(In 8th paragraph, fixes typographical error in group's name to read "Protect Colorado," not "Project Colorado")
HOUSTON, Oct 2 (Reuters) - Oil and gas companies and their supporters have poured $20.3 million since August into a campaign to defeat a Colorado ballot initiative that would limit new drilling in populated areas, according to state financial filings released on Tuesday.
Colorado, the sixth largest and one of the fastest growing U.S. oil producing states, votes on Nov. 6 on a citizens' petition that would require new wells be at least 2,500 feet (762 m) from homes, schools and parks.
The initiative, which received 172,834 signatures to get a spot on the ballot, was aided in part by the deaths of two men last year in an explosion caused by gas leaking from an abandoned well near a Firestone, Colorado home.
The state's oil production rose 26 percent in the 12 months through July as companies used higher prices to expand in the state's Denver-Julesberg basin.
Oil and gas companies with operations in the state, including Anadarko Petroleum Corp, DCP Midstream LP , and Noble Energy Inc, have donated more than $1 million each to Protect Colorado, a group fighting the citizens' initiative.
Anadarko did not respond to a request for comment, and DCP and Noble were not immediately available to discuss contributions.
The group still has more than $10.2 million in financial and service contributions available to fight the measure with just weeks to go before the vote, according to state filings.
A Protect Colorado spokeswoman said the financing - more 100 times that raised by initiative backers - is no guarantee of victory.
"It's really going to depend on voter turnout," said Karen Crummy, a spokeswoman for the group. "Most people assume this November will have a high turnout, no matter where you are."
Colorado Rising, which led the signature-gathering effort to get the measure on the ballot, currently has about $100,000 and has recruited about 2,000 volunteers to conduct pro-initiative phone calls and text messages, and to canvas neighborhoods, according to Anne Lee Foster, a spokeswoman for the group.
"We've seen many times where the industry has outspent communities and we've still won at the ballot box," said Foster.
Americans for Prosperity, a political advocacy group backed by billionaires Charles and David Koch, also has joined the opposition. Its Colorado state chapter is planning a digital campaign to communicate directly with voters, state director Jesse Mallory said in an interview.
Opponents say the set-back restrictions would curtail some 85 percent of new oil and gas development and harm the state's economy. Colorado mayors and other elected officials have called for a "no" vote.
"There are better ways to protect the health and safety of our communities while keeping our state's economy strong," said Denver Mayor Michael Hancock. (Reporting by Liz Hampton Editing by Marguerita Choy)