U.S. Geological Survey
Energy Resource Surveys Program
USGS Fact Sheet FS-025-97


Organic Geochemistry in
Future Energy Development

"Organic geochemistry is one of a number of disciplines used to determine the ultimate quantities of oil and gas that might be available from source rocks. U.S. Geological Survey (USGS) research helps to explain how these fluids are generated and how they move out of source rocks. These studies lead to a better understanding of how and where oil and gas accumulate and can perhaps provide an improved basis for estimating the total in-place oiland gas available to the explorationist."

Dr. Jim Palacas, U.S. Geological Survey

The increasing dependence on foreign oil and the decreasing domestic exploration effort in the United States are important national concerns.

More accurate and scientifically-based assessments of oil and gas resources in the United States are needed in order to improve our domestic energy policy. Assessments of undiscovered petroleum resources are currently based on statistical evaluations and geological analogues of discovered oil and gas resources. While these types of assessments represent the best of our current knowledge base, they typically refle ct only the amounts of oil and gas to be found by current and past technologies. These assessments do not reflect the ultimate resource potential.



Cross section through part of a sedimentary basin in which a hydrocarbon source rock layer has been buried to different depths. Due to increasing temperatures with increased burial depth, organic matter within this source rock `cooks', resulting in partial decomposition and petroleum generation (mature source rock). With further burial, organic matter decomposes to generate natural gas (over-mature source rock). Generated petroleum and natural gas are expelled from the source rock and migrate upward into porous overlying rock layers. If appropriate conditions exist, petroleum and natural gas are trapped and accumulate. If appropriate conditions do not exist, natural gas is eventually released to the atmosphere and petroleum seeps at the surface to form asphalt (tar) deposits.


USGS investigations comprise the principal long-term research effort, involving the role of organic geochemistry in the evolution of oil and gas, in the United States.

USGS research in organic geochemistry is an integral part of collaborative efforts between the USGS and a variety of other interested groups including the Department of Energy, the Environmental Protection Agency, the Bureau of Indian Affairs, the U.S. Forest Service, the Bureau of Land Management, the Department of State, and the Agency for International Development. Additional cooperative work has been undertaken with many State agencies and universities, as well as with industry groups such as the Gas Research Institute and oil companies, especially those that are independent. The USGS's lack of vested interest in the outcome of analytical research provides an important basis for cooperative efforts.

Organic geochemistry serves to improve our understanding of the quantities of oil and gas that have been generated and expelled by source rocks.

The geochemical program is wide in scope and includes identifying source rocks from which oil and gas may be generated, determin ing the amount of burial a specific source rock must experience in a given sedimentary basin before oil and gas generation commences, predicting the amount of generated oil and gas that may be expelled from a source rock, and correlating discovered oil and gas with their source rocks to understand migration pathways.

Recent USGS studies in organic geochemistry suggest that petroleum resources in fractured source rocks may be much larger than previously recognized.

Scientists know that good source rocks generate considerably more oil and gas than is found in conventional oil and gas fields. Scientists also accept the concept that most of the generated oil was expelled from the source rock and was free to migrate. This degree of efficiency in expulsion of hydrocarbon, coupled with seemingly inefficient accumulation, led to the conclusion that much of the hydrocarbon has escaped through leakage or has been dispersed in low concentrations. Recent USGS investigations, however, indicate that in many basins much of the hydrocarbon resource remains in the source rocks. An increasing body of evidence suggests that in most sedimentary basins mature source rocks generate at least one order of magnitude more oil and gas than has been found using current exploration strategies. A scientific understanding of the fate of this "surplus" oil is imperative for accurate oil and gas assessments and for initiating new domestic exploration.

USGS research in the origin of oil and gas has given the petroleum industry new directions for further exploration.

Experimental and field research at the USGS addresses fundamental aspects of oil and gas formation and identifies oil and gas source rocks in frontier and known petroleum-producing provinces. In some basins, identification of previously unrecognized source rocks has opened new exploration plays for the petroleum industry. For example, in the Powder River basin of eastern Wyoming, deep oil production has been shown to be derived from source rocks within the Powder River basin itself rather than from distant source rocks along the Wyoming-Idaho border. An understanding of source rock distribution and the migration pathways of the oil has significant bearing on exploration strategy and resource assessment in the basin.

The USGS maintains a state-of-the-art laboratory to conduct research and to undertake collaborative st udies.

USGS geochemists are actively researching controlling factors for oil generation within and expulsion from source rocks. This research is performed through laboratory experiments that simulate the natural processes of oil formation. The results provide a base for improved models used to determine the ultimate petroleum resource potential of a sedimentary bas in. In addition, other research emphasizes improved understanding of the molecular components of oil and gas that provide key correlations between petroleum types and their source rocks. This kind of knowledge is critical to calibrating and monitoring broad assessments of oil and gas resources.



The natural process of petroleum generation can be simulated in the laboratory by heat-ing source rocks under proper time and temperature conditions in the presence of liquid water. A, Crushed rock is placed in metal reactor vessels with sufficient water to insure that the fragments remain submersed in liquid water during the heating. The remaining air space in the reactor vessel is purged with helium to remove all traces of oxygen. If a rock is a potential source of petroleum, then generated oil will be expelled from the rock between 300°C and 365°C after 72 hours. B, The expelled oil accumulates on the overlying water surface where it can be collected for analysis. These experiments pro-vide data on petroleum formation as well as on natural gas formation, aqueous organic species generation, and mineral transformation.


Oil-source rock correlations can determine the source rocks for oils that may have migrated far from their point of origin. Sophisticated techniques are used to compare the distribution of biologically-derived compounds contained in oils with that of their possible source rocks. The composition of a sample of crude oil (left) is similar to an extract from possible source rock A (center) in the signature of tricyclic compounds (shown in red) and gammacerane (shown in blue). Possible source rock B, however, contains more tricyclic compounds and less gammacerane than the oil. Therefore, the oil is more likely to have been expelled from source rock A than source rock B.


Detailed organic geochemical studies at the USGS are used to determine ways to mitigate the impact of oil and gas on the environment, whether the occurrence is natural or manmade.

Domestic use of fossil fuels is likely to increase over the next few decades. Organic geochemical studies of natural and manmade leakage of hydrocarbon materials is an important new direction for USGS research. Release of methane from fossil fuels has important implications for atmospheric chemistry and global warming. USGS studies of methane leakage from coalbeds and natural-gas deposits provide useful data for atmospheric modeling and can provide a rational, scientific basis for policy formulation. USGS research on the characterization of oil and gas, and determination of their ultimate fate, helps to evaluate the impact on our environment.


For More Information:

Dr. Paul Lillis or Dr. Michael Lewan
U.S. Geological Survey
MS 977 Denver Federal Center
Denver, CO 80225
Phone: (303) 236-9382 236-9391
E-mail: plillis@usgs.gov