Charles Day "Chuck" Masters, 70, a government geologist who directed the first comprehensive national assessment of untapped oil and gas resources in the United States, died of a brain tumor Aug. 19 at his home in East Hampton, Conn. He moved there from Falls Church last year.

He retired in 1995 as chief of the world energy resources project at the U.S. Geological Survey after working on global assessments of undiscovered oil and gas. He had also served there as chief of energy resources and marine geology. The first national resources survey, published in 1975, was used in forming federal energy policies.

Dr. Masters was a native of Pawhuska, Okla., and a graduate of Yale University. He received a master's degree from Colorado University and a doctorate from Yale, both in geology. He served in the Navy in the Atlantic and Mediterranean during the Korean War.

He began his career with the company that became Amoco, working on assessments of petroleum-bearing rocks in the Midwest. He was chairman of the math and science department at West Georgia College before joining the Geological Survey in 1973.

His honors included a Meritorious Service Award from the Interior Department. He was a fellow of the Geological Society of America and a member of the American Association of Petroleum Geologists, the Sigma Xi scientific honorary society and the American Association for the Advancement of Science.

He was blood drive coordinator at Lewinsville Presbyterian Church in McLean and a volunteer with Habitat for Humanity. His interests included canoeing and gardening.

Survivors include his wife of 46 years, Anna Sandin Masters of East Hampton; three children, Cynthia Masters-Waage of Windsor, England, Eric Alan Masters of Great Falls and Carolyn Masters of Bethesda; a brother; and seven grandchildren.