May 6 (Reuters) -

Top executives at U.S. oil giants Exxon Mobil and

Chevron Corp ( CVX ) said on Monday that the U.S. needs to

clarify rules on energy subsidies to drive the rapid,

large-scale investments needed to fight climate change.

President Joe Biden has been trying to encourage energy

producers to slash emissions using technologies like carbon

capture and green hydrogen that are expensive and have yet to be

proven at scale. Biden's 2022 Inflation Reduction Act included

big subsidies for those technologies, along with incentives for

more solar and wind power, and electric vehicles.

"Even today, the IRA has not been translated into

regulation. And so, the certainty and the incentives for

investing in this space are still being developed and formed,"

Exxon CEO Darren Woods told a panel at the Milken Global

Conference in Los Angeles.

"And so as a business we've got to understand what are

the implications for the investments? What are the returns for

our shareholders, making sure it's competitive in our

portfolio?" he said.

U.S. climate envoy John Podesta criticized Woods during

the panel for "not going fast enough" to cut emissions at Exxon.

Chevron ( CVX ) CEO Mike Wirth, also at the conference, said the

world was not on track to decarbonize by 2050. Wirth cited the

challenge of meeting rising energy demand in developing

countries, and added U.S. rules for hydrogen subsidies were also

a headwind.

"Treasury is writing the rules and the rules are getting

very restrictive, and they discourage investment," Wirth said.

"They make it difficult to get comfortable with a multi-billion

dollar investment.. so we've had kind of these conflicting

signals."

Woods and Wirth advocated for global, market-based

systems - like a price on carbon - to encourage the advancement

and deployment of climate friendly energy technologies.

Woods also said on Monday that the company's acquisition

of Pioneer will mean more oil production at lower cost.

"We're bringing a unique set of technologies and development

capabilities to a very attractive set of acreage. So we will

produce more oil at a lower cost, which is good for the

economy," Woods said at the conference.