ABSTRACT
Accumulations of oil in fractures, stylolites, burrows, sandstone lenses, and chert nodules, deeply embedded within organic-rich, mature oil source beds of Late Devonian-Early Mississippian age are considered prima facie evidence of internal migration and expulsion. Most oil migrated internally and was expelled as a separate phase.
Within a given section in which chert and black shale are interbedded, contain the same type of organic matter, and have reached the same level of thermal maturity, black shales typically contain less hydrocarbons per weight percent TOC than cherts. The observed differences represent the amount of oil expelled from the black shales and range from 10 to 15 mg/g TOC for total bitumen, 7 to 8 mg/g TOC for saturated hydrocarbons, and 11 mg/g TOC for volatile hydrocarbons. Approximately 27-33% of the oil generated in these source rocks was calculated to have been expelled. In the main oil-producing region of central and southern Oklahoma, 22 billion bbl of bitumen and 16 billion bbl of saturated hydrocarbons were estimated to have been expelled from the Woodford formation.
Relatively efficient, separate-phase oil expulsion may be characteristic of very rich oil source rocks like those reported here. Such rocks would reach effective oil saturation and begin to expel oil as a separate phase at a relatively early stage of generation. Timing and efficiency of oil expulsion must then be influenced by the concentration and type of organic matter in the source rock, because these factors determine the volume of oil generated and, hence, the time when a source rock becomes oil saturated.
