Karachaganak field hosts world-class gas and liquid reserves in an Upper Devonian to Lower Permian isolated carbonate platform. A complex diagenesis over carbonate rocks extends from syndepositional to deep burial settings. Effective porosity in most parts of the reservoir is secondary in nature, such as vugs and fractures, with average porosity considered as low. The complexity of carbonate reservoirs is well known owing to the nature of their heterogeneity and cannot be resolved by conventional openhole logs because of limited vertical resolution and large azimuthal dependency.
Electrical 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. The use of a high-resolution electrical borehole image log helps place the tool probes at optimum formation and depth locations, thereby reducing risk and operating time. Analysis of pressure gradients can confirm subseismic reservoir barriers that had been earlier interpreted as structural or stratigraphic breaks.
Two case studies from nearby vertical Wells A and B are described. Well A has a fieldwide unconformity surface picked at a depth of X610 m MD, indicated by a drag pattern in the overlying zone. The carboniferous section in that well 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. Meanwhile, Well B has an unconformity within the internal Permian at a depth of X142.5 m MD, indicated by the drag pattern. The fieldwide unconformity and a stratigraphic barrier can be proved by a shift of pressure gradient at a depth of X608 m MD, indicated by a change of dip attitude.