OLIGOCENE-MIOCENE SEDIMENTARY ARCHITECTURE OF THE NORTHERN FALCóN BASIN (ONSHORE VENEZUELA)

Platform carbonates of Oligocene-Miocene age are important hydrocarbon reservoirs in e.g. Iran, China, Indonesia and Philippines. To better understand rock heterogeneities and the stratal architecture in such carbonate reservoirs, the Upper Oligocene to Early Miocene syn-rift sedimentary succession from the northern Falcón Basin (onshore Venezuela) is studied by means of sequence stratigraphy and lithofacies analysis. The northern flank of this back-arc basin is characterized by a kilometre-scale prograding delta system, which interfingers with carbonate deposits. Six transgressive-regressive sequences bounded by maximum regressive surfaces were recognized. Transgressive strata are characterized by metre-thick stratified branching coral-bearing beds passing basinwards to centimetre to metre-thick packstone to rudstone textures dominated by coralline algae and larger foraminifera. The scleractinian colonies are mainly found overturned and damaged. Coral rubble is abundant. The coralline algae and larger foraminifera tests forming the packstone to rudstone beds show high degrees of abrasion, fragmentation, bioerosion and encrustation. The matrix of these grain-supported textures corresponds to bioclastic silt. Thus, the skeletal components underwent important transport and reworking. On the other hand, regressive deposits are constituted by prograding siliciclastics with interbedded coral meadows, which developed on a fore-delta environment, occasionally within delta channels. Here, the corals occur as sparse isolated colonies in life position and as rubble. It is noteworthy that in this case study siliciclastic inputs did not inhibit coral development. Nevertheless, accumulation of terrigenous sediment limited the extent of the coral meadows and their life span. Finally, the rather recent discovery of a coeval super-giant gas field (Perla) in the Gulf of Venezuela could bring special interest from the hydrocarbon industry to the rocks analyzed. However, the Oligocene-Miocene sedimentary succession from the northern Falcón Basin is not an analogue of this offshore gas reservoir. La Perla field is constituted by a coarse siliciclastic-lean carbonate bank dominated by coralline algae and larger foraminifera where scleractinians are a minor component.