Lopez Obrador were to direct Pemex to cancel the contract, it would be forced to purchase from the consortium the sprawling Etileno XXI petrochemical facility currently valued at $1.26 billion, according to a contract annex seen by Reuters.
Neither Pemex or Braskem responded to questions about the valuation.
Conversely, if the new government opted to stick to the deal, it could only hope for more favorable ethane prices that might reduce its losses.
Under the terms of the 20-year-long contract, Pemex committed to selling ethane to Braskem-Idesa based on a formula that worked out to around 16 cents per gallon at the start of the deal in 2010. At the time, market prices for ethane were three times that, at 50 cents per gallon.
Current ethane prices hover around 40 cents per gallon.
A Pemex spokesman said the contract, which took effect in 2016, "responded to the market conditions of that time."
Before the facility began operations in 2016, Pemex produced more ethane than it needed, forcing it to inject excess supply back into its natural gas pipelines.
Pemex's production of ethane this year averages 88,000 bpd, but this is now insufficient to supply its own Morelos and Cangrejera petrochemical facilities that require a combined 66,900 bpd, plus the Baskem-Idesa contract obligation of 66,000 bpd.
As a result Pemex was forced to turn to ethane imports this year for the first time as domestic oil and gas production continues to fall, costing Pemex some $50 million during the first half of 2018, according to Reuters calculations, due to the cost of imported ethane at market rates compared to the cheaper formula in the contract with the Braskem-Idesa consortium.
If Pemex is left without enough ethane, it would have to shut down the so-called cracking plants at its two petrochemical facilities, and the cost of re-starting them after being idled one week would be some $2.6 million, according to comments from the head of Pemex's ethylene unit, Alejandro Cruz, at a board meeting in December.
In June, pricing agency Platts reported that Pemex entered into a $237.6 million contract with Swiss commodities trader Vitol to supply 720,000 tonnes of ethane to Pemex through 2020.
Both Pemex and Vitol declined to confirm the deal.
In 2016, Mexico's federal auditor determined that Pemex ethane exports during a 10-month stretch of that year could have yielded the company more than $100 million had it not been for the Braskem-Idesa contract.
Using official data, Reuters calculated a similar $100 million opportunity cost in 2017.
Both Pemex and Braskem declined to comment on the calculations.
Braskem said the contract was mutually beneficial, arguing that it helps cut Mexico's reliance on foreign plastics.
"We are not planning on undoing a positive contractual relationship that we've been building with Pemex and that brings benefits to all," said Sergio Plata, head of institutional relations for the Braskem-Idesa consortium.
Rocio Nahle, Lopez Obrador's pick to be Mexico's new energy minister, has said the incoming government will review the Braskem-Idesa contract for possible signs of corruption, part of a broader energy contract review.
The consortium's Plata said he was confident the contract will not be modified.
According to a transcript of a recent session of the board of directors of Pemex's ethane unit, acting director Rodulfo Figueroa, admitted that supplying the gas is "the most serious problem" it faces.
Lopez Obrador's incoming transportation minister, Javier Jimenez Espriu, is an alternate member of the Grupo Idesa board of directors but told Reuters the contract was reviewed by the separate board of the Braskem-Idesa joint venture.
Luis Miguel Labardini, a Mexico City-based energy consultant, said an even bigger problem for Pemex lies with whoever agreed to the contract's terms in the first place.
"We should give the benefit of the doubt to whoever negotiated this contract that they didn't act in bad faith," he said. "But they were negligent." (Additional reporting by Marianna Parraga and Ana Isabel Martinez; Writing by David Alire Garcia; Editing by Frank Jack Daniel and Clive McKeef)