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ABSTRACT

The Apiay field is located in the southwestern part of the Llanos Basin, Colombia. The main Apiay reservoirs are in Upper Cretaceous (Coniacian-Campanian) sandstones. Within the lower Tertiary strata (lower Oligocene), a secondary gas/ condensate reservoir occurs in the lowermost sandstones of the Carbonera Formation, informally named the “T1 sandstone.”

The T1 sandstone is sandstone-rich across the central part of the field and more mud-rich to the west, and is interpreted as representing the fill of a paleovalley. It comprises dominantly marginal marine strata deposited under brackish water conditions, as indicated by a detailed facies analysis, the trace fossil assemblages, and biostratigraphic data.

Lithofacies descriptions recognize the following depositional history within the valley (from bottom to top): a thin interval of fine-grained fluvial sandstone, which rests unconformably over black, shallow marine shales. These fluvial deposits are confined to the deepest part of the paleovalley and have a restricted distribution within the field. There is an abrupt transition upward into marginal marine, brackish deposits composed of medium-grained sandstones, that represent mainly intertidal sandstone bars. The upward change from fluvial deposition to marginal marine is interpreted to reflect an overall transgression during filling of the paleovalley. Marginal marine deposits are volumetrically the most important within the T1 sandstone reservoir.

The T1 sandstone can be divided into two flow units. Flow unit 1 shows poor reservoir quality (porosity 17%, permeability of 146 md); flow unit 2 shows good reservoir quality (porosities 14.5%-15.6%, permeabilities 309-397 md). A zone of lower porosity and permeability (13% and 197 md, respectively) was found within this flow unit.

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