A complex diagenesis over carbonate rocks extends from syndepositional to deep burial settings at Karachaganak field. The complexity of carbonate reservoirs are well known due the nature of their heterogeneity and cannot be resolved by conventional openhole logs because of limited vertical resolution and large azimuthal dependency. Borehole image logs provide both the small-scale resolution and azimuthal coverage to quantitatively resolve the heterogeneous nature and are used routinely to determine subseismic structural or stratigraphic events, as well as to optimize the selection of formation pressure and sampling points. Analysis of pressure gradients from good pressure sampling points can confirm subseismic reservoir barriers which had been earlier interpreted as a structural or stratigraphic break. Two case studies from nearby vertical wells A and B are described. A field-wide unconformity surface picked in both wells separates between Carboniferous and Permian formations. It is indicated by the drag pattern of the dips and a shift of pressure gradient. The carboniferous section in well A is composed of numerous shoaling upward cycles. Any of these cycle tops can act as a vertical permeability barrier and result in a step in the pressure profile.


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