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= Cusiana Field = Cusiana is a mature onshore oil field located in Llanos Basin on the Eastern Cordillera of the Andes Mountains, in Colombia. The Cusiana field was discovered in 1988 by the British Petroleum (BP) and became a commercial field in 1993. The Cupiagua field is a neighboring field that is often conjoined with the Cusiana field due to its close proximity and is collectively known as the Cusiana-Cupiagua Complex, one of the largest oil and gas fields of Colombia. This region has been searched by explorationists since 1960 however, commercial hydrocarbons were not discovered until 26 years later after to several failed attempts to strike oil and gas. A long geologic and tectonic history of subduction, rifting, and faulting shaped the Cusiana field into its current state.

Discovery history
In 1971, BP identified the frontal Cusiana thrust fault as an area of interest for drilling and spent $78 million on exploration over the next seven years but no commercial discoveries were found. In 1987, the first well, Cusiana-1, was drilled and confirmed the presence of hydrocarbons in the Mirador formation. The Cretaceous Carbonera Formation was also reached by Cusiana-1 but the fluid contents could not be confirmed due to technical difficulties. In 1989, Cusiana-2 was drilled ~5km from Cusiana-1 but was abandoned shortly afterwards due to casing hanger failure. After several failed attempts at exploration, in mid-1991 Cusiana-2A was the first well to penetrate the Guadalupe Formation which confirmed Cusiana as an oilfield giant. Cusiana-2A tested oil at a rate of 6500 BOPD and gas at 11.5 MSCFGD*, confirming the presence of commercial hydrocarbons.

*MSCFDG = million standard cubic feet of gas per day

Depositional history
The Cusiana field contains five formations within the petroleum system: Gatcheta (Cretaceous), Guadalupe (Late Cretaceous), Barco (Paleocene), Mirador (Eocene), and Carbonera (Paleogene-Neogene) Formations. The Gatcheta is the oldest of the five formations and is composed of shallow marine mudstones that were formed during post-rift thermal subsidence. The Upper Guadalupe Formation was deposited ~83 Ma and consists of ~400 ft of sandstone overlain by ~103 ft of mudstone. The Lower Guadalupe Formation is composed of shale but is not a part of the Cusiana petroleum system. At the end of the Cretaceous (approximately 64 million years ago), the Western Cordillera mountain range began to uplift which caused sea level to drop resulting in a hiatus in sediment deposition between the Guadalupe and Barco Formations. The Barco Formation (~60 Ma) is characterized by alternating layers of sandstone and mudstone ~270 ft thick that likely formed during sea level transgressions and high stand periods. Approximately 38 million years ago, the Mirador Formation formed and is described as coarse sandstones with interbedded mudstone and shale that thin to the east. Lastly, the Carbonera Formation contains delta and coastal plain mudstones that were deposited in the Late Eocene-Early Oligocene followed by alternating layers of sandstone and mudstone in the upper Oligocene caused by alternations of sea level transgressions and highstands. * Ma = million years ago

Tectonic history
In the Cretaceous, the Nazca oceanic plate subducted beneath the South American plate resulting in the formation of the Andes mountain range. Thermal subsidence in the Late Cretaceous occurred followed by shortening and uplifting of the Andes in Late Oligocene, which caused fault inversions. This field consists of the NE trending La Florida anticline bounded by the Cusiana and Yopal thrust faults in the subsurface. The faults and structures of the Eastern Cordillera and Llanos Basin have a distinct NE trend, a result of regional compression. The Yopal fault formed as a result of collision and compression of the Panama-Choco island arc and the North Andes during the Late Cretaceous. The Cusiana fault originally formed as a normal fault during the Paleocene but was inverted during the Andean shortening and uplift and was then displaced by NW trending compression from deformation of the Eastern Cordillera less than 5 Ma. Holocene deformation of the Eastern Cordillera migrated eastward resulting in thrust belts and frontal folds seen in the Cusiana area today.

Petroleum system
The source rocks are from the Cretaceous Gachetá Formation which generated most of the hydrocarbons in South America and consists of organic-rich mudstones. The Mirador Formation accounts for 66% of the reserves in the Cusiana field but hydrocarbons are reserved in the Barco and Guadalupe Formations as well. The three reservoirs are dominated by cemented quartz-arenites altered by diagenesis. The composition of the reservoirs resulted in low porosity (2-12%) but high permeability. The Upper Guadalupe Formation contains phosphatic litharenite, a sandstone high in phosphorous, which has similar characteristics to quartz-arenite but a slightly higher porosity (11-17%). The reservoir is sealed by the Carbonera Formation which serves as a regional seal for many other fields in Colombia as well. The petroleum traps of the area were formed during the Oligocene-Miocene and Miocene-Holocene deformation of the Eastern Cordillera and fault-bend folds. Hydrocarbons migrate to the traps through permeable channels at fault intersections. The reservoirs have an average depth of ~15,000 ft which contain light oil, gas, and condensate between 240-300ºF. Rather than an abrupt transition between phases, the hydrocarbons grade from gas at the peak of the field, to a critical point fluid in the middle of the reservoir, and to volatile oil in the lower reservoir.

Hydrocarbon production and recovery
Of the three reservoirs (Guadalupe, Barco, and Mirador), the Mirador Formation contains the most retrievable hydrocarbons. The Mirador Formation has a 66% STHIIP while the Guadalupe and Barco Formations combined only account for 34% STHIIP. As of December 2019, the Cusiana field produces 35,720 barrels of oil per day which accounts for 31% of hydrocarbon production in Colombia. Oil production peaked in 1998 at 310,000 barrels per day. However, from 1994 to 2009, the Cusiana field depleted approximately 640 Mb (million barrels per day) and from 1999-2020 there was a 23% production decline rate.

Gas and water injection is a common method used in the Cusiana field to enhance oil recovery by maximizing liquid recovery from the gas cap and oil leg while maintaining reservoir pressure. The composition, pressure, and temperature make Cusiana an ideal candidate for this recovery method. The first injection phase began in 1992 in which crestal gas injection and shallow water disposal wells were implemented in the main Mirador Formation. During the initial period, the field was responsive to the injections with high oil rates over 10,000 bpd but over the course of five years oil production declined and the Barco and Guadalupe Formations had a significant pressure drop. A new plan was developed in 1997 in which down structure water injection was used to enhance recovery in the Barco and Guadalupe Formations. Significant results were seen using this method in 1998 with peak production of 310,000 bpd however, by 2004 was water breakthroughs began to occur due to fractures in the reservoirs. From 2004 to 2009, gas injection was implemented in all reservoirs at low rates to avoid injection corridors, and over $300 million was spent on water injection lines and rehabilitation of shut-in wells. The new development plan resulted in decreased reduction rates and less water entry. As of 2010, down flank gas injection is still being used and expansion of water injection is under evaluation.