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 km2, 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.

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 40.000 Km2 comprises a series of north-northeast and south-

southwest 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).

Santa Cruz

Brazil

Argentina

Chile

Paraguay

Perú

Trinidad

La Paz

GUAPORE SHIELD

BOOMERANG

CHIQUITOS

BENI

MADRE DE DIOS

CHACO PLAINSFOOTHILLS

NORTHERN SUBANDEAN

SOUTHERNSUBANDEAN

EASTERNCORDILLERA

ALTIPLANO

Figure 1 Figure 2

Caranda Field

Rio Grande Field

PETACA

CAJONES

TAIGUATI

TUPAMBI

LIMONCITO

/ EL CARMEN

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).

0 10 20

2000 0

-8000 -12000

-4000

A A'

Figure 3

0

-5000

-10000

B B'

Figure 4

--5000

0

--10000

--15000

C C'

Figure 5

Eastern Cordillera Pescado Pintascayo

Bermejo Field

Ramos Field

Aguaragüe Field Field

Campo Durán Field

Field

TERTIARY

MESOZOIC

CARBONIFEROUS

UPPER DEVONIAN-LOS MONOS Fm

MIDDLE DEVONIAN HUAMAMPAMPA Fm

LOWER DEVONIAN-SILURIAN

SOUTHERN SUB-ANDEAN RANGES FOOTHILLS

0 10 20

0 10 20 Km

TERTIARY

MESOZOIC

CARBONIFEROUS

UPPER DEVONIAN-LOS MONOS Fm.

MIDDLE DEVIAN HUAMAMPAMPA Fm.

LOWER DEVONIAN-SILURIAN

TERTIARY

MESOZOIC

CARBONIFEROUS

UPPER DEVONIAN-LOS MONOS Fm

MIDDLE DEVONIAN HUAMAMPAMPA Fm.

LOWER DEVONIAN-SILURIAN

SOUTHERN SUB-ANDEAN RANGES FOOTHILLS

SOUTHERN SUB-ANDEAN RANGES FOOTHILLS

San Josecito Huayco San

Antonio

Domo

Pilcomayo Villamontes Iguitinti Sayurenda

Ipaguazu Aguaragüe San Alberto Field

Iñiguazi Domo Oso Castellón

Margarita Field

Km

Km Km

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 50.000 Km

2 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 Fields, 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 Field

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 silica-

cemented 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).

NNN

Falla B

Falla A

1 Km.1 Km.1 Km.

CHACO FM.

PETACA FM.

CAJONES FM.

TAIGUATI FM.

TUPAMBI FM.LOS MONOS FM.

A BSouth North

Figure 11: Seismic Line in the Caranda Field. See Figure 10 for location

Figure 10: Structural time map from top Tertiary Petaca Fm. .in Caranda Field.

Figure 9 : Remaining Gas Reserves in the Boomerang

Trend Play

N

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

References

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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. 545-555.

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.

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