Figure 1. Forecasting efficiencies based on good and bad luck as well as use of geophysical and geochemical technologies.
Hydrocarbon exploration has evolved through time as a result of the prospecting tools that have been developed. Early explorations needed to either step in oil or observe the burning of organics as evidence for a possible hydrocarbon accumulation. Cherry picking season ended some time ago, however.
Successful explorations now have a variety of tools available to them to aid their evaluation of prospects. This evaluation includes geological, geophysical, and geochemical data or suppositions. This process is now referred to as petroleum system evaluation. Basically, a petroleum system analysis says that there must be
or, quite simply, NO COMMERCIAL AMOUNTS OF HYDROCARBONS WILL BE FOUND.
Geological studies and intuition lead the way and geophysical analysis provides substantiation of geological work and the availability and size assessment of traps. However, without hydrocarbon charge, a large trap is like an empty cupboard.
Shell found many years ago that forecasting efficiencies could be enhanced when geochemical evaluations were included in their assessment of prospects. Murris (1984) showed forecasting efficiency was improved 63% over random drilling of prospects, whereas forecasting efficiency was improved 28% using trap size only (Figure 1).
Figure 1. Forecasting efficiencies based on good
and bad luck as well as use of geophysical and geochemical
technologies.
What are the tools
of geochemistry and how are they applied?
Geochemical evaluation focuses on identifying the source of oil
and whether sources are mature enough to generate hydrocarbons,
and what type of hydrocarbons are likely to be generated. In
addition the timing of generation can now be predicting using
source or oil derived kinetic parameters. This has been extended
by Humble to include the kinetics of
oil and gas generation by compositional fractions. In addition the
yield of gas and oil can be predicted to the point of GOR values
at various levels of maturity
(
et al., 1999). Bulk kinetic models
have trouble predicting the first oil generated, whereas true
compositional kinetics are able to predict this early oil. In
addition data on gas generation showing gas generation throughout
the oil window and its variation with kerogen type (or the oil
window) is now available
(
et al., 1999).
While good source rocks are not always encountered in drilling reservoirs, oil samples can tell us a great deal about the source rock. Oil studies enable description of the likelihood of different source rocks, their depositional environment, maturity, lithofacies, and, in some case, even the likely age of the source. Further, work is now being presented to show that source rock kinetics can be predicted from oil chemistry so the timing of hydrocarbon generation can be determined even without the actual source rock ( et al., 1999; et al., 1999).
Basin modeling utilizes geochemical data to optimize the understanding of burial and thermal histories. These data can be taken and extrapolated to seismic lines to model well locations to evaluate the best prospects for future drilling. In this case prior to drilling a well in a wildcat area, predictions as to hydrocarbon charge can be made accurately, but with sensitivity analysis to enable high grading of various prospects or well locations.
Conclusion
If you can step in oil, there is a chance oil is at least seeping
from an accumulation and that an active source rock is, at least,
present. However, if you can't step in it, your ability to find oil
can be improved 63% by appropriate application of geochemical
techniques. When integrated with geological and geophysical data,
you might just find the next hot play.
References
Murris, R. J., 1984, Introduction, Petroleum Geochemistry and Basin Evaluation, AAPG Memoir 35, G. Demaison and R. Murris, eds., pp. x-xii.
, , and , 1999, Prediction of Source Rock Hydrocarbon Generation Kinetics from Petroleum Asphaltenes, 19th International Meeting on Organic Geochemistry, Istanbul, submitted.
, , , , and D. A. Wavrek, 1999, Compositional Kinetic Parameters and Yields from Oil and Gas Prone Source Rocks and Oil Asphaltene Fractions, 19th International Meeting on Organic Geochemistry, Istanbul, submitted.
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