Gran Tierra Energy Updates Recent Reserve Additions and New Field Development Planning Progress



    CALGARY, CANADA, September 13 /CNW/ - Gran Tierra Energy Inc. (OTC
Bulletin Board: GTRE), a company focused on oil exploration and production in
South America, today announced that initial estimates of reserves from the
independent reserve auditor's report by Gaffney, Cline & Associates (GCA) for
two recent oil discoveries in Colombia reported August 13, 2007, have been
updated to incorporate a third party over-riding royalty on Gran Tierra
Energy's interests in certain properties in Colombia, including the Chaza and
Guayuyaco Blocks where the discoveries are located. These amendments do not
materially impact the scale of these discoveries which, as previously
reported, more than tripled Gran Tierra Energy's acquired proved, probable and
possible oil reserves base, and almost doubled its acquired proved oil reserve
base with its drilling program executed in the first half of 2007.

    For the Costayaco oil discovery, GCA has now allocated to Gran Tierra
Energy proved reserves of 2.7 million barrels of oil (3.0 million previously
reported), with additional probable reserves of 5.5 million barrels of oil
(6.0 million previously reported), and additional possible reserves of 7.4
million barrels of oil (8.2 million previously reported).

    The discovery of the Costayaco field in the Chaza Block, located in the
Putumayo Basin of Colombia and operated by Gran Tierra Energy, was the result
of drilling the Costayaco-1 exploration well in the second quarter of 2007. As
previously reported, this well tested 5,906 barrels of oil per day. A staged
development program is being planned. Test oil production is currently being
transported by truck to existing facilities. A flow line is being designed to
replace trucking operations and to accommodate full commercial production. Two
additional wells are currently scheduled to be drilled. Drilling will be
initiated late in the fourth quarter of 2007 and will carry on into early
2008. Additional drilling and infrastructure construction will be contingent
on these well results. In addition, a new 3-D seismic survey planned for the
fourth quarter of 2007 has been expanded to cover 70 square kilometers.

    For the Juanambu discovery, GCA has now allocated to Gran Tierra Energy
proved reserves of 0.1 million barrels of oil (0.1 million previously
reported), with additional probable reserves of 0.3 million barrels of oil
(0.3 million previously reported), and additional possible reserves of 0.9
million barrels of oil (1.1 million previously reported).

    The discovery of the Juanambu field in the Guayuyaco Block, also located
in the Putumayo Basin of Colombia and operated by Gran Tierra Energy, was the
result of drilling the Juanambu-1 exploration well early in 2007. As
previously reported, this well tested 778 barrels of oil per day. Application
for commerciality of this discovery is proceeding. Test oil production is
currently being transported by truck to existing facilities. A flow line is
being designed to replace trucking operations and to accommodate full
commercial production. A second Juanambu well is being planned for drilling in
2008.

    As a result of the adjustments and production for the first half of 2007,
Gran Tierra Energy's estimate of proved reserves, net of royalties, as of June
30, 2007, stands at 5.6 million barrels of oil (5.9 million previously
reported), with additional probable reserves at 7.0 million barrels of oil
(7.5 million previously reported), and additional possible reserves of 11.2
million barrels of oil (12.2 million previously reported), for a total of 23.8
million barrels of proved, probable and possible oil reserves (25.6 million
previously reported). This contrasts to Gran Tierra Energy's December 31,
2006, proved reserves of 3.0 million barrels of oil, probable reserves of 1.2
million barrels of oil, and possible reserves of 2.9 million barrels of oil,
or a total of 7.1 million barrels of proved, probable and possible oil
reserves.

    About Gran Tierra Energy Inc.

    Gran Tierra Energy Inc. is an international oil and gas exploration and
production company, headquartered in Calgary, Canada, incorporated and traded
in the United States and operating in South America. Gran Tierra Energy holds
interests in producing and prospective properties in Argentina, Colombia and
Peru. To date, Gran Tierra Energy has pursued a strategy that focuses on
establishing a portfolio of producing properties, and development and
exploration opportunities, through selective acquisitions, to provide a base
for future growth. Additional information concerning Gran Tierra Energy is
available at http://www.grantierra.com. Investor inquiries may be directed to
info@grantierra.com or 1-800-916-GTRE (4873).

    General Statements

    GCA's statement of Proved reserves have been prepared in accordance with
the definitions set out in Rule 4-10 of Regulation S-X of the United States
Securities and Exchange Commission. GCA's statement of Probable and Possible
reserves have been prepared in accordance with generally accepted petroleum
engineering principles provided for in the document entitled Standards
Pertaining to the Estimating and Auditing of Oil and Gas Reserves Information,
promulgated by the Society of Petroleum Engineers (SPE). Reserve definitions
utilized are those approved by the Society of Petroleum Engineers, World
Petroleum Council, American Association of Petroleum Geologists and the
Society of Petroleum Evaluation Engineers in March 2007 and issued under the
name of Petroleum Resources Management System in March of 2007.

    Forward Looking Statements

    The statements in this press release regarding Gran Tierra's expectations
relating to the initiation of drilling of additional wells in 2007 and 2008,
the plan for the 3-D Seismic survey and the plan to design a flow line to
replace trucking operations and to accommodate full commercial production are
'forward-looking statements' within the meaning of Section 27A of the United
States Securities Act of 1933, as amended, and Section 21E of the United
States Securities Exchange Act of 1934. Although these forward-looking
statements reflect the good faith judgment of management, forward-looking
statements are inherently subject to known and unknown risks and uncertainties
that may cause actual results to be materially different from those discussed
in these forward-looking statements including, but not limited to, in the case
of these forward-looking statements, unexpected delays in drilling due to
equipment failure or other unforeseen reasons; unforeseen events, such as
unexpected difficulties in drilling or unexpected discoveries, may cause Gran
Tierra Energy to reallocate resources, which may delay drilling, surveys,
design plans and production or alter the mix of development drilling and
exploration drilling in Gran Tierra Energy's portfolio; and unexpected costs
may decrease funds available to carry out Gran Tierra Energy's plans described
in the press release, which could delay or prevent execution of these plans.
Readers are urged not to place undue reliance on these forward-looking
statements, which speak only as of the date of this release. Gran Tierra
Energy assumes no obligation to update these forward-looking statements to
reflect any event or circumstance that may arise after the date of this
release, other than as may be required by applicable law or regulation.
Readers are urged to carefully review and consider the various disclosures,
including the risk factors, made by Gran Tierra Energy in its reports filed
with the Securities and Exchange Commission, in particular its quarterly
report on Form 10-Q filed on May 15, 2007, which attempt to advise interested
parties of the risks and factors that may affect Gran Tierra Energy's
business. If one or more of these risks or uncertainties materialize, or if
the underlying assumptions prove incorrect, Gran Tierra Energy's actual
results may vary materially from those expected or projected.




For further information:

For further information: Gran Tierra Energy Inc. Dana Coffield,
800-916-4873 President & Chief Executive Officer info@grantierra.com or
Investor Relations: Cameron Associates Al Palombo, 212-245-8800 Ext. 209
al@cameronassoc.com

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Gran Tierra Energy Inc.

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