5 August 2008

Gran Tierra Energy Encounters Good Oil Shows with Costayaco-5 in Colombia; Preparing To Test

Drilling and Testing Operations Update for Colombia and Argentina

CALGARY, Alberta, August 5, 2008, Gran Tierra Energy Inc. (AMEX: GTE; TSX: GTE), a company focused on oil exploration and production in South America, today provided details on it’s exploration and delineation drilling programs, and infrastructure development taking place in Colombia and Argentina.


Chaza Block: Costayaco Field

Costayaco-5 Drilling and Testing

Gran Tierra Energy completed drilling of Costayaco-5 on July 27, 2008 when total measured depth of 8,703 feet was reached in basement. The well was successfully cased on July 31, 2008. The company encountered good oil shows in cuttings in the Kg Sand Unit of the Rumiyaco Formation, the U Sandstone Unit of the Villeta Formation, the T Sandstone Unit of the Villeta Formation and the Upper Caballos Formation.

Electric logs, cuttings and shows indicate excellent reservoir with fair to good oil saturations in the primary Villeta T reservoir, with thicknesses comparable to previous wells in the field. No definitive oil-water contact is apparent in the Villeta T reservoirs, but testing will be required to confirm whether water is present in certain intervals. The Upper Caballos reservoirs also appear to have good oil saturations and thicknesses comparable to previous wells drilled in the field. The Lower Caballos reservoirs are well developed, but appear to be water bearing. The top of the Villeta T and Caballos were encountered at a shallower depth than expected and oil shows were encountered deeper than expected in both the Villeta T and the Upper Caballos reservoirs; both results have the potential to add significant reserves to the west flank of the field. Testing operations are expected to commence in mid-August and will take approximately one month to complete.

The recently reported Costayaco mid-year reserve update did not incorporate potential oil at the Costayaco-5 location as this well was drilled outside the control provided by previously drilled Costayaco wells. The report indicated that effective July 1, 2008, the Costayaco field had gross proved reserves of 20.5 million barrels of oil, 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. If hydrocarbon testing is successful, these new reserves will be incorporated in Gran Tierra Energy’s year-end 2008 reserve report.

Costayaco-4 Testing

Gran Tierra Energy is expected to commence testing of Costayaco-4 in early August, 2008; delayed due to the extended testing at Palmera-1 by the test rig. Log interpretations from data acquired after drilling indicate potential hydrocarbon pay in the Kg Sand Unit of the Rumiyaco Formation, in addition to 14 feet in the U Sandstone Unit of the Villeta Formation, 38 feet in the T Sandstone Unit of the Villeta Formation and 125 feet in the Caballos Formation. This is almost 25% more potential net pay than in any of the previous three wells in the field. Gran Tierra Energy expects this testing program to take approximately one month.

Additional Drilling

The drilling of Costayaco-6 and initiating of drilling of Costayaco-7 remain on the program for 2008, with a continuous delineation and development drilling campaign in the Costayaco field continuing through 2009. The details of the 2009 program will be finalized in the fourth quarter of 2008.


An 8 inch, 10 kilometer pipeline from the Costayaco field to the Uchupayaco Station on the existing pipeline system was completed on July 29, 2008 and is currently being tested. This new pipeline will have a capacity of approximately 25,000 barrels of oil per day. Initial throughput will be approximately 8,500 barrels of oil per day due to facility constraints further downstream in the existing pipeline system. The company is developing options to increase production utilizing trucks to by-pass infrastructure constraints and expects gross Costayaco field production to rise to approximately 13,000 – 15,000 barrels of oil per day by year-end 2008, with additional production capacity behind pipe.

The Costayaco field is located in the Chaza Block in the Putumayo Basin, where the company has a 50% working interest and is the operator, with Solana Resources holding the remaining 50% working interest.

Azar Block: Palmera-1 Tests First Oil

Gran Tierra Energy initiated re-entry operations of the Palmera-1 well in May 2008, an exploration well drilled in 1996 that had potential oil pay in the Kg Sandstone of the Rumiyaco Formation indicated on logs but was never tested. Drill stem testing operations were conducted in June and July after a liner was set and perforated from 7,876 to 7,878 feet measured depth in the Kg Sandstone.

Stabilized natural flow of 13-15 degree API oil at a rate of approximately 47 barrels of oil per day with no water was obtained. The reservoir and oil appear to be similar to the producing Miraflor field in the adjacent Santana Block operated by Gran Tierra Energy. The well has been completed with a production string and will be shut-in for two months to evaluate optimum artificial lift systems and production facilities. The company expects initial production rates from the well to be in the range of 150 bopd to 250 bopd gross with artificial lift, with early production to be transported by truck.

Gran Tierra Energy is operator of the Azar Block and has an 80% working interest. Under the terms of a farm-in agreement, Lewis Energy will earn a 50% share of the company’s interest by paying 100% of Gran Tierra Energy’s costs associated with the first three exploration periods, including the Palmera-1 workover and drilling of one exploration well. The Azar Block is subject to the Agencia Nacional de Hidrocarburos (ANH) contract terms, with attractive fiscal terms including a sliding scale royalty commencing at 8% and no back-in rights by the state company Ecopetrol.

Rio Magdalena Block: Popa-2 Testing Operations Continuing

Gran Tierra Energy initiated drill-stem testing operations of the Popa-2 exploration well in the Rio Magdalena Block in the Middle Magdalena Basin, on July 4, 2008. The company encountered oil and gas shows during drilling in the Cretaceous Monserrate Formation, the primary reservoir target in the Popa prospect, and in the underlying basement. Four drill-stem tests have been completed and three additional drill-stem tests have been added. Results of the testing are expected in late August.

Gran Tierra Energy, with a 100% working interest, is the operator of the 144,670 acre Rio Magdalena Block. Under the terms of a recently completed farm-in agreement, Omega Energy Colombia will earn a 60% share of the company’s interest by paying 100% of the costs associated with drilling, testing and completing the Popa-2 well. In the event of a commercial discovery, Ecopetrol S.A. has a right to back in for a 30% working interest, to be split proportionally between Gran Tierra Energy and Omega Energy Colombia.


Surub’i Block – Proa.x-1 Exploration Well

Gran Tierra Energy began drilling the Proa.x-1 exploration well on July 9, 2008. The well is planned to be drilled to a depth of 12,992 feet to test the Cretaceous Palmar Largo reservoir section, which is productive in the Palmar Largo field approximately 2 miles to the northeast in the adjacent Palmar Largo Block. The company expects drilling to be completed in late August.

In addition, Gran Tierra Energy obtained provincial ratification of a new contract for the 90,688 gross acre Surubi Block with the Formosa Province on July 29, 2008. This new contract includes a 10-year contract extension to August 15, 2026. As part of this revised agreement, Gran Tierra Energy has assigned a 15% working interest to REFSA (Recursos Energ’eticos Formosa S.A.), the provincial government company, while retaining an 85% working interest and operatorship. REFSA will be carried by Gran Tierra Energy during drilling of the Proa.x-1 well. In the event of success, Gran Tierra Energy will be reimbursed for all of the costs incurred during drilling from 50% of the net production assigned to REFSA. The provincial royalties payable on production in the revised contract vary from 12% for cumulative production below 750,000 cubic meters of oil (approximately 4.7 million barrels), to 14% for cumulative production between 750,000 to 1,500,000 cubic meters of oil (approximately 9.4 million barrels), and to 16% for cumulative production above 1,500,000 cubic meters of oil.


Commenting on the drilling and testing progress, Dana Coffield, President and Chief Executive Officer of Gran Tierra Energy Inc., stated, “These are outstanding preliminary results from Costayaco-5. Successful testing of oil from this well has the potential to significantly add to the total reserve potential of the Costayaco field, which has already grown significantly in size as delineation drilling has shown in the first half of 2008. With our first produced oil on the nearby Azar Block, and the promising testing taking place in the Rio Magdalena Block, and exploration drilling continuing in Argentina, the third quarter of this year is proving to be particularly exciting for Gran Tierra Energy.”

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 www.grantierra.com. Investor inquiries may be directed to info@grantierra.com or 1-800-916-GTRE (4873).

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 Energy’s plans and expectations for reserves, production, drilling, testing and infrastructure, including the timing of commencement and completion of these events, 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 oil and gas; and unexpected problems due to technical difficulties, operational difficulties, and weather conditions. 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 for the period ended March 31, 2008, 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 http://www.sec.gov. The forward-looking statements contained herein are made as at the date of this press release. Subject to its obligations under applicable law, Gran Tierra Energy does not undertake an obligation to update forward-looking or other statements in this release. Gran Tierra Energy’s forward-looking statements are expressly qualified in their entirety by this cautionary statement.