Gran Tierra has 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.

Production Plateau Plateau production of 19,000 barrels of oil per day (BOPD) gross for the Costayaco field, scheduled to be attained in the fourth quarter of 2009, has now been established. Production has been averaging approximately 16,700 BOPD net after royalty from the Costayaco field since attaining plateau production in early August. An increase in royalties expected at the end of the third quarter of 2009, once cumulative production reaches 5 million barrels for the Costayaco field, will subsequently result in a net after royalty production level of approximately 13,000 BOPD from the Costayaco field. This is in addition to expected production of approximately 1,200 BOPD net after royalty from Gran Tierra Energy's other operated fields in the Putumayo Basin, and approximately 1,000 BOPD net after royalty production from Argentina.

Costayaco-9 Gran Tierra Energy has completed drilling and logging operations for Costayaco-9, a vertical development well located 1,950 feet southwest of Costayaco-8. Well logs indicate that the T Sandstone of the Villeta formation lies completely within the field's oil column and that the Caballos formation encountered an oil-water contact consistent with the established Caballos oil-water contact for the field.

The well reached a total measured depth (MD) of 8,580 feet on August 17, 2009 after drilling through the same reservoir sequences encountered in previous wells drilled in the field. Log interpretations from data acquired after drilling indicate the presence of reservoir sandstones in the T sandstone beginning at 8,197 feet MD with an approximate net pay thickness of 22 feet, as well as in the Caballos sandstone beginning at 8,321 feet MD with approximately 46 feet of net pay. These results are consistent with pre-drill estimates.

Completion and testing operations are currently underway at Costayaco-9 with the drilling rig. Test results are expected in the latter part of September.

Costayaco-10 Costayaco-10 is scheduled to be drilled next, with drilling expected to begin in late September. It will be drilled approximately 2,500 feet southwest of Costayaco-9. The main objective of Costayaco-10 is to add production capacity to assist maintaining the gross production plateau of 19,000 BOPD from Costayaco field. In addition, it is expected to convert probable and possible reserves into proven and probable reserves. This is the last well planned to be drilled in the Costayaco field in 2009.

Tags: Gran Tierra Energy Inc.