The Cesar-Ranchería Basin is an underexplored region, in which there are some exploratory wells drilled since 1947, based on surface information. Seismic acquisition began only in 1979. At the present time, 57 wells have been drilled; four of them have produced non-commercial quantities of hydrocarbons. In Cesar sub-basin, El Paso- 3 well presented oil production in the Molino Formation. The Cesar F-1X well, drilled in 1980, produced gas in noncommercial quantities from rocks of the Aguas Blancas Formation. In the Rancheria sub-basin, the Molino-1X well showed non commercial gas amounts, from the Hato Nuevo Formation. The Papayal-1 well showed no commercial oil and gas from Lagunitas and La Luna formations. Well DST tests allowed the recovering of crude samples with API gravities between 27 º and 42 º.
Molino, La Luna and Aguas Blancas formations (Figure 2) have excellent hydrocarbon source potential due to its high content of type II and III kerogen. TOC varies ranges between 1% and 4.5%; vitrinite reflectance between1 and 2%.
According to geochemical modeling, oil migration began in the Oligocene and has continued until present time.
Migration routes are mainly thrust faults that cut the Cretaceous- Cenozoic cover.
The main reservoirs are fractured limestones from La Luna Formation and the Cogollo Group (Aguas Blancas and Lagunitas formations), which have delivered gas productions in DST (Drill-Stem Test) analysis. The Paleogene formations (Cuervos and Cerrejón) containing thick beds of coal constitute methane reservoirs (CBM).
The Molino, Cerrejón and Los Cuervos formations act as regional seals located above La Luna Formation. Local seals are mainly interbedded with pelitic rocks in Aguas Blancas, Lagunitas and La Luna formations.
According to seismic interpretations, the main traps for hydrocarbons in the Cesar-Ranchería Basin are found in folds associated with thrust faults (Figure 3). Similarly, the unconformities between La Luna-Molino and Molino- Cerrejón formations represent potential oil and gas stratigraphic traps.