Santa Cruz - Tarija Basin

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Introduction

(Fig. 1) Location map and geographic features of Santa Cruz - Tarija Province.[1]
(Fig. 2) Petroleum systems and assessment units of Santa Cruz.[1]

Santa Cruz - Tarija is a province of Bolivia, Argentina and Paraguay. The Tarija Basin or Chaco-Tarija basin is surrounded by a warm climate and has high subsidence due to the active oceanic plate subduction westward. This basin covers most of the province and even leans a little north into Peru. A reservoir known as Los Monos accounts for most of the basin. It has accumulated roughly 2500m of continental sediments dating back to the carboniferous and jurassic periods. The Tarija Basin is comprised of Devonian and Silurian shale source rocks. It has gained massive attention from the oil industry because of its large volumes of hydrocarbons that have been retrieved from its reservoir rocks.[2] Dating in 1995, this province is dominated by Nine hundred eighty-two wells, with 715 in Bolivia, 229 in Argentina, 36 in Paraguay, and two in Brazil. Bolivia got to drilling first in 1914 and failed with their first exploratory well. Argentina's drilling has been described as "modest but continuous" varying in wells since the 1940s anywhere from 1-3 to 1-9 a year. Currently, Argentina has 27 fields with 58 successfully completed wells. Bolivia has had a shaky history of drilling starting in 1958 with 10-20 wells till the 70's then 20-40 for the next decade and most recently varying from 5-25.[1] Figure 1 shows the province's outline and Figure 2 shows the Basin's outline in accordance with the province.

Depositional History

The province is mostly covered by Devonian and Silurian shale source rocks that were deposited in a semi-restricted marine basin. Both horizons are preserved throughout the area with thicknesses ranging from many hundred to several thousand meters each, but Devonian source rocks are typically thicker. Source rock quality possibly deteriorates to some degree because of lithologic variability near depositional basin margins and perhaps over the central Chaco high, which was prominent and exposed by the Carboniferous period.

Geology

The 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 Devonian warmer climate and increased subsidence from active oceanic plate subduction to the west resulted in several thousand meters of marine sediment, including the middle Devonian Los Monos shale source rocks.[1]

Primary Geologic Risks and Uncertainties

Every basin has its geologic risks and uncertainties it is just a matter of whether the risk is worth the possible reward in terms of petroleum exploration. Judgments under uncertainty are also susceptible to biases, which commonly influence geologic estimates of (1) prospect target size, (2) discovery probability, and (3) cost of finding. The concept of expected value, apparently originated in 1654 by the French mathematicians Fermat and Pascal offers an effective way to evaluate risk ventures. The basic procedure is to multiply the probability of each possible outcome by its economic consequence and add all the products. For example, consider a simple game in which a coin is flipped one time. If the coin lands heads, the player wins $20,000; if the coin lands tails, the player wins nothing. The question is, how much would you be willing to pay for one chance to play this game? The expected value of this risk venture is $10,000 (½ × $20,000 + ½ × 0). However, most players would not pay $10,000 to play this game—many would pay $1,000-$5,000. Some risk-prone or affluent players might pay as much as $9,500; some highly conservative or non-affluent players might not be willing to chance even a small loss on the gamble.[3] It is important to have an understanding of risk in the petroleum business because funneling everything you have into a venture that isn't going to return the required profit in time is devastating.

(Fig. 3) Structural view of the Boroigua fault that runs through the Margarita field.[4]

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 Foreland Basins and Foreland Central Chaco High. 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. The Thrust Belt Foreland region has a lower thermal gradient than the surrounding basement highs, but its gradient is higher than that in the fold and thrust belt. All potential reservoir rocks are present, and stratigraphic on-laps and truncations border the sub-basin margins where thermal gradients are higher. Northwest-southeast trending extensional fabrics and associated structural closures are expected. The Foreland central shows hydrocarbons but no established production. It has been a persistent structural high, with shallow burial histories but 14 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.[1]An example of a risk would be one of the main drilling challenges which is the Bororigua Fault that runs through the Margarita field. This fault runs in an N-S direction principally (see fig. 2 for a topographical overview) involving the top formations (~3,000 meters) and divides the basin into two distinct drilling scenarios.The main stratigraphic difference between drilling on either side of this geological feature is the Carboniferous Formations on the eastern side is encountered at around 1,500–1,700m. On the western side, the Carboniferous Formations occur at a far shallower depth, resulting in more difficult drilling due to the larger borehole sizes utilized at these depths.[4]

Political Aspects

Santa Cruz holds a stable government with active and previous oil companies. The province has a history of cooperation with outside industries.

Petroleum and Facility Engineering aspects

Existing fields are typified by alternating reservoir and seal rocks in post Ordovician sandstones and shales on anticlines. Thick Devonian and Silurian shale source rocks, depositionally and erosionally confined to this province, at a minimum have generated 4.1 BBOE known ultimate recoverable reserves (as of 1995, 77% gas, 15% condensate, 8% oil) into dominantly Carboniferous reservoirs with average 20% porosity and 156 md permeability.[1]

Future Petroleum Potential

There is still much to be discovered of the Tarija Basin which can be followed by uncertainty and risks, but that is a definite in any exploration. Further hydrocarbon discovery in the fold and thrust belt is expected. In the foreland basin, higher thermal gradients and variable burial history – combined with the presence of unconformity and on-lap wedges – create potential there for stratigraphic traps and pre-Andean, block-fault and forced-fold traps. Reserve growth probably will occur in existing fields of both subAndean trends with future detailed delineations of minor faulting and reservoir compartmentalization.

Some Potential Areas of Exploration:

  1. Stratigraphic horizons that are under-explored in the foreland basin of the Santa Cruz – Tarija Province
  2. The Forced Folds that are overlying extension block faults around the old basin margins and central chaco high of Paraguay
  3. The western sub-Andean trend

These are all under-explored reservoirs which can have some risks and uncertainties accompanied with them.

Petroleum Elements

Source Rock

The development of a new stratigraphic framework  in the Tarija Basin allowed the review of the hydrocarbon source potential within the typically recognized Devonian sequence sets. Nearly 1000 rock samples data were evaluated for source rock typing and maturity assessment. The samples were collected from several wells and outcrops from a broad area comprising the thrust belt, Chaco Plains and Izozog High, between the Santa Cruz Elbow and the Ramos gas field in Northwestern Argentina Source Rock maturation is decided under many factors like Andean loading history, post-Devonian isopachs, and Regionally variable thermal gradients. Poor to moderate, and occasionally good source rock qualities have been recorded in most of the evaluated samples.[5]

(Fig.4) Cross sectional view of anticlinal trap[6]

Trap Styles

A trap is described as a configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate.[7] The discovered fields in this basin are mostly on sub-Andean and thrusted anticlines. The Silurian and Devonian detachment surfaces allow for the deformation to be thin-skinned and laterally extensive. There are extensional Paleozoic block faults in the basin which create new force-fold structures that can preserve or destroy pre-existing ones. The Tarija Basin is mostly comprised of anticlinal traps which are beneficial in hydrocarbon extraction due to their large size in nature. This trap style is necessary if earliest phases of oil migration were to be caught. There are two concepts that are known to exist from seismic data which are barley explored and developed. The first one is simple stratigraphic truncation against local and regional highs and the second one is forced folds overlying extensional fault blocks of various ages.[1]

Seal

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. Seal thicknesses range from as little as approximately ten meters for some inner 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.[1]

Migration

Due to the prominent Silurian and Devonian shale detachment horizons, migration routes are mostly created laterally. There has been reports on the existence of two stratigraphic zones of high fluid pressure that likely enable expulsion in both upward and downward directions, explaining charge in older reservoirs when no lateral juxtaposition occurs. The presence of both mature and immature source rocks on uplifted thrust segments suggests that both vertical and limited lateral migration are required to charge the folded and faulted structures.[1]

Reservoir

One petroleum system, Los Monos – Macerate, with Siluro-Devonian shale source rocks and Carboniferous siliciclastic reservoirs dominates this basin. Most discovered fields are in sub-Andean reservoirs ranging from Silurian to Tertiary in age.[1] The thick Los Monos shales are the proven source rock, where as the Santa Rosa and Huamampampa quartzites are the main reservoirs for fields in the southern tip of the basin in Argentina.The Los Monos shales entered the generation window during the Mesozoic, but the last maturation pulse occurred during the Neogene. The expulsion is related to Andean tectonics. The explodes hydrocarbons followed two migration paths, resulting in two main petroleum systems: one characterized by downward migration to the fractured reservoirs of the Huamampampa and Santa Rosa formations, and the other by upward migration charging Carboniferous to Tertiary reservoirs. Hydrocarbons in the latter migration pattern reached the younger reservoirs by following complex paths through the paleovalley fills and faults.[8]

References

  1. Baby, P., Moretti, I., Guillier, B., Limachi, R., Mendez, E., Oller, J., and Specht, M., 1995, Petroleum system of the northern and central Bolivian sub-Andean zone, in Tankard, A.J., Suarez S., R., and Welsink, H.J., eds., Petroleum basins of South America: American Association of Petroleum Geologists Memoir 62, p. 445-458.
  2. Ahlfeld, F., 1946. Geología de Bolivia. Revista Museo de La Plata, 3:5-370. La Plata, Argentina. Google Scholar
  3. P. Allen, J. Allen Basin Análisis: Principles and Applications Blackwell Scientific Publications, Oxford (1990) 451 p.Google Scholar
  4. Beer, J.A., and Lopez P., O., 1993, Paleozoic expulsion in Bolivia – its influence on field size and distribution: American Association of Petroleum Geologists Bulletin, v. 77, no. 2, p. 306.
  5. S. J. Lindquist.The Santa Cruz - Tarija Province of Central South America: Los Monos – Machareti(!) Petroleum System. Retrieved Dec 5, 2022, from https://pubs.usgs.gov/of/1999/ofr-99-0050/OF99-50C/OF99-50C.pdf

External Links

https://www.britannica.com/science/Devonian-Period

https://en.wikipedia.org/wiki/Andean_foreland_basins#:~:text=The%20Andean%20foreland%20basins%20or,as%20the%20Amazonian%20foreland%20basins.

https://www.sciencedirect.com/topics/earth-and-planetary-sciences/diapir

https://glossary.slb.com/en/terms/s/seal