Gas Plays The three proven gas plays are: 1) Southern Sub-Andean Ranges, 2) Foothills and 3) Boomerang trend, Figure 1.

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1 Different Gas Plays with a Huge Potential in the Tarija Basin of Bolivia and Argentina Belotti, H.J. (Pecom Energía S.A.), C.E. Cruz (Pluspetrol E&P) and R. Giraudo (Repsol- YPF) Introduction Three proven gas plays have been identified in the Sub-Andean Thrust Belt and Foothills of the Tarija Basin in Bolivia and northwestern Argentina (Fig. 1). In this gas province, which encompasses an area of approximately 113,000 km 2, over 52 TCF of proven and probable gas reserves have been discovered from 76 fields. Almost 39 TCFG (75 %) of those were discovered in 3 fields during the last 4 years. The remaining undiscovered reserves expecting to be found in this area are in the order of 60 TCFG. Figure 1 Figure 2 MADRE DE DIOS Brazil Perú PETACA NORTHERN SUBANDEAN La Paz Trinidad BENI GUAPORE SHIELD BOOMERANG CAJONES TAIGUATI ALTIPLANO EASTERN CORDILLERA Caranda Santa Cruz CHIQUITOS Rio Grande CHACO PLAINS TUPAMBI LIMONCITO Chile SOUTHERN SUBANDEAN Paraguay Argentina / EL CARMEN Gas Plays The three proven gas plays are: 1) Southern Sub-Andean Ranges, 2) Foothills and 3) Boomerang trend, Figure 1. 1) Southern Sub-Andean Ranges Covering an area of around comprises a series of north-northeast and southsouthwest oriented structural trends. Thin-skinned deformation originated a tectonic eastward verging in sequence wedge. Shortening was transmitted from the basal detachment located at the Silurian Kirusillas shales to the foreland. Important thickness variations were developed within the incompetent Los Monos shales; instead, the overlying Carboniferous and Tertiary units are deformed passively (Belotti et al., 1995; Giraudo et al, 1999).

2 Main gas reservoirs in the Sub-Andean thrust belt occur in the Devonian Huamampampa and Santa Rosa fractured marine silica-cemented quartz-arenites (Figure 2), deposited during global lowstand episodes, (Stark, 1995). A huge volume of Huamampampa sands was constrained to a coastal ramp setting as lowstand prograding shoreface (Fernandez Seveso et al, 2000). The arenites contain a large numbers of natural fractures connecting the low matrix and micro-fracture porosities. As a result, the average porosity of the system ranges from 2 to 6%. While in Ramos and Aguaragüe fields (Figure 3), the intensively fractured zones are associates to the crest and front limb of the structures, in the San Alberto and San Antonio fields (Figures 5 and 6), the fractured zones are also present in the back limb, due to the higher steep (60-70 ) of the western flank. A series of east-west balanced structural cross-sections show the main structural trends of this play (Figures 3 to 5) (Giraudo et al, 1999; Giraudo and Belotti, 2000). A SOUTHERN SUB-ANDEAN RANGES A' Eastern Cordillera Pescado Pintascayo Bermejo Ramos Aguaragüe Campo Durán TERTIARY MESOZOIC CARBONIFEROUS UPPER DEVONIAN-LOS MONOS Fm MIDDLE DEVONIAN HUAMAMPAMPA Fm LOWER DEVONIAN-SILURIAN B SOUTHERN SUB-ANDEAN RANGES Figure 3 B' Castellón Domo Oso Iñiguazi San Alberto Aguaragüe Ipaguazu TERTIARY MESOZOIC CARBONIFEROUS UPPER DEVONIAN-LOS MONOS Fm. MIDDLE DEVIAN HUAMAMPAMPA Fm. LOWER DEVONIAN-SILURIAN Figure 4 C San Josecito SOUTHERN SUB-ANDEAN RANGES Huayco Margarita San Antonio Domo Pilcomayo Villamontes Sayurenda Iguitinti C' TERTIARY MESOZOIC CARBONIFEROUS UPPER DEVONIAN-LOS MONOS Fm MIDDLE DEVONIAN HUAMAMPAMPA Fm. LOWER DEVONIAN-SILURIAN Figure 5

3 Proved + Probable remaining gas reserves distribution in the Southern Sub-Andean Ranges Gas Play is shown in Figure 6 (source: Cámara Boliviana de Hidrocarburos, January 2001). Almost 45.7 TCFG have been discovered from 18 fields and 39 TCFG of those were discovered in 3 fields (Margarita, San Alberto-Itaú and San Antonio) during the last 4 years. The contribution of this play to the total gas reserves discovered in Northern Argentina and Bolivia is 85.6%. The remaining undiscovered reserves expected to be found in this Play range around 55 TCFG. Figure 6: Remaining Gas Reserves in the Southern Sub-Andean Play. Figure 7: Remaining Gas Reserves in the Foothill Play 2) The Foothills Covering an area of nearly comprises a series of north-northeast and south-southwest oriented structural trends at the front of the Sub-Andean Ranges (Figures 1, 3, 4 and 5). An internal detachment, located at the base of Los Monos shales, generates a series of eastward-verging gentle fault propagation folds, as a result of the transmission of the shortening from the Sub-Andean ranges to the east (Figure 5). Gentle anticlines associated to fault propagation folds detached at the Silurian Kirusillas Fm. conform the biggest structure. Two of the most important gas fields discovered between the 50 and the 60, Río Grande and Campo Duran s, belong to this play (Figures 3 and 8). Main gas reservoirs in the Foothill occur in the fluvial Carboniferous Taiguati and Tupambi formations and in the Tertiary Petaca Fm. (Fig. 2). The sandstone s porosity ranges from 14 to 20 %. Figure 8: Time Structure Map from top Carboniferous Taiguati Fm. In Río Grande

4 3) The Boomerang trend The sharp bend of the Sub-Andean Thrust Belt in the Santa Cruz Elbow (Figure 1) generates a structural setting with gentle strike-slip related anticlines, that bears a series of gas fields. Within this hinge zone, the Silurian-Devonian sedimentary wedge thins rapidly over a basement structured by Silurian extensional faults. The east-west orientation of the hinge zone coincides with the east-west trend of the Silurian sequence, resulting in the configuration of the Boomerang Hill, oblique to the Andean tectonic transport direction (Welsink et al., 1995). Proved + Probable remaining gas reserves distribution in the Boomerang Trend gas play is show in Figure 9 (source: Cámara Boliviana de Hidrocarburos, January 2001), where 25 fields have almost 3.7 TCFG. The contribution of this play to the total gas reserves discovered in Northern Argentina and Bolivia is 6.8 %. The remaining undiscovered reserves expected to be found in this Play is less than 1 TCFG. The fluvial sandstones of Tertiary Petaca Fm., the shallow water Cretaceous Cajones and Yantata Fm. with porosities ranging between 12 to 20 % and the fractured marine silicacemented quartz-arenites of Santa Rosa Fm. are the main gas reservoirs in the Boomerang Trend. The Caranda anticline located near to the Andean Emergent Thrust Front, is the biggest anticline of this play (Figures 1, 10 and 11). Figure 9 : Remaining Gas Reserves in the Boomerang Trend Play N N South A B North CHACO FM. PETACA FM. CAJONES FM. Falla B Falla A TAIGUATI FM. TUPAMBI FM. LOS MONOS FM. 1. Figure 10: Structural time map from top Tertiary Petaca Fm..in Caranda. Figure 11: Seismic Line in the Caranda. See Figure 10 for location

5 Source Rocks and Timing Middle Devonian (Eifelian-Emsian) Los Monos and Huamampampa black shales (Figure 2) are the main source rocks in Southern Sub-Andean Ranges and in the Foothill of south Bolivia and North Argentina. The Average TOC is about 1.2 % and the average S1 + S2 is 5 mg HC/g. In the Foothill of Central Bolivia the probable source rocks are the dark gray shales of Lower Devonian (Lochkovian) Santa Rosa and Icla formations, recording the highest organic contents. In the Boomerang Trend, the Middle Devonian Limoncito and Silurian El Carmen dark gray to black shales are the main source rocks (Moretti et al., 1995; Disalvo and Villar, 1999; Cruz et al., 2001). The Tertiary units deposited during the Andean tectonics triggered the maturation process. Expulsion from the source rocks to the neighboring structural traps occurred primarily within in the last 10 my (Dunn et al., 1995, Moretti et al.1996). Figure 12: Los Monos Fm. Huamampampa Fm. (!) Petroleum System Event Chart Southern Sub-Andean Ranges Figure 13: El Carmen/Robores Fm. Petaca Fm. (!) Petroleum System Event Chart in the Boomerang Play

6 References Belotti, H. J., L. L. Saccavino, and G. A. Schachner, 1995, Structural styles and Petroleum Occurrence in the sub-andean Fold and Thrust Belt of Northern Argentina, in A. J. Tankard, R. Suarez S. and H. J. Welsink, Petroleum Basins of South America: AAPG Memoir 62, pp Cruz, C. E., Albariño L. M., Sylwan and C. A., Villar H. J., 2001, Source Rocks and Hydrocarbons South of The Santa Cruz Elbow, Bolivia and Northwestern Argentina. Expanded Abstract, 2001 AAPG Annual Convention, Denver. Disalvo A. and H. J. Villar, 1999, Los Sistemas Petrolíferos del Area Oriental de la Cuenca Paleozoica Noroeste, Argentina. IV congreso de exploración y desarrollo de Hidrocarburos, Actas I, pp Dunn J., K. Hartshorn and P. Hartshorn, 1995, Structural Styles and Hydrocarbon Potential of the Sub-Andean Thrust Belt of southern Bolivia, in A. J. Tankard, R. Suarez S. and H. J. Welsink, Petroleum Basins of South America: AAPG Memoir 62, pp Fernandez Seveso, F., M. C. Vistalli and R. F. Viñes, 2000, Reservoir correlation in the Silurian-Devonian Chaco Basin, Boletín de Informaciones Petroleras, Argentina. Giraudo, R., R. Limachi, E. Requena and H. Guerra, 1999, Geología estructural de las regiones Subandino y Piedemonte, Bolivia. Actas del IV Congreso de Exploración y Desarrollo de Hidrocarburos, Mar del Plata, Argentina. Giraudo, R. and H. J. Belotti, Reducing Exploration Risk in a New and Growing Regional Energy Market in South America. Abstract, 2000 AAPG Annual Convention, New Orleans, USA. Moretti I., E. Díaz Martínez, G. Montemurro and M. Pérez, 1995, The Bolivian Source Rocks. Sub Andean Zone, Madre de Dios, Chaco. Revue de l Institut Français du Pétrole 50, pp Moretti I., P. Baby, E. Méndez and D. Zubieta, 1996, Hydrocarbon generation in relation to thrusting in Sub Andean Zone from 18 to 22 S, Bolivia, Petroleum Geoscience, Vol. 2, pp Stark, D., 1995 Silurian-Jurassic stratigraphy and basin evolution of Northwestern Argentina, in A. J. Tankard, R. Suarez S. and H.J. Welsink, Petroleum Basins of South America: AAPG Memoir 62, pp Welsink, H. J., A. Franco M., and C. Oviedo G., 1995, Andean and Pre-Andean Deformation, Boomerang Hills Area, Bolivia, in A.J. Tankard, R. Suarez S. and H. J. Welsink, Petroleum Basins of South America: AAPG Memoir 62, pp