(Adds quotes, context on reservoir unitization)
HOUSTON, Oct 19 (Reuters) - Mexico's oil regulator will likely add another auction in 2018 featuring conventional onshore oil and gas blocks, the head of the National Hydrocarbons Commission (CNH) said on Thursday, potentially teeing up a third tender in an election year.
The bid terms will be announced later this year or in early 2018 while contracts will likely be awarded by the summer, said Juan Carlos Zepeda on the sidelines of a forum in Houston.
The onshore tender is in addition to a deepwater Gulf auction expected to attract in January some of the world's biggest producers, as well as a March shallow water auction.
A landmark 2013 constitutional energy reform championed by President Enrique Pena Nieto paved the way for the auctions, in which private firms can bid to operate oil and gas fields on their own. Before the reform, state-owned company Pemex had a monopoly on hydrocarbons production.
Depending on the winner, Mexico's July 2018 presidential election could alter the pace and scope of future auctions, which are organized and supervised by the CNH, while the energy ministry designs the contracts and sets the schedule.
Zepeda added that so-called non-conventional blocks to produce shale oil and gas are also being analyzed for inclusion in an additional separate auction.
The CNH has run eight oil auctions to date, awarding 72 exploration and production contracts to more than 60 companies. The contracts are seen generating almost $61 billion in investment over their lifetime.
The 64 blocks to be offered in the two upcoming offshore auctions account for more than 65 percent of Mexico's estimated resources. Along with the January bidding round, Pemex could also find a partner for the promising Nobilis-Maximino deeepwater project close to the U.S. maritime border.
A development plan for another large deepwater project, Trion between Pemex and Australia's BHP Billiton, has not yet been submitted to the regulator, Zepeda said, but it is expected before year end.
New regulation to establish how operators of two different blocks should produce oil from a single shared reservoir was recently finished by authorities and is now under public consultation, said Aldo Flores, Mexico's deputy energy minister.
"The final version (of the regulation) should be ready by November," Flores said.
The well Zama-1 containing over 1 billion barrels of oil in place discovered in July by U.S. firm Talos Energy and its partners in Mexico's shallow water could extend into a Pemex area, Zepeda said.
"The first unitization case could be Zama, but it has not yet been officially presented (to authorities)," Zepeda said.
The reservoir unitization regulation will establish the need to nominate a single operator to produce oil in shared reservoirs even keeping two separate companies or consortia for each one of the blocks. The energy ministry will have the final word if the parties do not agree on how to develop the field.
(Reporting by Marianna Parraga; Writing by David Alire Garcia; Editing by Susan Thomas and Meredith Mazzilli)