THERMAL GRADIENTS

Santa Cruz – Tarija thermal gradients vary regionally. The foreland has higher gradients than the sub-Andean thrust belt. Gradients in basement uplifts like the central Chaco high are higher than in the foreland sub-basins. Sub-Andean gradients range from 11-19 °C/km (0.62-1.07 °F/100 ft) (Petroconsultants, 1996). The northwestern margin of the province at Boomerang Hills is typified by 25 °C/km (1.4 °F/100 ft) (Baby and others, 1995). The northeastern province margin at the Sierras Chiquitanas averages 36 °C/km (2 °F/100 ft) (Williams, 1995). Wiens (1995) notes "high" thermal gradients of 7-9 °C/km (0.4-0.5 °F/100 ft) (sic) on the Paraguayan central Chaco high where thermal maturity locally reaches metamorphic grade and local magmatic activity is postulated. Wien’s reported gradients are probably misstated too low by a factor of 10.

Where thermal gradients are 25 to 40 °C/km (1.4 to 2.2 °F/100 ft), 2.5-4 km of overburden thickness is needed for maturation (Baby and others, 1995; Williams, 1995). Lesser overburden is required in the foreland around the hotter central Chaco high. The sub-Andean with lowest thermal gradients is presently mature to oil with 5.5 km overburden (Dunn and others, 1995).

Silurian sources are now overmature in outcrops west of this province and likely mature for gas in deepest parts of the foreland. If traps were present and not subsequently lost, Silurian-sourced oil accumulations could be preserved in old Carboniferous-aged extensional blocks. Silurian capability for recent gas generation is doubtful because of the long period of time its source rocks were at a mature stage for oil (Figure 8a).

Because of overburden variability, central sub-Andean Devonian source rocks could have expelled as early as 270 Ma; whereas southern Bolivian counterparts adjacent to Argentina probably expelled about 150 Ma and northern counterparts 10 Ma (Beer and Lopez P., 1993). Devonian source rocks in the sparsely explored western sub-Andean would have had an expulsion history favorable for entrapment in Andean-age structural traps. The Argentinian sub-Andean probably did not expel significant hydrocarbon until late Tertiary (Andean) or later time, but local maturation could have begun as early as the Jurassic period (Starck, 1995). Devonian source rocks on margins of hotter foreland basement highs could be presently in the oil window.

TRAP STYLE OF OIL AND GAS FIELDS

Trap Location and Development

Santa Cruz – Tarija discovered fields are dominantly on sub-Andean anticlines and thrusted anticlines (Table 1, Figure 2a). Silurian and Devonian detachment surfaces enabled the deformation to be thin-skinned and laterally extensive. Deformation progressed eastwardly through time, and consistent changes in deformational style – related both to maturity of deformation and to subtle stratigraphic variations impacting detachment surfaces – occur from west to east (Belotti and others, 1995; Dunn and others, 1995). The underexplored and more oil-prone western sub-Andean is geologically older, structurally more mature and complex, and has an extra level of detachment than the eastern sub-Andean. Major accumulations in Argentina exist where fault-bend folds overlie major step thrusts (Belotti and others, 1995). Stratigraphic complexities probably impacted hydrocarbon distribution and complicate field development locally. Reserve growth probably will occur in existing fields of both sub-Andean trends with future detailed delineations of minor faulting and reservoir compartmentalization.

Historic Santa Cruz – Tarija extensional trends are parallel with the Brazilian shield front (northwest/southeast) and the Michicola arch (east-northeast/west-southwest). Unconformities and sedimentary onlap wedges on the old Brazilian shield also play important roles in trapping hydrocarbons at this northern basin edge and in contributing to the creation of structural closures here during Andean compressional deformation (Welsink and others, 1995).

Extensional Paleozoic block faults were created and/or rejuvenated periodically in the Chaco-Tarija basin (Welsink and others, 1995), creating new force-fold structures and either preserving or destroying pre-existing ones. Such traps were necessary to potentially collect earliest (Carboniferous) phases of oil migration (Figure 7). Only one well northeast of Boomerang Hills has tested this pre-Andean trap concept successfully to date.

Although earliest compression and shortening began in late Cretaceous to Paleocene time in the western Andes Mountains, major folding and trap development occurred here 10 Ma or less (Miocene and Pliocene). The continuing eastward migration of the orogenic complex through time (Horton and Decelles, 1997) and the regional variability of thermal gradients have created a mechanism for a continuum of maturation and entrapment, both in younger structures and in onlapping stratigraphic wedges. Foreland high thermal gradients result in shallow maturation profiles and potentially recent accumulations if seals are adequate.

Underdeveloped and underexplored trap concepts known to exist from seismic data in the easterly foreland area are 1) simple stratigraphic truncation against local and regional highs, and 2) forced folds overlying extensional fault blocks of various ages.

Reservoir Distribution of Oil and Gas

Most ultimate recoverable reserves are in fields with multiple producing horizons, but 43% can be allocated to individual producing strata based on porosity, net thickness, and water saturation considerations (Table 1) (Petroconsultants, 1996). Twenty-five percent of ultimate recoverable reserves are definitively within Carboniferous reservoirs, 11% in Devonian counterparts, and just 2-3% each in the Silurian, Tertiary and Cretaceous section, with a trace in Permo-Triassic rocks. Twenty-seven percent of Santa Cruz – Tarija known recoverable reserves are in multiple horizons within the two largest fields, and Carboniferous sandstones contribute significant production to at least one of those fields.

Discovery history

In the decade following the Bermejo field discovery of 1924, generally 1 to 3 fields were discovered per year by Argentina and/or Bolivia. Following an unsuccessful period until 1947, both countries alternated finding a field every year or two until around 1960 when Bolivia’s success increased in conjunction with its increased exploration drilling (Figure 6). Periodically, as many as five fields were discovered in one year. Argentina, although at a slower drilling schedule, also continued sporadically to add one or two discovered fields annually to its small sub-Andean area through the present. Paraguay’s two contiguous productive areas were discovered in 1959 and the mid 1990s.

RESERVOIR ROCK

Geographic and Stratigraphic Location

Known Santa Cruz – Tarija reservoirs include Silurian and younger rocks. Siluro-Devonian reservoirs exist in the entire sub-Andean trend and comprise the only known production in Paraguay (northwestern area). Amalgamated channel-like sandstones with predictable trends abound in what was the Carboniferous depositional basin (excludes the central Chaco high), and such sandstones contain most reserves (Figure 1, Figure 7 and Table 1). Carboniferous strata produce in two Bolivian fields east of the mountain front and within the entire sub-Andean trend, south of where they are regionally truncated (just south of 17° south latitude and south of the Boomerang Hills).

Permo-Triassic reservoirs are rare but evenly dispersed along the Bolivian sub-Andean fold and thrust belt between 18° and 21.5° south latitude. Cretaceous and Tertiary reservoirs also are similarly distributed, with significant representation in Boomerang Hills (where Siluro-Devonian sandstones produce along their truncation edge). The Argentinian portion of the fold and thrust region is dominated by Devonian and Carboniferous reservoirs, although two fields also have Tertiary completions.

Physical and Depositional Characteristics

Lower Paleozoic rocks in Santa Cruz – Tarija are marine to deltaic sandstones and shales. Multiple provenance areas surrounded the paleo-basin at different points in time, although the persistent northern Brazilian shield and eastern Asuncion arch often supplied rather limited volumes of sediment compared to the western Arequipa massif. Deepest parts of the marine basins were farther northwest into Peru (and toward the paleo-Pacific Ocean). Despite potentially great thicknesses, pre-Silurian sandstones are considered non-reservoir rocks because of low porosity and permeability resulting from their burial and tectonic histories. Silurian and Devonian reservoirs have as much as several hundred meters net-pay thickness in sub-Andean folds, and generally several meters to tens of meters net-pay thickness along the Boomerang Hills truncation (Petroconsultants, 1996).

Carboniferous proximal mountain glaciers supplied as much as 2 km sedimentary strata to a basin eventually filled – from southeast to northwest – with fluvial to submarine siliciclastic channel deposits. Sixty to more than 80% of sub-Andean production has been attributed to Carboniferous reservoirs (Eyles and others, 1995; Franca and others, 1995). Orientation of anastomosing sand-filled channels was impacted by basement structure. Individual channels on outcrop are tens of km wide and several hundred km long, and amalgamation is common (Eyles and others, 1995). Significant Carboniferous deposits are lacking on the Paraguayan central Chaco high (Carboniferous Izozog arch). Sand trends are southeast-northwest in the Curupaity sub-basin adjacent to the Brazilian shield and south-north in the Carandaity sub-basin adjacent to the present sub-Andean fold belt. Carboniferous reservoirs average several meters to tens of meters net-pay thicknesses, with a recorded maximum of 115 meters (Petroconsultants, 1996).

Reservoirs younger than Carboniferous also have several meters to tens of meters net-completion thicknesses. Typically thin Permo-Triassic reservoirs are marine to continental in origin, and thicker Cretaceous and Tertiary counterparts are progressively more continental in character with younger age.

Reservoir properties

Compared with younger reservoir rocks, pre-Carboniferous horizons have low overall porosity and permeability (Table 1) and probable fracture assisted deliverability, particularly in southern sub-Andean fields. At the northern sub-Andean truncation edge (Boomerang Hills), Siluro-Devonian production is from sandstones with field averages of 10-15% porosity and as much as 50 md permeability. Farther south in the sub-Andean fold belt, the porosity minimum is 4% and the permeability minimum is tenths of millidarcies. Average porosity for Carboniferous fields ranges from 10-26%, and average permeability from 11-450 md. The few reported Permo-Triassic reservoirs are characterized by 21-25% porosity and 200 md permeability. Producing Cretaceous fields average 17-25% porosity with tens to 200 md permeability, and Tertiary fields average 15-25% porosity and several to 271 md permeability.

Underexplored reservoir rocks

All stratigraphic horizons are underexplored in the foreland basin of the Santa Cruz – Tarija Province because of the paucity of well penetrations there. Forced folds overlying extensional block faults (some reactivated periodically and inverted) and truncation/onlap relationships exist around the old basin margins and the central Chaco high of Paraguay. The western sub-Andean trend also is sparsely drilled, but reservoirs there are likely to be older because of increased tectonic uplift and resultant erosion in a westerly direction through the fold and thrust belt.

SEAL ROCK

Regional and local Paleozoic marine shales, regularly alternating with sandstone reservoir rocks, provide the necessary sealing capacity for many known Santa Cruz – Tarija hydrocarbon accumulations (Figure 7). Seal thicknesses range from as little as approximately ten meters for some intraformational strata to more than 1000 meters for regional horizons such as the Los Monos source rock interval. The ultimate seal for the most important Devonian reservoir rocks in Ramos (the largest Santa Cruz – Tarija field) is a 2-km-thick "diapir" of highly contorted and high-pressured Los Monos shale in the core of the fold, which creates a box-fold geometry for younger strata above (Belotti and others, 1995).

Some Carboniferous seals are actually poorly sorted glacial diamictites tens of meters to hundreds of meters thick. Somewhat lesser-quality marine to marginal marine shales also are adequate seals within the Mesozoic and Cenozoic section – particularly the Triassic Ipaguazu Formation, which has some evaporite content, and the Miocene Yecua Formation shales.

ASSESSMENT UNITS

Santa Cruz – Tarija has at least six geologically distinct areas – three in the fold and thrust belt and three in the foreland basin – that are grouped here into the following three assessment units: Sub-Andean Fold and Thrust Belt (#60450101), Foreland Basins (#60450102), and Foreland Central Chaco High (#60450103) (Figure 2b).

Sub-Andean Fold and Thrust Belt #60450101

Within the thrusted sub-Andean folds, the western half is geologically older, structurally more complex, less densely explored, and more oil prone than its eastern counterpart. Some investigators might also sudivide the eastern sub-Andean along a north-south transect, based on differences in burial history and expulsion timing, as previously discussed. Reservoirs of all ages produce in the fold and thrust belt. Production in the sub-Andean trend north of the Carboniferous truncation (Figure 2b) is limited to non-Carboniferous reservoirs, and it is characterized with relict extensional fabrics that were reactivated during the Andean compressional event. This small area north of the Carboniferous truncation also has established production, but a slightly different field-size distribution than the remaining fold and thrust belt; it is here combined with the sub-Andean assessment unit because of its limited areal extent.

Foreland Basins #60450102

The foreland region contains a sub-basin (Curupaity) parallel with the Brazilian shield on the province northeastern margin and extending southward along the province eastern margin (Figure 2a and Figure 2b). Although the basinal axis has a lower thermal gradient than the surrounding basement highs, its gradient is higher than that in the fold and thrust belt. All potential reservoir rocks are present, and stratigraphic onlaps and truncations border the sub-basin margins where thermal gradients are higher. Northwest-southeast trending extensional fabrics and associated structural closures are expected. Another sub-basin (Carandaity) is parallel with and just eastward of the sub-Andean folds (Figure 2a and Figure 2b). Both these sub-basins have limited production and are combined into this one frontier assessment unit where all reservoirs – including those of Carboniferous ages – are likely preserved.

Foreland Central Chaco High #60450103

The third assessment unit is hypothetical, with hydrocarbon shows but no established production. It has been a persistent structural high, with shallow burial histories but significantly high thermal gradients, at least in its most recent past. Although age equivalents of the source rocks are present, they might have more terrigenous character and poorer source-rock quality here. Carboniferous reservoirs are absent. Traps and seals are more questionable in quality than in the "Foreland Basin" assessment unit. Most of this assessment unit is within Paraguay, and its boundary coincides with the zero edge of Carboniferous strata (Figure 2a and Figure 2b).