East venezuelan basin

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Introduction

The East Venezuelan Basin is a major sedimentary basin in northeastern Venezuela. It is a depression in the igneous-metamorphic rock basement filled with Mesozoic and Cenozoic sediments to a thickness of as much as 40,000 feet. The basin has a general east-west trend and is some 800 kilometers long and 250 kilometers wide. The basin lies between several geological structures. It is contained to the south by igneous and metamorphic rocks of the Guiana Shield, to the north by metamorphic rocks of the Andes, to the east by the oceanic crust of the Atlantic Ocean, and to the west by the Espino Graben.

The East Venezuelan Basin is the second largest hydrocarbon-producing basin in Venezuela with proven reserves of 36 billion barrels.

History of the Basin

The oil-bearing basins of Eastern Venezuela developed through an unusual confluence of Atlantic, Caribbean, and Pacific plate tectonic events. Mesozoic rifting and passive margin development created ideal conditions for the deposition of hydrocarbon source rocks. In the Cenozoic, west-to-east movement of the Caribbean plate along the northern margin of Venezuela led to the maturation of those source rocks in several extended pulses, directly attributable to regional tectonic events. The combination of these elements with well-developed structural and stratigraphic fairways resulted in efficient migration of large volumes of oil and gas, which accumulated along the flanks of thick sedimentary depocenters.

The first hydrocarbons were discovered in 1937, located within the Oficina Formation in Cabimas.

To reflect the areal distribution of formations and the regional geologic structure, the region is divided into physiographic provinces. These provinces are the Northeastern Mountain, Northwestern Mountain, Foothills, Coastal Plains, Delta, Mesa, and Rolling Plains.

Primary Geologic Risks and Concerns

A concerning instability risk within the Pedernales Field of Eastern Venezuela was observed in a study. The risk is mostly associated with high-angle, cross-dip wells. "Bedding-parallel shear" is the principal mechanism for instability and the reduced strength properties acting on the bedding planes can result in enhanced, and sometimes catastrophic, instability. This instability can create "corners" to the breakout, forming a square borehole. Essentially, a common drilled well trajectory in the Eastern Venezuelan Basin may be considered as encountering "complex geologic conditions".

Petroleum System

Source Rock

The East Venezuelan Basin contains several hydrocarbon source rocks within its formations. Several formations include the Oficina Formation, Upper Cretaceous Guavinita Member of the Tigre Formation, Querecual Formation, and the San Antonio formation.  

The Querecual and San Antonio Formations, also known as the Guarico Group, are believed to have contributed over 90% of the discovered hydrocarbons in the basin, excluding the Orinoco Heavy Oil Belt. Campanian-age marine shales, calcareous shales and bituminous limestones were deposited under anoxic conditions in a shelfal setting as part of a depositional system that stretched across northern South America. The organic matter in these formations is typically Type II, with measured hydrogen index up to 700 mg hydrocarbon/gm organic carbon and total organic carbon of up to 8%. The range has suffered large amounts of shortening and possible long-distance transport.

The Tigre formation consists of marine shales and limestones with Type II, mainly marine amorphous kerogen. TOC ranges from 1-2.5% indicating potential, viable source rocks for exploration. It is indicated that this formation resulted in the gas in this basin.

The Oficina Formation also contributes to hydrocarbon accumulations in the Guarico Sub-Basins. Shales and coals within this formation contain Type III/I organic matter capable of generating both oil and gas with total organic carbon values that exceed 5%.

Migration

Migration pathways for oil and gas are directly linked to the maturation analysis. A study was conducted to model vertical migration along thrust faults, coupled with strata-parallel migration within reservoir-prone units. Migration is mainly strata-parallel, from north to south, and aligned with the orientation of the major depositional fairways. The alignment of normal faults in the Oficina Trend are likely to have had the effect of diverting southward-directed migration toward the southwest.

Trap and Seal

The Orinoco Heavy Oil Belt of the Eastern Venezuelan Basin involves stratigraphic trapping provided by onlap and by tar plugging. However, in Eastern Venezuela, the Furrial Trend indicates fault-related traps. Faults play an extensional role during Jurassic rifting and subsequently suffered inversion and strike-slip reactivation. This created anticlines as well as fracture porosity and permeability, and influenced the distribution of sandstone reservoirs, unconformities and related truncation traps. These faults also provide migration paths as well as lateral seals.

Reservoir

The reservoirs within the Oficina Formation consist mainly of stacked distributary channels and bay fill sands with porosity ranging from 20-35% and permeability from 50-1000md. Short distant lateral pinch-outs are common, but reservoir pressures and oil gravities indicate that many of the channels are in communication.

Petroleum Potential

Venezuela ranks first in national oil reserves that have yet to be exploited as of 2020. The Eastern Venezuelan Basin has a prolific Upper Cretaceous petroleum system that hosts discovered accumulations in excess of 90 billion BOE. This indicates that new and under-explored plays exist in the basin.

Oil Quality

Variations in source quality, maturation, migration pathways, and post-emplacement processes have directly contributed to extreme variability in the quality of Eastern Venezuela crude oils. East Venezuela have a vast range of properties such as heavy, high sulfur oils of the Orinoco Heavy Oil Belt with ranging API of 5-20 degrees and sulfur levels of 1-6%. In contrast, the oils of the Oficina and Anaco Trends are relatively light with API levels of 30+ degrees and low sulfur contents less than 0.5%.


Further Reading

[6] Venezuelan Hydrocarbon Habitat

[7] Sequence Stratigraphy of Venezuelan Basin

[8] Venezuela Sedimentary Basins

References

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This page is currently being authored by a student at the University of Oklahoma. This page will be completed by Dec 7, 2022.

  1. [1]Structure,Stratigraphy,HydrocarbonPotential
  2. [2]HydrocarbonSystemsofNortheasternVenezuela
  3. [3]VenezuelanHydrocarbonHabitat
  4. [4]ResourcePotentialOfEasternVenezuela
  5. [5]PrincipalReservoirsAndCompletionPractices