Drilling and Testing Operations Update for Colombia and Argentina



    CALGARY, Alberta, Aug. 6 /EMWNews/ -- Gran Tierra Energy

Inc. (Amex: GTE; TSX: GTE), a company focused on oil exploration and

production in South America, today provided details on its exploration and

delineation drilling programs, and infrastructure development taking place

in Colombia and Argentina.






    COLOMBIA



    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.



    Infrastructure



    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.



    ARGENTINA



    Surubi 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

Energeticos 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.



    SUMMARY



    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 (Amex: GTE) and the Toronto Stock Exchange (TSX: 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

http://www.grantierra.com. Investor inquiries may be directed to

[email protected] 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.






     Contacts:



     Dana Coffield                                 Al Palombo

     Gran Tierra Energy Inc.                       Cameron Associates

     President & Chief Executive Officer           Investor Relations

     (800) 916-4873                                (212) 245-8800 Ext. 209

     [email protected]                           [email protected]