Province Boundary

The Santa Cruz – Tarija Province is a subequant, 400,000-sq-km area, comprising parts of southeastern Bolivia (57% of the province), western Paraguay (33% of the province), northern Argentina (5% of the province), and Brazil (5% of the province) (Figure 2a and Figure 2b). Maximum north-south extent is 700 km in the western (Andes) mountains, with an east-west dimension averaging 700 km.

Geographic Setting

A salient of the thin-skinned fold and thrust belt, called "sub-Andean" and located east of the Main Andean frontal thrust, comprises the western part of the province. Excluded are – each successively farther west – Bolivia’s thick-skinned, thrusted, basement-involved eastern Cordillera (oriental), Bolivia’s altiplano, and Chile’s western Cordillera (occidental) (Figure 3). "Basement" here includes non-reservoir, Cambro-Ordovician and older rock. The sub-Andean trend is approximately ten times the area of the productive Absaroka trend in the western United States fold and thrust belt.

Included to the northwest is the thrust front bend at Boomerang Hills (Figure 2a), an oblique-ramp transfer zone caused by the non-orthogonal relationship of a northward-truncating Paleozoic sedimentary wedge to eastward compressional forces. Included to the south is a portion of Argentina’s Northwestern basin. The Chaco plain is a Quaternary veneer over the Chaco-Tarija foreland basin (northeast Curupaity and southwest Carandaity sub-basins separated by the Paraguayan central Chaco high). The Brazilian (Guapore) shield deforms the north-northeastern province edge with block faulting (Sierra Chiquitanas) and exposes Precambrian section, as does the Asuncion arch near the Paraguay River on the east. To the south is the subsurface Michicola arch.

Political Entities

Governments of the Santa Cruz – Tarija Province are relatively stable, with current or former national oil companies and a history of international cooperation with industry and academia.

Geologic Setting

Santa Cruz – Tarija has a history of episodic extension and compression resulting from the complex tectonic history along the western margin of South America. Sedimentation patterns and tectonic style intertwined repeatedly, and structural elements were reactivated. The Brazilian shield and Asuncion arch were persistently exposed.

"Basement" is heterogeneous terrane accreted onto southwestern Gondwana during late Precambrian through early Paleozoic time when it was in a polar setting (Williams, 1995; Eyles and others, 1995). The Paleozoic intracontinental Chaco-Tarija basin of Bolivia and Paraguay overlies the suture of the Arequipa massif to the Gondwanan Brazilian shield. Extensive Cambro-Ordovician rifting resulted in deposition of a maximum 7 km of marine rocks, influenced by northwest-southeast and northeast-southwest structural trends (Wiens, 1995). Much of the Arequipa source area would eventually translate southward (Carboniferous) to become Patagonia and part of Antarctica (Williams, 1995).

The late Ordovician Ocloyic orogeny resulted from island arc accretion to the Pampean terrane south of Chaco-Tarija. The orogeny interrupted rift deposition with inversion and resulting unconformity at the Ordovician-Silurian boundary. Silurian Chaco-Tarija rifting resumed at a slower rate, and marine conditions became suitably anoxic for deposition of the Kirusillas / El Carmen shale source rocks.

Devonian warmer climate and increased subsidence – from active oceanic plate subduction to the west – resulted in another several-thousand meters of marine sedimentary record, including the middle Devonian Los Monos shale source rocks. More tectonic activity to the southwest (late Devonian Chanic orogeny) was recorded as another Chaco-Tarija unconformity when the Chilenian terrane was emplaced and Arequipa began its translation southward.

Carboniferous major global plate interactions signaled the completed assembly of Pangea and formed several intracratonic highlands in South America, including the Asuncion arch to the east and the remaining Arequipa massif bordering Chaco-Tarija to the west. At 30° south latitude, these mountains were often glaciated, and the Carboniferous rock record in Chaco-Tarija comprises "glacially influenced" (terminology of Eyles and others, 1995) fluvial to turbidite deposits filling topographic relief as great as 500 meters.

Warmer Permian climate coincided with another orogenic episode of uplift and inversion in southwestern Gondwana. Chaco-Tarija experienced both deposition and erosion, and a restricted marine seaway with some carbonate deposition extended from the northwest. Triassic deposition was sparse, and a subsequent hiatus lasted from 210 Ma through the Jurassic period to 130 Ma.

The late Jurassic Araucanian orogeny marks the opening of the south Atlantic, separating Africa from South America. Uplift of the southern Michicola arch, the northern terminus of a major Cretaceous (Lomas de Olmedo) graben system south of this province, resulted in the erosion there of much Paleozoic section, including the source rocks. A similar situation existed at Boomerang Hills to the north (Figure 4). Subsequent impact during Andean (Tertiary) thrusting was to laterally terminate the thin-skinned thrusts and folds where these truncations and loss of prominent detachment surfaces occur.

Cretaceous extension affected Chaco-Tarija with southeasterly directed transgression. Late Cretaceous to late Tertiary, west-to-east propagating thrusts (Andean orogeny) created the Andes mountains, loading the easterly Chaco plain into a flexural foreland basin and forebulge/backbulge complex that migrated eastwardly through time (Horton and DeCelles, 1997; Dunn and others, 1995) (Figure 5). The sub-Andean part of the deformation initiated in late Oligocene time. A wedge of Cenozoic continental sedimentation subsequently filled the foreland. Buried, growing thrust structures underlie the westernmost Chaco-Tarija basin east of the Andes Mountain topographic front. Tectonic shortening ranges laterally from tens to several hundreds of kilometers.

Province Exploration History

Nine hundred eighty-two total wells penetrated this province as of 1995 (Petroconsultants, 1996), with 715 in Bolivia, 229 in Argentina, 36 in Paraguay, and two in Brazil (both stratigraphic tests).

Bolivia’s first wildcat exploratory well, along the central mountain front, failed in 1914 (Figure 6). Exploration activity continued ten years later with another well (oil show) in the same area, and two others at the southern border with Argentina (one with an oil show and the other a discovery of Bermejo field). One Argentina wildcat failure in 1920 was followed by discovery of the Agua Blanca field, on trend with Bermejo, in 1926. The largest field in the province, Argentina’s Ramos (containing both oil and gas), was discovered in 1928.

Argentina’s total drilling (both exploratory and development wells) has been modest but continuous – increasing through the early 1940s to approximately 5 wells/year, waning for the next twenty years with 1-2 wells/year, increasing in the mid to late 1960s with 3-6 wells/year, waning again through most of the 70s decade, and regularly represented by 1-9 wells/year for the last 15 years. Argentina has 27 fields with 58 successfully completed wells (Petroconsultants, 1996).

In contrast, Bolivia’s total drilling (both exploratory and development wells) ceased from the early 1930s until the late 1940s. From 1958 through the mid 70s, drilling surged with 10-20 wells/year, doubled to 20-40 wells/year for the next decade, and has varied from 5-25 most recently. The second largest province field, Bolivia’s Rio Grande (containing both gas and condensate), was discovered in 1961. The first Boomerang Hills discovery was in 1968. Bolivia currently has 77 fields with nearly 350 successfully completed wells (Petroconsultants, 1996).

Paraguay, in the foreland-basin part of this province, was first drilled in 1947, three years after seismic data first were acquired. Although drilling shows encompass a large area, just two producing fields exist in the northwesternmost corner of the country.