Merrill Robert K. Source and Migration Processes and Evaluation Techniques

American Association Of Petroleum Geologists, 1991. — (Treatise of petroleum geology).Geologists who map organic facies can project the geographical distribution of petroleum source rocks. These maps provide a foundation for calculating the timing of petroleum generation and for determining realistic migration pathways to potential reservoir traps. In some geographical areas, geologists may find organic facies useful for reconstructing stratigraphic sequences. Organic facies are the criteria for evaluating the physical, chemical, and biological processes that control the distributions of organic sediments in geological time and space.Petroleum source rocks represent uncommon paleoenvironrnents. Their organic content is a function of biological productivity, oxygenation in the water column and in near-bottom sediments (preservation), and mineral supply (dilution). Biological input varies from hydrogen-rich algal/bacterial constituents to hydrogen-poor woody fragments. Physical and chemical processes alter biological material before its entombment in rocks. The single most important process affecting the preservation of oil-prone, hydrogen-rich, and therefore oil-generative sedimentary organic matter is oxygenation. Oxygen in the water column, soils, and subaqueous sediments encourages both biological and abiological destruction of hydrogen-rich molecules. Demaison and Moore (1980) discuss oxygen-restricted paleoenvironments where hydrogen-rich,petroleum-generative organic matter is best preserved.Transported and precipitated minerals dilute the relative concentration of organic matter in sediments, and this initial organic matter concentration may ultimately determine whether the rock expels petroleum. Petroleum expulsion requires, among other factors, enough generated petroleum to saturate and exceed the available source rock (usually shale) porosity, especially where available porosity decreases significantly with burial-related compaction. Based on experience, Peters (1986) suggests that this threshold generally occurs at total organic carbon (TOC) values of approximately 0.5 wt. %. Palciauskas (this volume) describes the factors that drive petroleum expulsion.