The vertical compartmentalization of reservoirs: An example from an outcrop analog, Crotone Basin, southern Italy

The lower Pliocene shallow-marine successions deposited within half-graben subbasins exposed in the northern Crotone Basin (southern Italy) are good examples illustrating the variable development of small-scale cycles (2-15 m [6.6-49 ft] thick) within the hanging-wall blocks of normal growth faults. Shallow-marine small-scale cycles forming the successions deposited near the immediate hanging wall of half-graben sub-basins show a clear alternation between well-cemented shell-rich intervals and weakly cemented siliciclastic sandstones. In contrast, the shallow-marine successions deposited in hanging-wall dip-slope locations and in the footwalls show a poorly preserved cyclicity related to lower subsidence rates and a higher degree of cycle amalgamation. The vertical alternation between well-cemented carbonate-rich intervals and less cemented sandstones creates a marked heterogeneity in the permeability of these shallow-marine half-graben fills, which are sealed by impermeable offshore and lagoonal mudstones and may be considered as an outcrop analog of potential reservoirs. As a consequence of the well-developed cyclicity, therefore, the potential reservoir is vertically compartmentalized. This feature tends to vanish toward the hanging-wall dip slopes, where the cyclicity is remarkably less appreciable. The correct prediction of geometry and effective volume of reservoirs and, in particular, the recognition of their vertical compartmentalization have an important impact on hydrocarbon exploration and production. The developed conceptual model of compartmentalized half-graben fill may be useful in several contexts to study fault-bounded basins from a reservoir point of view.