Scientific Research in Understanding the Origin of Petroleum
Research on the origin of petroleum has traditionally focused on issues related to the petroleum industry. This research has provided earth scientists with valuable insights on the evolution of sedimentary basins, and the workings of the Earth’s upper crust. While empirical and conceptual schemes have address immediate industry issues, a considerable amount of scientific understanding remains to be undertaken on the origin of petroleum. This is not a criticism of pragmatic research, but rather recognition that more scientific research on the origin of petroleum and what it reveals about the Earth’s upper crust remains to be studied. There are at least six research topics that immediately come to mind, and they include 1) kerogen structure, 2) bitumen‐oil immiscibility, 3) kinetics of oil generation, 4) primary gas generation, 5) pyrobitumen formation, and 6) thermal maturity indices. 1) Kerogen structure models to date consist of rigid covalent‐bonded macromolecules that do not take into account the partial decomposition of weak non‐covalent kerogen bonds responsible for generation of polar‐ rich bitumen, which is an intermediate to oil generation. The character of these weak non‐covalent bonds and how they are manifested in the conversion of living precursors to kerogen remains to be determined. 2) Dissolved water in polar‐rich bitumen is considered to be a source of hydrogen and a cause of the hydrocarbon‐rich oil being immiscible in the bitumen from which it is derived. It remains to be determined how this dissolved water and its chemistry facilitate oil immiscibility in bitumen and mechanisms by which H2O‐derived hydrogen and oxygen are made available. 3) The non‐isothermal curve‐fitting method to determine oil generation kinetics assumes multiple first‐ order reactions for temperature‐programmed S2 pyrolysis yields, which include bitumen, oil, and gas products. The S2 is a product of generated volatiles being swept by an inert carrier gas into a detector. However, volatilization and inert carrier gases are not operative in the subsurface during natural petroleum formation. Although this approach is quick and requires little sample, it is empirical with no theoretical grounds upon which to build. These empirically derived kinetics frequently do not provide reasonable predictions when extrapolated to geologic conditions. More research on the kinetics of specific petroleum phases (oil, bitumen, and gas) is needed. 4) Primary gas associated with the thermal decomposition of kerogen to bitumen and subsequent thermal decomposition of bitumen is not typically responsible for significant natural gas accumulations. However, its role in enhancing oil expulsion and releasing H2S and CO2 for mineral diagenesis and souring of natural gas accumulations remains to be determined. 5) Thermal cracking and cross‐linking reactions result in the formation of pyrobitumen during thermal decomposition of kerogen to bitumen, bitumen to oil, and oil to gas. The proportionality of these two overall opposing reactions determines the gas‐ and oil‐generating potential of a source rock (i.e., petroleum charge). Laboratory pyrolysis experiments show that hydrogen availability, pressure, and heating rate control this proportionality. This proportionality remains to be determined for natural thermal maturation under various subsurface conditions. 6) Vitrinite reflectance is universally considered the standard thermal‐maturity index. However, absence of vitrinite, occurrence of solid bitumen, and sometimes subjective measurements make this index less precise than usually given credit. Although accurate vitrinite reflectance is a good thermal maturity indicator, it is not always a good indicator of stages of petroleum formation. Proxies for vitrinite reflectance have compounded the issue with poorly calibrated relationships that may change with source rocks and their organic facies. More research is needed in determining chemical compositions of vitrinite and other macerals for more precise determinations of thermal maturity and stages of petroleum formation.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019