Matthew R. Simmons, 67, who founded Simmons & Co. International in 1974, died on Sunday in Maine.

Simmons founded the Houston-based energy investment bank and had become an industry go-to expert and frequent critic. He routinely filled energy conference halls with his insights and criticism about oil and natural gas issues.

Simmons started the investment bank in May 1974 to focus on the oil services industry, and by 1981 the firm employed 13 people. While some financial services firms abandoned the energy sector through the 1980s, Simmons & Co. persevered and began to work on bankruptcy workouts and debt restructurings.

In 1995 Simmons & Co. redefined itself to offer "total energy services," with a focus on companies with natural gas plants, pipelines and other gathering systems, as well as the downstream sector. In 1998 an office was opened in Aberdeen, Scotland. Two years later the firm completed its first investment banking assignment in the exploration and production sector, now a major part of the firm's business.

Simmons often spoke at energy trade shows, and his bold comments and popular speeches were frequently posted on the firm's website. Among other things Simmons was a proponent of the "peak oil" theory; in his book Twilight in the Desert he offered detailed research that he believed indicated that the world was running out of fossil fuels.

Simmons retired as chairman emeritus of the bank in June to devote his full time to the Ocean Energy Research Institute, which he founded in 2007. The institute is a think tank and venture capital fund that focused on the challenges of the U.S. offshore renewable energy industry, including wind energy.

"Since founding Simmons & Co. International 36 years ago, I have truly enjoyed being a part of the firm's growth and success," Simmons said in June. "In the three years since I founded Ocean Energy, my passion for renewable energy R&D [research and development] and investment has continued to grow and this venture requires my full attention."

According to reports, Simmons died of a heart attack at his home in North Haven, ME. He is survived by his wife Ellen and their five daughters. Arrangements were pending. However, the family asked that instead of flowers, donations may be made to the Ocean Energy Research Institute.

"We are deeply saddened by the unexpected loss of a true visionary and friend," said Simmons & Co. CEO Michael E. Frazier. "As a pivotal figure in the lives of many of our employees, and countless others across the energy industry, Matt will be sorely missed. Our thoughts and prayers are with his family during this difficult time."

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