A Weatherford spokesman in Iraq declined to comment, and the company's Houston headquarters did not respond to repeated requests for comment.
The material, which uses gamma rays to test flaws in materials used for oil and gas pipelines in a process called industrial gamma radiography, is owned by Istanbul-based SGS Turkey, according to the document and officials.
An SGS official in Iraq declined to comment and referred Reuters to its Turkish headquarters, which did not respond to phone calls.
The document, dated November 30 and addressed to the ministry's Centre for Prevention of Radiation, describes "the theft of a highly dangerous radioactive source of Ir-192 with highly radioactive activity belonging to SGS from a depot belonging to Weatherford in the Rafidhia area of Basra province".
A senior environment ministry official based in Basra, who declined to be named as he is not authorised to speak publicly, told Reuters the device contained up to 10 grams (0.35 ounces) of Ir-192 "capsules", a radioactive isotope of iridium also used to treat cancer.
The material is classed as a Category 2 radioactive source by the International Atomic Energy Agency, meaning if not managed properly it could cause permanent injury to a person in close proximity to it for minutes or hours, and could be fatal to someone exposed for a period of hours to days.
How harmful exposure can be is determined by a number of factors such as the material's strength and age, which Reuters could not immediately determine.
The ministry document said it posed a risk of bodily and environmental harm as well as a national security threat.