Gran Tierra Energy Provides Mid-Year Update to Costayaco Field Reserves, Colombia

    Proved, Probable and Possible Reserves, Net After Royalty, Increase to
    Million Barrels of Oil

    CALGARY, Alberta, July 29 /CNW/ -- Gran Tierra Energy Inc. (Amex:   GTE;
TSX: GTE), a company focused on oil exploration and production in South
America, today announced the results of a mid-year reserve update for the
Costayaco Field in the Chaza Block located in the Putumayo Basin of Colombia.
Gran Tierra Energy holds a 50% working interest and is the operator of the
Costayaco Field and the Chaza Block.  Solana Resources Limited holds the
remaining 50% working interest.
    An independent reserve evaluation of the Costayaco field conducted by GLJ
Petroleum Consultants effective July 1, 2008, using 3-D seismic and drilling
results from four wells (Costayaco-1 through -4, not including test results of
Costayaco-4 as these test results are not yet available) indicates that the
Costayaco field has gross proved reserves of 20.5 million barrels of oil
(MMbbl), gross proved plus probable reserves of 34.9 million barrels of oil
and gross proved plus probable plus possible reserves of 61.4 million barrels
of oil.  The following table summarizes gross reserves for the field, in
addition to Gran Tierra Energy's gross and net reserves:

                                                 Gran Tierra    Gran Tierra
    Light and Medium Oil                           Energy         Energy
    (SEC Compliant)             Gross Reserves     Working       Net After
                                                  Interest        Royalty
                                                  Reserves       Reserves
    Reserves Category                MMbbl          MMbbl          MMbbl
          Producing                   9.64           4.82           3.39
          Developed Nonproducing      3.58           1.79           1.07
          Undeveloped                 7.33           3.66           2.21
    Total Proved                     20.54          10.27           6.67
    Total Probable                   14.37           7.19           4.32
    Total Proved Plus Probable       34.91          17.46          10.99
    Total Possible                   26.48          13.23           7.91
    Total PPP                        61.39          30.69          18.90
    Gran Tierra Energy's reserves, net of royalty, compared to year end 2007
for the Costayaco Field as of July 1, 2008 are as follows:
    -- Proved reserves are 6.67 million barrels of oil compared to 3.27
       million barrels of oil at year end 2007, an increase of 104%;
    -- Probable reserves are 4.32 million barrels of oil compared to 3.32
       million barrels of oil at year end 2007, an increase of 30%;
    -- Possible reserves are 7.91 million barrels of oil compared to 2.60
       million barrels of oil at year end 2007, an increase of 204%;
    -- Total proved, probable and possible, net after royalty, reserves in the
       Costayaco Field are 18.90 million barrels of oil compared to 9.19
       million barrels of oil at year end 2007, an increase of 106%.
    Commenting on the reserve update, Dana Coffield, President and Chief
Executive Officer of Gran Tierra Energy Inc., stated, "The Costayaco Field in
Colombia continues to exceed our expectations.  Drilling in the first half of
2008 has added to our understanding of the reserve potential of the field.
Pending test results from Costayaco-4 and drilling results from Costayaco-5
are expected to add material new information to the reserve potential as we
continue to delineate and develop the field with a continuous drilling program
through the balance of this year."
    About Gran Tierra Energy Inc.:
    Gran Tierra Energy Inc. is an international oil and gas exploration and
production company operating in South America, headquartered in Calgary,
Canada, incorporated in the United States, and trading on the American Stock
Exchange (GTE) and the Toronto Stock Exchange (GTE). The company holds
interests in producing and prospective properties in Argentina, Colombia and
Peru. The company has a strategy that focuses on growing a portfolio of
producing properties, plus production enhancement and exploration
opportunities to provide a base for future growth. Additional information
concerning Gran Tierra Energy is available at
Investor inquiries may be directed to or 1-800-916-GTRE
    Cautionary Statement:
    Possible reserves are those additional reserves that are less certain to
be recovered than probable reserves.  There is a 10% probability that the
quantities actually recovered will equal or exceed the sum of proved plus
probable plus possible reserves. The estimate of reserves for individual
properties may not reflect the same confidence level as estimates of reserves
for all properties, due to the effects of aggregation.
    Forward Looking Statements:
    The statements in this news release regarding Gran Tierra plans and
expectations for testing and drilling, and expectations as to results of these
testing and drilling operations, are forward looking information, forward
looking statements or financial outlooks (collectively, "forward-looking
statements") under the meaning of applicable securities laws, including
Canadian Securities Administrators' National Instrument 51-102 Continuous
Disclosure Obligations and the United States Private Securities Litigation
Reform Act of 1995.  These forward-looking statements are subject to risks,
uncertainties and other factors that could cause actual results or outcomes to
differ materially from those contemplated by the forward-looking statements.
Although, Gran Tierra Energy believes that the assumptions underlying, and
expectations reflected in, these forward-looking statements are reasonable, it
can give no assurance that these assumptions and expectations will prove to be
correct. Important factors that could cause the results or outcomes discussed
herein to differ materially from those indicated by these forward-looking
statements, include, among other things:  Gran Tierra Energy's operations are
located in South America, and unexpected problems can arise due to guerilla
activity, technical difficulties and operational difficulties which impact the
production, transport or sale of its products, geographic, political and
weather conditions; and the usefulness of the information from the test
results from Costayaco-4 and drilling results from Costayaco-5 may be
different than Gran Tierra Energy predicts.  Further information on potential
factors that could affect Gran Tierra Energy are included in risks detailed
from time to time in Gran Tierra Energy's Securities and Exchange Commission
filings, including, without limitation, under the caption "Item 1A - Risk
Factors" in Part II of Gran Tierra Energy's Quarterly Report on Form 10-Q,
filed with the Securities and Exchange Commission on May 12, 2008.  These
filings are available on a Web site maintained by the Securities and Exchange
Commission at The forward-looking statements contained
herein are made as at the date of this press release. Gran Tierra Energy does
not undertake an obligation to update forward-looking or other statements in
this release except as may be required by law. Gran Tierra Energy's
forward-looking statements are expressly qualified in their entirety by this
cautionary statement.


     Dana Coffield                                 Al Palombo
     Gran Tierra Energy Inc.                       Cameron Associates
     President & Chief Executive Officer           Investor Relations
     (800) 916-4873                                (212) 245-8800 Ext. 209                 


For further information:

For further information: Dana Coffield, President & Chief Executive
Officer of Gran Tierra Energy Inc., 1-800-916-4873,; or
Investor Relations, Al Palombo of Cameron Associates, +1-212-245-8800, Ext.
209, Web Site:

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