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AAPG Geoscience Technology Workshop

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Karst Evolution, Development and Interpretation in Carbonate Reservoirs and Impact on Fluid Flow Circulation in Reservoir Modelling

Abstract

Karst processes can greatly influence HC potential in carbonate reservoirs, hence their understanding is of relevant importance for processing the reservoir interval properties during the modelling phase. Nevertheless, the predictability of occurrence of karst features is usually quite difficult as they are the result of a more or less extensive and long-lasting exposure of carbonates to meteoric waters. Often, 2D and 3D seismic interpretation also associated with multi-attribute analysis do not clearly identify these features due to the size of karst elements and the seismic resolution. Within this context, the use of analogues could strongly improve the understanding of the karst structure and evolution and reservoir connectivity. The experience gained by working on several examples of karts reservoir in Southern Italy, Pre-Caspian Basin and Barents Sea helped in recognize and classify the most significant karst elements resulting from karstification processes, their spatial distribution and variability and their impact in the reservoir fluid flow. In the Pre-Caspian Basin, we tried to understand the evolution of a Permo-Carboniferous carbonate platform in the northern portion, focusing on the processes of karstification that, whilst enhancing the characteristics of potential carbonate reservoirs, may also result in complex fluid flow regimes more difficult to predict. The 3D seismic data available allowed the implementation of a multi-attribute approach in characterizing structures and morphologies of interest. The results of this study revealed that karst features can be readily detected and identified via seismic attributes analysis; however scale is fundamental for understanding if karst features are real or not during seismic interpretation. In general, the results have a positive impact on the understanding and reduction of the uncertainty associated with behaviors of fluid flow within karst-related structure and can be apply to analogues fields in the Pre-Caspian Basin and other regions having hydrocarbon potential within karstified carbonate reservoir. A series of field exposures has been chosen in a quarrying district in southern Italy as analogue for a field in the Barents Sea. The karst system is the result of several phases of subaerial exposure since Cenomanian until today, affecting a Lower Cretaceous substrate. The detailed description has been performed on the virtual outcrops, identifying the various elements of subsurface karst, like dissolved fractures, vertical shafts, horizontal conduits, etc. The upper part (epikarst) is showing a very variable thickness and the dissolved fractures are usually filled with abundant Terra Rossa deposits. The vertical transfer zone (vadose zone) shows mostly vertical dissolved fractures, often evolved as shafts, filled again with Terra Rossa. In this case, the vertical karst features are usually discontinuous as they are developing irregularly along fractures. The phreatic zone can be identified by the presence of rounded horizontal conduits and by horizons of enhanced dissolution, probably related to freshwater/seawater interaction. The position of the horizons of enhanced dissolution is not limited to the deepest part of the quarries, but their vertical repetition is suggesting an evolution of the level of groundwater table. The observed features are helping to understand more in detail the geometry of a karst system, especially in its complexity. We observed that the boundary between the vadose and the phreatic zone is never sharp, but is controlled by a variable groundwater level developing high-K 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.