This investigation is directly relevant to various applications associated with the safety aspects of underbalanced drilling operations where de-oxygenated air may be co-injected with oil-based drilling fluid. However; de-oxygenated air often still contains up to 5% oxygen by volume. This residual oxygen can react with oil during the drilling process, v forming potentially hazardous oxidized hydrocarbons and compromising the safety of drilling operations. This article examines the conditions and processes by which oxidation reactions occur and may be helpful in reducing risk in drilling operations. This project characterizes the oxidation behavior of several oils and a typical oil-based drilling fluid (it atmospheric and elevated pressures using thermogravimetry (TG) and pressurized of differential scanning calorimetry (PDSC). Tests performed on mineral matrix (core) from the oil reservoirs showed no reactivity in both inert and oxidizing atmospheres. In an inert atmosphere, tests on all hydrocarbon samples showed only vaporizing environment, the tests on hydrocarbons showed several oxidation regions. The presence of core had no effect on the behavior of the hydrocarbons tested in an inert atmosphere but accelerated the higher temperature oxidation reactions of the oil samples. The oil-based drilling fluid exhibited the opposite effect-the presence of core material retarded the oxidation reactions. This is perhaps due to the presence of an oxygen scavenger reacting with oxygen-containing clays present in the mineral matrix. In all tests performed on mixtures of hydrocarbon and core in oxidizing atmospheres, elevated pressures resulted in acceleration of the lower and higher temperature reaction regions.