Facies and Sedimentary Environments of the Tertiary Sediments of the Ayacucho Area, Orinoco Oil Belt, Venezuela, South America
Celia Bejarano, Alvaro Cardena, Simon Perez, and Alejandra Rinaldi
Proyecto Faja Petrolifera del Orinoco, Caracas, Venezuela
The Orinoco Oil Belt (OOB) constitutes the southern margin of the Eastern Venezuelan Basin, and runs parallel to the present Orinoco River along a surface of 55000 KM2. The OOB comprises four main areas named west to east Boyaca, Junin, Ayacucho and Carabobo, which has been divided into 27 blocks for the purposes of reserve quantification.
In Ayacucho area, the major reservoir unit is the fluvial-deltaic Oficina Formation of early to middle Miocene age. The most extensive and oil bearing sand bodies are located in the lower to middle parts of the formation. The reservoir sand bodies generally consist of unconsolidated quartz sandstones with excellent porosities (20 to 38 %) and permeabilities (1,600 to 40,000 md).
To understand the complex and rich facies variation of the Tertiary sediments occurring across the Ayacucho block, and to position those in time and space, an integrated study of core analysis, bioestratigraphic and lithostratigraphic correlation was conducted. The results were essential for the construction of the geological and reservoir engineering models for the area. Herein, a review of the main sedimentary facies and environments of deposition derived from the study of more than 15 cored-wells in the study area is presented.
The Eastern Venezuela foreland basin was formed as a consequence of the oblique collision of the Caribbean and South American plates that took place during Oligocene. The OOB to the south of the deformation front, remained unaffected by compression and as part of a peripheral forebulge associated with the development of this important petroleum basin. This broad paleohigh was crosscut by rivers, and as uplift continued, down cutting kept pace with uplift, helping the development of broad deeply incised fluvial valleys. Therein, Cretaceous and Paleogene sediments underwent erosion.
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