Total Organic Carbon (TOC) Analysis

Abstract

Total organic carbon (TOC) is a measurement of the organic richness of sedimentary rocks. Its utility for screening potential petroleum source rocks is based on the source of known petroleum reserves. This source is organic matter derived from a variety of biological origins which has been deposited and buried through geological time (Tissot and Welte, 1984; Hunt, 1979). While some constituents of petroleum are directly incorporated into petroleum (biological markers or biomarkers), the bulk of petroleum is formed from the thermal decomposition of organic matter through time. The reactions resulting from increased thermal stress yield the large variety of carbon-containing compounds of which oil and gas are composed. Since the oil or gas potential of a formation is related to its carb n content, TOC analysis is typically the first screening analysis completed to evaluate the general potential of a formation to generate hydrocarbons. Additional analyses confirm the source potential and elucidate source characteristics such as the level of thermal exposure (maturity) and the likelihood of oil, gas, or a mixture of the two being generated (kerogen type). Organic carbon is distinguished from inorganic carbon by its derivation. Organic carbon is derived from biogenic matter, whereas inorganic carbon is derived from mineral matter. Inorganic carbon is an oxidized form of carbon often combined with calcium, magnesium, or other elements commonly found in carbonate rocks. Carbon constitutes 75-95 wt. % of hydrocarbons by molecular weight, with an average value near 83 wt. %. The nonhydrocarbon organic constituents of petroleum (nitrogen, sulfur, and oxygen containing compounds) and the bulk of the asphaltene fraction as well as the bitumen and kerogen of petroleum source rocks also contain high percentages of organic carbon.