Mexico City, Mexico — Major international oil firms including Exxon Mobil, and Royal Dutch Shell are showing interest in the initial phase of a bidding round for exploratory oil and gas blocks in the shallow waters of the Gulf of Mexico that will be assigned by the government midyear, a top energy official said.

The recent plunge in oil prices appears to not to have affected the shallow-water phase of the bidding round because of modest production costs, while a later phase involving more costly production in shale-rock formations will be trimmed back to offer only the most attractive of the so-called unconventional resources, said Juan Carlos Zepeda, head of the "National Hydrocarbons Commission."

The commission is overseeing what Mexico is calling "round one," since it is the first of its kind since an energy overhaul last year that ended the 76-year government monopoly on oil exploration and production by national firm Petróleos Mexicanos, or Pemex.

"Even in this price environment, the round is moving forward quite well," Mr. Zepeda told journalists during a tour of "data rooms" where oil companies can see seismic and other data on the areas before bidding.

Among the seven companies that have been authorized into the data rooms, after paying fees, are Exxon Mobil, Chevron, Shell, Ecopetrol and BG Group, the commission said. A total of 30 companies have shown some interest in the process short of paying for entry into the data rooms, the commission said.

Mr. Zepeda said the shallow-water round is in an area of the Gulf of Mexico where there already is significant oil production, and where costs are less than $20 a barrel, making them attractive even in the current environment of depressed prices.

A later phase of the bidding round this year involving shale-rock formations and so-called tight oil that is complicated and expensive to extract will be trimmed back to include only the most attractive areas due falling oil prices, Mr. Zepeda said. The commission, along with the Energy Ministry, will decide by March or April which riskier project areas will be trimmed, he added.

The highly anticipated bidding phase near the end of the year for blocks in the mostly unexplored deep waters of the Gulf of Mexico will go forward as planned since oil and gas there won’t be produced for at least eight years, and prices will likely have recovered, Mr. Zepeda said.

The Mexican legislature passed the final details of the energy overhaul last year after 10 straight years of declining oil production by Pemex. Supporters of the move argued that Mexico was running out of the "easy oil" that Pemex was best at producing and needed private expertise and money to tackle more complicated fields. Opponents said it was a sellout to multinational companies that eventually would gut Pemex.