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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Geochemical Logic and Techniques for
Unconventional Gas Exploration
By
Humble Geochemical Services
Source rocks have been oil
productive for many years, typically
from highly fractured units or from adjacent porous intervals.
In addition source rocks may be good resources for gas if they
have the optimal organic richness and
thermal maturity necessary to have
converted both residual kerogen and any
retained
oil
to gas. Obviously, completion
engineering is a critical component of
extracting the gas from these systems, but
commercial rates will not be achieved if a
minimum level of conversion or thermal
maturity has not been reached.
While gas is generated in the oil
window from all kerogen types
(whether
oil
or gas prone), the presence of
black
oil
components
will occlude the limited permeability of a tight shale system,
resulting in low flow rates and precipitous decline rates. Thus,
even though gas shows are present in the
oil
window and measured
gas contents can appear commercial, it does not necessarily
indicate the likelihood of commercial shale gas production. Gas
window thermal maturity is a critical component of producibility
as the presence of higher molecular
black
oil
components will
occlude the limited permeability of a tight source rock and result
in low gas flow rates.
Thermal maturity should be assessed by both visual and chemical means. Visual methods such as vitrinite reflectance are the most common means and are widely used. Oftentimes, however, vitrinite reflectivity is not the best indication of the presence of producible gas from tight rocks. These data should be complemented by data obtained through chemical techniques. These techniques include Rock-Eval Tmax, kerogen transformation ratio, gas composition, carbon isotopes, and residual liquids fingerprinting. Using these techniques, the extent of organic matter conversion to gas can be accurately determined. The key point is that indications of thermal maturity may not necessarily agree with the extent of kerogen conversion, nor provide a good indication of the presence of problematic compounds. Gas risking plots can then be constructed to ascertain if all data provide a consistent assessment of gas producibility.
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