Molecular Thermal Maturity Indicators in Oil and Gas Source Rocks

Abstract

Many detailed chemical parameters have been proposed as indicators of thermal maturity in oil and gas source rocks. Certain classical maturity parameters involving carbon preference indices and compound class ratios, such as hydrocarbons/extract yield and extract yield/total organic carbon, are less commonly used today, having been complemented with detailed molecular parameters. Among these parameters, the molecular distributions of metalloporphyrins, cyclic hydrocarbons, low-molecular-weight hydrocarbons, and gases are most commonly used. Recent instrumental advances have allowed the routine measurement of molecular ratios in geochemical organic matter, stimulating the development and use of biological markers, such as steranes, hopanes, and metallated tetrapyrroles, as thermal maturity indicators. Increased chromatographic resolution of source rock hydrocarbons has also led to the use of low-molecular-weight hydrocarbons, methylphenanthrenes, and aromatized steranes as maturity indicators. In this paper, we discuss these developments, emphasizing the applications and the pitfalls of using molecular maturity indicators.Measurements of source rock thermal maturity attempt to describe the progress of the sum of the chemical reactions that convert sedimentary organic matter into oil and gas. Such measurements can be made on a molecular basis, using both soluble organic matter (C15+ hydrocarbons) and gases/gasoline range hydrocarbons (C15−). The biological markers, including both aliphatic and aromatic hydrocarbons, are probably the most “ideal” molecular maturity indicators currently available, although their range of applicability is generally limited. Gases and gasoline range molecules (particularly their isotopic characteristics) also have great potential as thermal indicators, although an understanding of migrational fractionation is critical. While no single thermal maturity indicator will ever be the ultimate “ideal” parameter, molecular indicators will undoubtedly increase in importance in coming years, owing largely to their specificity and to the increasing ease of obtaining this type of data.KeywordsVitrinite ReflectanceLight HydrocarbonOrganic GeochemistryMaturity ParameterHydrogen IndexThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.