By Alexandra Alper and Marianna Parraga
RIO DE JANEIRO/MEXICO CITY, March 26 (Reuters) - Oil and gascompanies are gathering in Mexico and Brazil this week for whatmay be two of their last chances to compete for some of theworld's biggest reserves ahead of elections that could dial backenergy reforms in both countries.
An auction of rights to explore and produce oil from shallowwaters off Mexico and an onshore and offshore auction round inBrazil are taking place months before presidential elections inLatin America's top two economies. Leftist contenders havepledged to reverse or slow the flow of private money into theoil industry in both countries.
The uncertainty of the elections could spur some companiesto win bids to lock in access to new exploration areas.
"There is indeed a feeling that these could be the last(auctions) for a while," an executive for one company registeredto bid in both countries said on condition of anonymity.
Other firms could hold off competing until they see ifelections bring any changes to contracts or terms.
Mexico's leftist firebrand candidate Andres Manuel LopezObrador, who is leading in opinion polls, has said he willreview over 90 contracts signed since Mexico passed legislationin 2013 ending the 75-year monopoly of state energy firm Pemex.
This month Lopez Obrador plans to ask President Enrique PenaNieto to cancel two auctions planned for the second half of theyear if he wins the July election.
Support for Lopez Obrador is pegged at around 29.5 percentcompared with 21.2 percent for his nearest rival, according topolls.
In Brazil, the most likely leftist contender in thepresidential race, Ciro Ferreira Gomes, has warned off potentialoil company bidders, saying he would expropriate energy assetsbought by private investors if he wins the October election.
Gomes, a former governor, is trailing rightist candidateJair Bolsonaro in polls where former President Luiz Inacio Lulada Silva, who has vowed to run but is likely to be barred due toa corruption investigation, is not offered as a candidate.
"The companies might now want greater rates of return orthey could get to a point where they will not want that level ofrisk," said Alfredo Alvarez, head of the North Latin Americaenergy group for consultancy EY in Mexico.
Most oil executives and analysts told Reuters they are notexpecting the reforms to be fully reversed in either country.But many see a slower bidding calendar and changes in contractterms as real possibilities.
"The speed of the auctions could be slower and even stop,but the reform is protected in the constitution, which requirestwo-thirds of the Senate and the lower house to change," PemexChief Executive Officer Carlos Trevino said earlier this monthin Houston.
[For a graphic including blocks on offer in both auctions,click on https://tmsnrt.rs/2pzobJo]
OFFERING BIG CUT OF OIL
Brazil's top-quality geology and history of honoringcontracts have outweighed worries about political instability.Bids in Thursday's round and in a subsequent auction in June forpresalt blocks could be so big they may damage the profitabilityof any awards, consultancy Wood Mackenzie said.
In September, Exxon Mobil paid a record $678 millionfor a Brazilian block with partner Petrobras in aconcession-based auction. In another sign of strong demand,firms competing in October to buy stakes in the country'scoveted presalt oilfields promised to give the government asmuch as 80 percent of the oil produced after costs.
For Thursday's auction in Brazil, 21 companies includingChevron, BP, Exxon, Royal Dutch Shell,Statoil, and Total have registered to bid ina round where 70 blocks are being offered.
Majors are likely to vie for a handful of blocks in theprolific offshore Campos and Santos basins, with minimum signingbonuses of up to $575.5 million. Some are rumored to havedesirable presalt geology.
In Mexico, 14 firms and 22 consortia composed of majors andmedium-size companies qualified to bid in this week's auction,which will offer 35 blocks in the Gulf of Mexico's shallowwaters. The government expects at least seven to be awarded, theenergy minister said.
Pemex is likely the round's dominant player because it ownsthe infrastructure and has accumulated a vast knowledge of thegeology.
(Reporting by Marianna Parraga, Ana Isabel Martinez and DavidAlire in Mexico City, and Alexandra Alper in Rio de JaneiroEditing by Simon Webb, Gary McWilliams and Jeffrey Benkoe)
