In 1920, Ellisor switched professions again and started working for the
Humble Oil & Refining Company. She was specifically hired by
Wallace Pratt to design a subsurface laboratory which works to show the development of petroleum and natural gas as well as other minerals. This is where she was really able to make a name for herself in the study of petroleum geology as well as making many firsts for women in geology. Ellisor created and opened the laboratory to focus on focusing on
Tertiary and
Cretaceous time periods. In just a few months after opening the laboratory, she made a major discovery of
foraminifera in one of the company's wells at
Goose Creek. Her study of foraminifera indicated
Oligocene-aged coral reefs on the Damon Mound salt dome in
Brazoria County. Some of her specimens are contained in the NPL's Type Collection. She was one of the first to see the potential for using foraminifera to correlate rocks from drilling cores. These findings would be the basis of some of her most famous writing for years to come. and
Esther Applin Ellisor became one of the first female
stratigraphers, a leading economic
micropaleontologist in the US, Despite her contributions, she was never credited for her discovery in foraminifera in Edgar Owen's
The Trek of the Oil Finders, which exclusively credited J.A Udden, Edwin T. Dumble,
Joseph A. Cushman, and Jesse J. Galloway. Yet, the four could only conclude their findings only by the research of Ellisor herself, as well as her co authors
Esther Applin and
Hedwig Kniker. Ellisor, along with Applin and Kniker all proposed the idea of using microfossils to locate and find petroleum. She died at the age of seventy-two on September 22, 1964, in Galveston, Texas. ==Awards and honors==