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Facies Architecture and Its Implications: Oligocene Carbonera Fm., Llanos Foreland Basin, Colombia


Oligocene strata In the Llanos basin, comprise 4 units: lower, and upper Early Oligocene, and lower and upper late Oligocene. Each unit contains a couplet of thick basal sand-rich subunit and an upper shaly one. The objective here is to characterize the temporal-spatial variation of the productive Oligocene sands, specially the upper Early Oligocene one (C7), in context of foreland development and sea level oscillations. This unit hosts heavy oil reservoirs in the Rubiales and other fields. Analysis of 118 wells, 16 seismically calibrated regional transects, and 45 maps, helped to document regional facies variation in the Oligocene strata. Thickening of the Oligocene from proximal east to distal west coincided with a progressively finer granulometry westward. Significantly thick blocky proximal reservoir sands, including some incised valley fill deposits, in the east, pass laterally into finer facies toward west. The sand unit of Early Oligocene (C7) shows thickness variation across the area; the thickest basal sands (upto 400′ thick) with minor shaly intercalations occur as a belt closer to the eastern border near Guyana shield, whereas a thinner belt (upto 200′ thick) occurs westward. Further west, the 200′ sandy belt laterally passes into a silty shale facies containing only a few thin sand beds. Our upper Early Oligocene gross depositional environment (GDE) map shows 3 facies belts, broadly paralleling the lithofacies variation mentioned above. The two C7 sand belts are mainly of fluvial origin, whereas its coeval westerly shalier unit varies from proximal brackish to restricted marine outward. The Oligocene units show a marked easterly back-stepping character in all basin-transverse sections. Oligocene 0′ line distribution map reveals that following initial lower Early Oligocene regression, 3 succeeding Oligocene intervals experienced progressive eastward marine incursions coinciding with stacked retrogradational sandy units. MFS-related shales of sand/shale couplets, provide seals for the C7 sand reservoirs westward. However, hydrodynamic factors were responsible for entrapment of oil in easternmost C7 reservoirs lacking effective seals. Punctuated Oligocene uplift of the Eastern Cordilleras likely triggered the cyclic marine transgressions in the foreland basin. The significance of this study is that it provides an understanding of the temporal spatial distribution of the productive Oligocene sands in relation to seal, source rock and entrapment.