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.

ABSTRACT The upper Eocene La Paz Formation is the primary petroleum reservoir in the Provincia Field, located in the eastern foothills of the northern Middle Magdalena Valley Basin, Colombia. The average thickness of the La Paz reservoir is 800 feet in the Provincia field. A facies analysis of this unit was conducted using 332 feet of core, which were calibrated with well logs, core petrophysical data, and used for well log facies associations predictions in six wells. Four facies associations are recognized and represent deposition in braided trunk channel, minor channel (distributary or crevasse), crevasse splay and floodplain subenvironments. The minor channel and crevasse splay deposits comprise avulsion deposits. The avulsion deposits are volumetrically important and comprise nearly half of the reservoir (49.1%). Recognition of significant avulsion deposits in this study suggests that channel avulsion and preservation of thick avulsion sequences may be common in braided stream settings and are not exclusively characteristic of meandering systems. The best reservoir facies in the La Paz Formation are subarkosic sandstones (porosity 16.7%, permeability 409.2 md), deposited in braided trunk channels and comprise 30.6% of the total reservoir thickness. Sandstone facies in the avulsion deposits have poor reservoir quality (porosity 12.9%, permeability 29 md) and act as fluid flow retardants (or lateral plugs). Floodplain deposits comprising 20.3% of the reservoir are impermeable and are the main vertical barriers. Based on the depositional model, a 2-D reservoir zonation was constructed for the La Paz reservoir. Six flow units were defined, ranging from 50 to 140 feet in thickness, each separated by floodplain deposits. Within the flow units, interbedded avulsion deposits and trunk channel deposits produce high reservoir heterogeneity. Understanding all of the depositional elements in this type of fluvial system has helped to create realistic reservoir characterization that could be applied to reservoir simulation models.