Oriente Basin (Ecuador)

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Introduction

The Oriente Basin is a petroleum basin located in eastern Ecuador, known as the Oriente region. The Oriente Basin is part of a group of foreland basins that are located east of the Andes mountains in South America called the Sub-Andean basins. It is one of the productive Sub-Andean basins and currently produces approximately 300,000 barrels per day. Exploration of the Oriente basin began in 1921, but it wasn't until 1937 that Shell Exploration drilled the first well. This area is also known as the Putumayo-Oriente-Maranon province. The majority of this province is covered with the Amazon Rainforest. The location and economics of this region make access difficult and complicates road building, storage, transportation, and more. The Oriente Basin Cretaceous oil reservoir located in the eastern portion of the basin accounts for most of the discoveries in Ecuador over the last decade. This makes it one of the most important targets for oil exploration in South America.

(Fig. 1) Map of Oriente Basin.

(Fig.2) Simplified tectonic map of the Oriente Basin.

Depositional History

There are large reserves of petroleum in the sedimentary rocks of the Oriente of Ecuador. The sedimentary rocks were deposited on a continental shelf during the Cretaceous period. Although the petroleum was not created in these rocks, it was created in the fine-grained clastic sediments of a contemporaneous continental-rise prism deposited in deep water farther west. The rise sediments were metamorphosed and are now are part of the metamorphic rocks of the eastern Cordillera of the Andes Mountains. At the first time of deformation of the continental-rise sediments, which were caused by subduction during the Maestrichtian period, the majority of the petroleum in the northern part was driven upward and eastward parallel with bedding. Due to this, much of the petroleum entered the shelf rocks laterally. In the south, more complex deformation of the continental-rise sediments prevented the petroleum from escaping. The trapped petroleum was converted to graphite by metamorphism. A single carbon Analysis of metamorphic rocks from the south end of the Eastern Cordillera suggests that the graphite content also diminishes as you move southward.

Primary Geologic Risks and Uncertainties

There are a variety of risks and uncertainties associated with petroleum exploration. On the geologic side, there are risks of being incorrect on the dynamic history of structural and stratigraphic evolution of a basin, of being incorrect on the paleo-thermal development of a basin, being incorrect on hydrocarbon kinetic evolution, of being incorrect on primary and secondary hydrocarbon migration, and consequently being incorrect on accumulation sites and amounts of hydrocarbons retained at such sites. All of these risks and uncertainties must be evaluated prior to a corporation committing assets for hydrocarbon exploration in any particular area of the world because each influences the company's likely profitability.

When oil is produced, it is extracted in a mixture of water and gas. The water in that mixture is called formation water or produced water. Most of the oil production in Ecuador is developed within the Amazon region. This area is home to indigenous tribes, many different species, and thriving landscapes. Some argue that oil activities only affect the area of the operation point. However, in the Amazon, some rivers cross the entire region flowing through the area of those tribes, species, landscapes, and oil operation points. These rivers connect the whole region and can spread the negative effects of the Oil industry to those areas that are beyond oil operation points. Ecuador currently produces around 2 million barrels (320,000m^3) of formation water every day. The formation water from all the wells is directed to a central processing station where it is collected, separated, treated, and stored before its final disposal.

Petroleum Elements

Source Rocks

The Oriente Basin has several types of potential Source rocks. The most important of which are the Cretaceous Chant Formation and the Triassic-Jurassic Pucara Group. Geologists and Scientists have determined that the crude oil in the northern area of the basin came from the Cretaceous Chonta Group source rocks. While oil in the southeastern part of the basin came from the Triassic-Jurassic Pucara Group source rocks. The major oil generation source rock in the basin is the bituminous shale and carbonates of the Upper cretaceous Chonta formation. It generally contained type I and type II kerogen in the northwestern part of the basin. While type III kerogen can be found in the southeastern part of the basin. The total organic carbon (TOC) is between 1% and 2%. The TOC is about 1% in the east and raised to 4% in the western part of the basin, and reached up to 10% in some local areas. The hydrocarbon generation potential is usually between 2 mg/g and 7mg/g. The mature source rock is predominantly in the central and western parts of the basin, but is still immature in the east. The source rock hit peak oil generation during the Miocene period. The most important reservoir in the basin is the Middle Choanta delta-shallow marine sandstone, which is interbedded with source rock and formed a high efficiency self-generation and self-accumulation assemblage. The sandstone becomes thicker over time moving from west to east. Which has a porosity of 12%-20% and permeability of and permeability of 39.48×10–3–7 896.00×10–3 μm2. The overlaying Neocene Cachiyacu Formation mudstone is caprock. Another important set of source rocks in the basin is the organic-rich carbonates in the Triassic-Jurassic Pucara group. It mainly has type II kerogen with TOC between 1% and 12%, but mostly between 1% and 6%. The hydrocarbon generation potential is normally between 0.5 mg/g and 2.0 mg/g.This set of source rock reached its hydrocarbon generation peal during the Paleocene and Eocene period.

Seal

Over time interbedded Cretaceous reservoir rocks and seals were deposited over the majority of South America. The top transgression during the Turonian-Santonian was marked by deposition of the Napo, Villeta, and Agua Caliente formations. The primary hydrocarbon seals for the Lower Cretaceous Caballos, Hollin, and Cushabatay reservoirs are imbedded shales, and shales of the overlying Villeta, Napo, and Raya formations. Upper reservoir seal of the Hollin formation in the Oriente Basin is a condensed sequence of limestone and shale of the Napo Formation. Napo, Chonta,and Villeta reservoir seals are imbedded marine shales, and overlying Cretaceous and Tertiary shales. The broad intervals of seal rocks for Hollin-Napo are generalized in the events chart (Fig 3).

(Fig.3) Events chart for conventional oil and gas in the Hollin-Napo.

Trap

The majority of traps in the Oriente Basin are structural traps. Since Late Cretaceous, the Pacific plate continued to thrust into the South American plate. The powerful extrusion stress was released in the western area of the basin, which resulted in thrust/fold traps and basement involved traps. The extrusion stress shifted to the east, and the central part of the basin has weal extrusion function which resulted in the formation of extrusion and drape anticlines. In the eastern part of the Oriente basin, the extrusion stress from the west was blocked by the Gondwana shield which reversed the former normal fault and formed some thrust anticlines. All of these traps are distributed in the NW-SE direction. We could potentially expect lithological traps and stratigraphic traps to develop in the eastern and central areas of the basin.

Migration

The hydrocarbon migration in the basin has long distance and step-up migration features (Fig. 4). It has Lower Cretaceous Cushabatay sandstone with high porosity and high permeability. This gives a lateral migration channel for large-scale migration. These lateral migration distances can reach more than 100 km. There is also a vertical channel for hydrocarbon migration that is created by the successive fault activation. Through this the hydrocarbon could migrate to the shallow reservoir in slope, which is a characteristic of step-up migration.

(Fig. 4) Hydrocarbon migration model of Oriente Basin.

(Fig. 5) Reservoir thickness maps are a traditional tool used to identify the best areas of the field for a given reservoir and for calculations, but the do not capture the reservoir heterogeneities quality variation.

Reservoir

The contents of the reservoirs vary but the majority are clastic formations. Potential respective reservoir and source rocks are marine sandstones and shales of the Napo and Chonta Formations. The primary source rocks in the Oriente Basin are the Lower Cretaceous (Aptian and Albian) Caballos Formation and the overlying Lower and Upper Cretaceous (Albian to Campanian) Villeta Formation. Oil is also produced from the Eocene Pepino Formation. Fourteen fields produce oil from the Caballos Formation. The overlying Villeta formation is mainly Limestone, Shale, and sandstone that had been deposited in a shallow marine setting within a shallow shelf environment. Twenty-six fields produce oil from the Villeta "U," "T," "N" sandstones. The Vivian Formation in the northeast part of the basin is productive in 26 fields. The formation has variable depositional environments across the region and is primarily fluvial. The informally named primary reservoir is the "Main Hollin" which is categorized as an unconventional reservoir. The gross sandstone thickness in the field is nearly 150m (500ft) and consists of fine- to coarse-grained, cross-bedded quartzarenite to cubarkosic sandstones. The lower part of the "Main Hollin" consists of a thick series of braided stream deposits with excellent reservoir quality. The upper part of the Main Hollin formation is interpreted to have been deposited in a sandy meandering river system. The point bars of the meandering river deposits are excellent quality sandstones. The estimated thickness of the individual point bars is 15 to 20m (50 to 65ft). The Hollin in the western Oriente basin consists of five successive depositional sequences: three sequences in the Main Hollin sandstone, and two in the upper Hollin formation. Geologists interpreted poorer reservoir quality sandstones and siltstones in the upper part of the Hollin formation to have originated as crevasse splay, levee, and chute-fill deposits.

Porosity and Permeability

Porosity is the percentage of void space in a rock. It can be defined as the ratio of the volume of the voids or pore space divided by the total volume. It is written as a decimal fraction between 0 and 1 or as a percentage. The porosity of most rocks varies from less than 1% to 40%. Permeability is defined as how easily liquid and gas passes through something. An example of this is how fast water flows through a porous rock. Permeability is important because it determines the flow of fluid through the rock matrix. Porosity and Permeability are not closely correlated, but permeability will usually correlate with porosity. Porosity is associated with the storage of water, while permeability is associated with groundwater movement and flow.

The Upper T glauconitic sandstone contains high content of glauconite, which is a type of clay mineral with high radioactivity and density. Therefore, it is usually interpreted as shale sandstone or sandy shale without effective porosity. Porosity in the glauconitic sandstones at Tarapoa Block is primarily intergranular. Within framework grains, however, feldspar and glauconitic pellets have experienced partial dissolution (Fig 6). Porosity reduction is largely the result of pseudo matrix occlusion, pore-filling carbonate and overgrowth quartz cements, and natural grain compaction. Medium to coarse-grained sandstones offers a low to moderate visual porosity with good interconnectivity among adjacent pores, whereas the very fine to fine-grained sandstones off less visual porosity and poorer interconnectivity among adjacent pores. Porosities generally range from 8% to 18% and average about 13%. Permeabilities vary over a wide rang from 0.1 to 2000 md but average about 155 md.

(Fig. 6) Porosity types and diagenetic components of the Upper T glauconitic sandstones at Tarapoa Block. (A) Glauconitic sandstone (φ = 14%, k = 2.3md) showing the textural overview of moderately sorted, fine-grained.(B) The entire sample (φ = 8%, k = 0.06md) exhibits pervasive with ferroan calcite cementation (stained purple). (C) Sandstone (φ = 17.9%, k = 76.6md) displays the wide array of quartz overgrowths (white arrows) that cement detrital quartz grains together. Minor amounts of dolomite (Do) cements quartz grains. (D) SEM photomicrograph showing relatively clean, open pore throats typical of the fine-grained portions of the sandstone. (E) Diagenetic quartz overgrowths cement the detrital quartz grains together. (F) SEM photomicrograph showing the preservation of a partially dissolved plagioclase feldspar (Pf).

Engineering Aspects

Current Oriente operations involve over 300 producing wells, regional oil refineries, secondary pipelines, transfer lines and gas lines, and the network of roads that serves the industry.

Future Petroleum Prospects

The majority of the Oriente Basin has already been drilled and has no future potential. The Northern and Eastern portions of the Oriente Basin have reached an advanced stage. Although, the central, western, and Southern portions of the basin are relatively unexplored and could still have great future production potential.

Potential Exploration Areas:

1. The drape and extrusion traps in the central western portion, petroleum came from the Cretaceous formation source rock. Favorable Target, but drilling costs will be high due to the deepest buried depth being up to 4500m deep.

2. The thrust anticline and extrusion traps in southern and central portion. Targets have a buried depth of 3300-4000m.

3. The Upper Cretaceous lithological traps in northern and eastern portions. This is a mature exploration area but still has some exploration potential.

References

1. PETROL. EXPLOR. DEVELOP., 2010, 37(1): 51–56.

2. Mathalone J M P, Montoya M. Petroleum geology of the sub-Andean basins of Peru. In: Tankard A, Suárez Soruco R, Welsink H J. AAPG memoir 62: Petroleum basins of south America. Tulsa: AAPG, 1995. 423–444.

3. Valasek D, Aleman A M, Antenor M, et a1. Cretaceous se- quence stratigraphy of the Maranon-Oriente-Putumayo Basins, northeastern Peru, eastern Ecuador, and Southeastern Colom- bia. AAPG Bulletin, 1996, 80(8): 1341–1342.

4. Pindell J L, Tabbutt K D. Mesozoic-Cenozoic Andean paleo- geography and regional controls on hydrocarbon systems. In: Tankard A, Suárez Soruco R, Welsink H J. AAPG memoir 62: Petroleum basins of South America. Tulsa: AAPG, 1995. 101–128

5. Higley D K. The Putumayo-Oriente-Maranon Province of Colombia, Ecuador, and Peru Mesozoic-Cenozoic and Paleo- zoic petroleum systems. USA: U S Geological Survey, 2001. 1–31.

6. Wang Qing, Zhang Yinghong, Zhao Xinjun, et al. Petroleum geological characteristics and exploration potential analysis of Maranon Basin, Peru. Petroleum Exploration and Develop- ment, 2006, 33(5): 643–647.

7. Higley , D. K. (n.d.). The Putumayo-Oriente-Maranon Province of Colombia, Ecuador, and ... - USGS. Retrieved May 1, 2022, from https://pubs.usgs.gov/dds/DDS-63/DDS-63.pdf

8. Carrion, J. (2017, March 15). Producing the limit, by re-defining the rock typing classification in a mature oilfield: Case study in the Auca Field, Oriente Basin, Ecuador. OnePetro. Retrieved May 1, 2022, from https://onepetro.org/SPELAMA/proceedings/17LAMA/2-17LAMA/D021S012R002/195301

9. Zhao, Y.-B. (2018, October 18). Reservoir characteristics and hydrocarbon accumulation of the glauconitic sandstone in the Tarapoa Block, Oriente Basin, Ecuador. Journal of Petroleum Science and Engineering. Retrieved May 1, 2022, from https://www.sciencedirect.com/science/article/pii/S0920410518309276#fig10

10. Understanding porosity and density. Wisconsin Geological Natural History Survey. (n.d.). Retrieved May 1, 2022, from https://wgnhs.wisc.edu/maps-data/data/rock-properties/understanding-porosity-density/

External Links

https://archives.datapages.com/data/specpubs/memoir62/29balkwi/images/29balkwi.pdf

https://pubs.geoscienceworld.org/aapgbull/article/37/10/2303/33842/Oil-Explorations-in-the-Oriente-of-Ecuador-1938

https://pubs.geoscienceworld.org/aapgbull/article/59/7/1166/37001/Origin-of-Petroleum-in-the-Oriente-of-Ecuador1

https://sp.lyellcollection.org/content/50/1/89.short

https://www.sciencedirect.com/science/article/abs/pii/S2352801X21000229

https://www.sciencedirect.com/science/article/abs/pii/S0195667102910281

https://www.sciencedirect.com/science/article/pii/S0040195101002177?casa_token=lK8D9lApQ7sAAAAA:ScNQ9YJi8jlhFmAQ6ZSuEX7Yd_bsb55vYCdKxqNbE6HmsE_TFgthyEQCVbZnWUdIipvz-dLE

https://pubs.usgs.gov/dds/DDS-63/DDS-63.pdf