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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Worldwide Distribution of Major Carbon Dioxide
Deposits: Geologic Setting and Gas Isotopic Evidence
of Mantle Sources in Areas of Crustal Extension and
Transtension
University of Houston
Deposits of CO2-rich gas (>50%) are present worldwide but in limited areas. One hundred twenty-one (121) have been identified and classified worldwide, but many others remain to be identified and studied. If encountered while drilling for oil and gas, CO2 can be either an expensive nuisance or an economic resource. Traditionally, explorationists have only wanted to know how to avoid CO2 deposits or at least learn how to calculate the risk of finding them. In certain areas there is now a desire to find CO2. Evidence of the source of the CO2 deposits is in their geologic setting and in the gas itself.
Geologic Setting
The 121 known deposits of CO2 are typically
located in areas of tectonic extension. They
are distributed among: cratonic arches, 14;
cratonic basins with basic igneous intrusions,
47; continental rifts, 13; areas of transtensional
basins related to tectonic escape, 12;
back-arc basins, 13; cross
-trends in foreland
basins just in front of thrust sheets, 12; platebounding
strike-slip faults, often near basaltic volcanism, 8; and
in thrust belts, 2. Although the geologic settings of these deposits
suggest mantle CO2 rising with mantle-derived basalts into the
crust, other evidence is needed to support a reliable model for a
source of CO2 deposits.
Gas Composition and Stable Isotopes
Evidence of source is available from the gas itself in the stable isotopic ratios of carbon in CO2 and in the content and isotopes of noble gases, especially of helium 3He/4He. The isotopic ratio of carbon 13 to carbon 12 reported as difference in parts per thousand from the PeeDee belemnite standard (d 13C 0⁄00 PDB) can aid in distinguishing different sources of CO2. The ratio can vary from below -10 0⁄00 for CO2 derived from organic matter to 0 0⁄00 for CO2 from calcined limestone. Mantle CO2 is around -5 but overlaps with CO2 from metamorphism of limestone. The ratio 3He/4He as compared to a standard that is the ratio in air (Ra) is another useful measure. Helium 3 is a marker that can document access to the mantle as proved by values on the midoceanic spreading centers. The use of both these isotopes will be shown in the talk.
Example of a Typical CO2 Deposit
A detailed study was made of a typical CO2 deposit, Bravo Dome
Field, New Mexico, U.S.A., which contains 283
billion cu. meters (10 trillion cu. ft.) of 99%
CO2. It is a combination structural
-stratigraphic
trap, with Permian Tubb Formation
sandstone pinching out on a basement nose
and sealed above by anhydrite. Gases were
specially sampled and analyzed, revealing a
dynamic gas deposit in which the noble gas
content varies systematically across the field
from near mantle values in the west, far above
the gas-water contact, to higher
concentrations in the east at the gas-water contact. We interpret
that CO2 entered the lowermost sandstone on basement at the
west side of the field from a basalt dike below, sweeping the connate
water of the sandstone down-dip as the trap filled. The field
is a window to the mantle because mantle gases are preserved to
the west, while in the east, atmospheric and crustal noble gases
enter the CO2 from the water below. The CO2 of the deposit is
dissolving down-dip into the water. That the CO2 of Bravo Dome
Field is clearly of magmatic origin is shown by d 13C values of -
3.7 to -5.1 0⁄00 PDB in the CO2 gas, by the relationships of noble
gas concentrations, by the isotopic ratio 3He/4He being as high as
4.26 Ra, and by the high CO2/3He ratio. This will be illustrated by
maps
, charts and graphs.
End_Page 17---------------
Exploration Use and Conclusions
Examination of other CO2 deposits worldwide will be shown to illustrate that a general model of generation of CO2 deposits is possible; the use of the model in exploration to either avoid or find CO2 will be explained.
We conclude that, in general, CO2 trapped in sedimentary rocks
came from the mantle. Fractures in the crust in areas of extension
allow basic magma to rise. CO2 is expelled from the magma
and enters porous reservoirs in sedimentary sections
and, where
adequate traps and seals are present, forms CO2 deposits. CO2 is
unrelated to hydrocarbons, migrating separately and at different
times.
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