--> Re-Thinking Reservoirs: The Case of the T2 Sands in the Southern Llanos Basin of Colombia

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Re-Thinking Reservoirs: The Case of the T2 Sands in the Southern Llanos Basin of Colombia

Abstract

Abstract

The southern Llanos basin comprises one of the most prolific heavy oil provinces in Colombia where the main exploratory target is a thick (> 300 ft) sequence of sandstones operatively known as the T2 unit which was considered as a single cronostratigraphic unit of Eocene age and as a single flow unit. Recent comprehensive studies which include sedimentology, stratigraphy, biostratigraphy, petrology and geochronology using surface columns and cores showed that the operational T2 actually is formed by three different geochronological intervals that include Upper Cretaceous, Paleocene and Eocene sandstones separated by regional unconformities. Geochronological divisions of T2 suggest a strong structural control in the deposition related to successive extensional and contractional events, generating lateral west to east variations in thickness for each time interval. Reservoir time divisions also explains changes in rock quality, hydrocarbon production and pressure trends that are directly related to the differences in depositional environments and provenance and to the existence of regional intra T2 seals associated to Cretaceous-Paleocene unconformity.

Chronological and quality reservoir variations have being recognized and interpreted also through stochastic seismic inversion in which differences into the reservoir properties are reflected in the “elastic behavior” of the seismic data. Finally understanding of the T2 unit as a multistage reservoir has led to the ongoing proposal of a new play concept in which the reservoirs of Upper Cretaceous, Paleocene and Eocene age may conform a regional giant stratigraphic trap, charged during multiple Paleogene events.