The Karachaganak field is a massive reef carbonate structure. The main reservoir is of the late Devonian-Carboniferous age, where sequence stratigraphic cycles of progradation and aggradation defining the growth stages of the carbonate build-up have been revealed. Vertical and horizontal semiconductive barriers was identified in the reservoir during the field development. It was assumed that these barriers are located at the boundaries of the changing depositional cycles, which took place during the reef structure growth. According to the simulation results on a sector model of the reservoir it was determined that the pressure barriers can be developed due to different fracture intensities observed in the reservoir and not because of rock property as such. The reason for the different fracture densities may be associated with compaction during primary diagenesis and may have a sync-depositional nature, which can be seen on carbonate structure outcrops.

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