Plateau Production of Approximately 19,000 Barrels of Oil per Day Gross
Established Early

CALGARY, Aug. 31 /CNW/ - Gran Tierra Energy Inc. (NYSE AMEX: GTE; TSX: GTE), a company focused on oil exploration and production in South America, today provided a reserve update, effective July 31, 2009, for the Costayaco field located in the Chaza Block of the Putumayo Basin of Colombia. Gran Tierra Energy also announced that it has completed the drilling and logging of Costayaco-9 and established plateau production in the Costayaco field ahead of schedule. Gran Tierra Energy is the operator of the Costayaco field and the Chaza Block with a 100% working interest.

"Attaining plateau production in the Costayaco field marks a major milestone in the history of our company," said Dana Coffield, President and Chief Executive Officer of Gran Tierra Energy. "Following our discovery in 2007 of Costayaco, one of the largest light oil field discoveries in Colombia in the last decade, we are on track to complete the development of the field by the end of this year. With well established reserves, and strong cash flow expected to be generated from Costayaco's plateau production in the coming years, Gran Tierra Energy is well positioned to focus on its upcoming 14 well exploration program in Colombia and Peru in late 2009 and 2010."

Costayaco Reserves Update

GLJ Petroleum Consultants have provided an independent reserve evaluation of the Costayaco field effective July 31, 2009. Taking into consideration 3D seismic, and drilling results from eight wells (Costayaco-1 through Costayaco-8), the reserve evaluation indicates that as of July 31, 2009, the Costayaco field had gross proved reserves of 19.7 million barrels of oil (MMbbl), gross proved plus probable reserves of 30.3 MMbbl, and gross proved plus probable plus possible reserves of 43.5 MMbbl.

Cumulative production from January 1, 2009 to July 31, 2009 was approximately 2.186 MMbbl gross, or 1.912 MMbbl net after royalty. Accounting for production, proven and probable reserves increased during 2009 due to additional development drilling (Costayaco-8), and technical revisions resulting from routine and special core analyses. These revisions include an increase in oil saturation estimates and an increase in the oil recovery factor for the field. The reduction in possible reserves was due to the previously announced results for Costayaco-7.

Gran Tierra Energy's reserves as of July 31, 2009, net after royalty, compared with year end 2008 for the Costayaco field, accounting for production during the period, are as follows:

-   Total net after royalty proved reserves (calculated in accordance
    with United States Securities and Exchange Commission rules) were
    14.16 MMbbls at July 31, 2009 compared with 15.29 MMbbls at year end
    2008, a decrease of 7%. The decrease is due to production, partially
    offset by upward technical revisions and reserve additions from
    Costayaco-8;
-   Total net after royalty proved plus probable reserves (calculated in
    accordance with Canadian requirements (NI 51-101)) were 21.31 MMbbl
    at July 31, 2009 compared with 21.49 MMbbl at year end 2008, a
    decrease of 1% due entirely to production; and
-   Total net after royalty proved plus probable plus possible reserves
    (calculated in accordance with Canadian requirements (NI 51-101))
    were 30.37 at July 31, 2009 compared with 36.25 at year end 2008, a
    decrease of 16%, with one third of the decrease due to production.
The following table summarizes gross reserves for the field, and Gran
Tierra Energy's gross and net reserves. Proved reserves are based on SEC
reserve definitions with a July 31, 2009 posted price.