Insight on the Complexity of a Paleokarst Reservoir: Examples From a World-Class Outcrop Analogue (Southern Italy)

Abstract

Karst reservoir are widely distributed throughout the world, spanning from Cambrian to Neogene. The predictability of a karst reservoir is usually quite difficult as they are the result of a more or less extensive and long-lasting exposure of carbonates to meteoric waters. The use of analogues could strongly improve the understanding of the karst structure and evolution. A research project has been carried out focusing on the paleokarst features that are impacting the dynamics and modelling of an hydrocarbon reservoir. A series of analogues has been studied in a quarrying district of southern Italy, representative of the karst profile of some oilfields in the Adriatic Offshore. The Lower Cretaceous limestones of the Apulian Platform have been subject to polyphased karstification since Cenomanian until Messinian, developing a complex karst pattern, with infillings of multiple ages. The vertical karst profile we observed is the classical one, with the succession of unconformity, epikarst with variable thickness, the vertical transfer zone (vadose zone) and the phreatic one. We observed that the boundary between the vadose and the phreatic zone is never sharp, but is controlled by variations of the groundwater level developing highly dissolved horizons that act as high porosity/high permeability horizontal streaks. The impact of these features is an increased connectivity of the reservoir also above the main phreatic zone, leading to very high horizontal permeabilities, draining limited areas of the reservoir. The presence of these horizontal streaks is of fundamental importance, as it demonstrates that in the vadose zone there is the possibility of strong lateral connectivity within the reservoir; a very similar situation is also observed in modern karst systems where cross flow between conduits could be controlled by “old” phreatic horizons. Karst reservoirs are complex and their dynamic zonation can help to model them properly. The estimation of permeabilities in these systems are fundamental to avoid an increase on the water breaktrough. The presence of extensive, but spatially limited, features with very high permeabilities and porosities are in fact improving the drainage of the reservoir volumes and also better connecting the fracture networks. These features are usually underestimated in karst reservoirs, while their impact is dramatic on production.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017