--> Abstract: Petroleum Systems of the Eastern Venezuelan Basin, by S. C. Talukdar; #91004 (1991)

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Petroleum Systems of the Eastern Venezuelan Basin

TALUKDAR, SUHAS C., DGSI, The Woodlands, TX

The Eastern Venezuelan basin (area 165,000 km) is well known for its Orinoco Heavy Oil belt and many large accumulations of medium and light oils and gas in the Maturin and Guarico subbasins.

A single petroleum system is responsible for all the oil accumulations in the Orinoco Oil belt and the Maturin subbasin and the majority of oil deposits in the Guarico subbasin. Source rocks for the system are the calcareous shales and limestones of Upper Cretaceous Querecual and San Antonio formations. These rocks were deposited in anoxic to near-anoxic conditions over the entire shelves of the northern passive margin of the Cretaceous South American continent and contain marine type II organic matter.

Geologic reconstruction and basin modeling have shown the development of huge generating areas along the northern parts of the Guarico and Maturin subbasins during early stages of foreland basin development in lower/middle Miocene times. Expelled oils migrated southward and updip for long distances (150-300 km) along undisturbed homoclinal surfaces successively through the Upper Cretaceous, Oligocene, and Miocene sandstone carrier beds under progressively younger Tertiary marine shales. The oils finally accumulated in Miocene sandstone stratigraphic traps. Biodegradation of originally mature medium to light oils in shallow lower/middle Miocene reservoirs formed the heavy and extra-heavy oils of the Orinoco Oil belt.

The oils in the Maturin and Guarico subbasins are found in structural traps within many different reservoirs (Cretaceous sandstones and limestones; Eocene, Oligocene, Miocene, and Pliocene sandstones). They are mainly mature and medium to light gravity oils but some heavy oils were formed in shallow reservoirs by biodegradation and in deep reservoirs by deasphaltenation. New oil and gas generating areas developed to the south of the deformed and uplifted earlier kitchen beneath several thrusts formed during middle/upper Miocene deformations of the northern foreland basin. The Cretaceous sourced oils generated during upper Miocene-Recent times migrated vertically along thrusts and faults that caused disruptions of regional carrier beds and seals and accumulated into different stratigra hic and structural levels.

The essential elements and critical processes of this petroleum system span Upper Cretaceous-Recent times. Its stratigraphic extent includes Upper Cretaceous to Tertiary formations. A second petroleum system within the Eastern Venezuelan basin occurs in the Tertiary and gave rise to less significant terrestrial oil accumulations in the Guarico subbasin. It is not discussed.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)