Can Karachaganak field, in production since 1984, represent a good analogue for Kashagan field which is approaching the production start-up? In fact the two carbonate buildups developed in the same geological context, the Pre-Caspian-Basin margins, during the same stratigraphic interval, Visean-Bashkirian. While their depositional facies are comparable, their internal architecture, as inferred from seismic, is different. Karachaganak, affected in its early stage by tectonic, developed initially with aggrading mound complexes followed by prograding clinoforms; it is characterized by biohermal deposits passing, in its upper part, to cyclic, grain-dominated platform interior sediments. Kashagan started developing with a retrograding pattern followed by prograding and then aggrading patterns; it is characterized by a large platform interior, made of grain-dominated cyclic deposits, surrounded by a narrow biohermal rim and slope. The diagenetic overprint consists of marine cementation, dolomitization and later dissolution in Karachaganak while, in Kashagan, an early diagenesis, dominated by cyclic subaerial exposures and karst was followed by late burial cementation. The resulting reservoir qualities reflect these different geological backgrounds. Karachaganak, quite heterogeneous, is dominated by microbial boundstone, in situ and breccias; it shows low porosity but locally high productivity when affected by micro-fractures and vugs. These characteristics are consistent with the wide range of well performances historically observed (orange!). Kashagan instead appears dominated by two rock types: platform cyclic grainy porous rock, having moderate productivity and cemented poor rim boundstones affected by outstanding productivity because of the presence of both karst and solution-enlarged fractures. These data support the expected bi-modal performances (apple!).


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