Carabobo Field

Despite no surface indications of petroleum, exploration led to a discovery well drilled in 1936, 25 miles north-northwest of Ciudad Bolivar. Drilled to a depth of 3,855 feet, 7 °API gravity oil was discovered in the Miocene tar sands. Nine months later, oil was struck again, this time in the current Giant Temblador field, 93 miles east-northeast. Preliminary exploration ended in 1967, and in 1987, Venezuelan consultant Anibal Martinez determined that the belt extends 285 miles westwards from Puerto Ordaz. Four fields were proposed from the six main producing areas in the belt. Venezuelan President Hugo Chavez's administration named the four fields from west to east; Boyacá, Junin, Ayacucho, and Carabobo, with 36 licensing blocks.^[4]

In 2010, the Carabobo field was split into three joint venture projects; Carabobo-1, Carabobo-2, and Carabobo-3.^[5]

Due to the nature of extra heavy crudes, the oil produced from the Carabobo field has extremely high viscosities. Typical heavy crudes produced are around 10,000 cp, which is the viscosity of corn syrup. Recoveries in this area are naturally very low. As a result, emerging technologies have recently made recoveries high enough to produce the extra heavy crudes economically. Specifically, enhanced oil recovery methods have dramatically improved recoveries. Methods such as immiscible gas injection, polymer flooding, and in situ combustion, better known as fireflooding, have all seen limited success in the Carabobo field. However, steam injection and microbial enhanced oil recovery (MEOR) have seen the most success.^[6]

Microbial enhanced oil recovery, specifically, has received special attention. In 1999, Bognolo reported that biosurfactants not only reduced the viscosity of the Venezuelan heavy crude oils, but also increased their mobility in transport lines. Currently, the biodegradation of the heavy crudes has become an integral tertiary recovery method in the field, as it is a cost-efficient and eco-friendly way to drive the residual oil trapped in the reservoirs.^[7]

Summarize Fact Check

Carabobo is one of the four fields in the Orinoco Oil Belt. Located on the southern border of the East Venezuela Basin,^[2] the Orinoco Oil Belt extends 375 miles along the Orinoco River in the eastern part of Venezuela.^[8] The East Venezuela Basin is a foreland basin south of a fold belt formed from the progressive collision of the Caribbean Plate and the passive margin of northern South America. Thrust faults associated with the fold belt resulted in the burial of Cretaceous and possibly older source rocks into the thermal window for oil.^[2]

The Orinoco oil belt has many unique features from a geological point of view. All of these unique features are related to the tectonic events that formed the field. This deposition includes the Querecual and San Antonio formations that would become source rocks for the Orinoco Oil Belt.^[9]

While the Guyana shield underwent erosion, the sediments were continually deposited in the East Venezuela Basin. This led to the burial of the Querecual and San Antonio formations, pushing them into the thermal window for oil.^[9]

Below is a figure of the stratigraphy of the Carabobo field. Three periods of deposition are prevalent, including the formation of the source rocks, reservoir rocks and seal. Faulting from the thrust belts are found throughout the field, and are known to trap oil. However, most of the oil can be found in the Miocene sandstones that are sealed by the Miocene mudstones and Guyana Shield.^[9]

The source rocks, the Querecual and San Antonio formations, were formed in the Upper Cretaceous, and were organically rich. After burial, these source rocks were pushed into the thermal window for oil, and began to migrate updip into the Miocene sandstones.^[9]

A stratigraphic pinch out of the Miocene Oficina formation sandstones against the Miocene Carapita mudstones created a trap for the oil of the Orinoco Belt to migrate updip from the deeper basin. A classic example of long distance migration, the oil from the Upper Cretaceous Querecual formation source rock travelled about 300 kilometers.^[10]

In the Carabobo field, the reservoir tar sands are characterized by several depositional sequences of Miocene sandstones and mudstones. The reservoirs consist of the Oficina, Freites, and Merecure formations. Faulting and shale dispersion is common in these formations, leading to severe heterogeneity. This heterogeneity in the reservoirs severely reduce recovery efficiency in the field. Reservoirs have an average thickness of 60 meters and range in depth from 150 to 1,400 meters.^[2]

The main source rocks for the Carabobo field are the Upper Cretaceous Querecual formation and San Antonio formation.^[9]

The oil in the Carabobo field resides in the Oficina, Freites and Merecure sandstone formations. Formed in the Miocene era, these formations consist mostly of sands formed by the erosion of the uplifted Guyana Shield. The sands were carried along north flowing rivers of the time period and deposited on the southern margin of the East Venezuela Basin. Stratigraphically trapped, the reservoir contains extra heavy oil with an average gravity from 4 to 16 °API.^[9]

The Oficina reservoir rocks are trapped by a stratigraphic pinch-out with the Guyana Shield. Faulting is prevalent in the field, and known to occasionally trap oil.^[10]

The Querecual is believed to have matured during the Oligocene (~33.9 to 23 million years ago), proceeding from north to south. During the fluid migration, the oil produced from the Upper Cretaceous Querecual formation underwent a drastic change. The oil was eventually trapped in the Oficina formation by the Guyana Shield and Carapita mudstones, but had to travel about 300 kilometers. Due to the long distance migration, the lighter fractions of the oil evaporated. Microbial activity further aided the transition to extra heavy oil.^[9]