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Diagenetic Processes and Their Impact from the Petrophysical Properties in Kashagan Carbonate Platform Reservoir (Carboniferous, Kazakhstan)

Ronchi, Paola 1; Ortenzi, Andrea 1; Borromeo, Ornella 1; Claps, Michele 2; Zempolich, William G.2
1 Eni E&P, San Donato Milanese, Italy.
2 Agip KCO, San Donato Milanese, Italy.

The Kashagan (offshore Kazakhstan) huge isolated carbonate platform developed from Visean to Bashkirian in the Precaspian Basin; the sedimentation is organized in shallowing upward cycles marked at the top by subaerial exposure surfaces. This diagenetic study, based on about 2000 samples from 1600 m of cores from 11 wells, integrated sedimentology, petrography, fluid inclusion, stable isotopes and trace elements data and reconstructed the pore-system evolution in the margin and in the platform interior.

The early diagenesis includes marine cement fringes followed by meteoric dissolution and cementation. The meteoric diagenesis, related to cyclic subaerial exposure surfaces, more evident in the Bashkirian sequence, is still detectable in the older sequences by petrographic and geochemical signature. The main karst surfaces, correlated along the whole platform, are evidenced by: non luminescent cements, root traces, sharp δ13C depletion, fresh or brackish fluid inclusion calcites.

In the inner platform the porosity follows a cyclic pattern due to interaction of different factors: the fresh water dissolution more intense and preserved in the middle part of the cycle, cementation in the cycle top and compaction processes that destroyed the porosity at the cycle top and base. The diagenetic processes and resulting porosity pattern are more complex in the platform margin areas because the sedimentary cyclicity is less defined, and because of the stronger overprint of burial diagenesis.

The burial late diagenesis in the inner platform is represented by some calcite cementation that not altered deeply the porosity network. In the margins burial exotic fluid diagenisis is detected. These fluids were able to enter the rim through slope and margin facies and karst/flank margin fracture network; they caused dissolution, cementation and local dolomitization. The exotic fluid cementation is characterised by distinct petrographic and geochemical features: bright luminescence, high Mn content, depleted δ18O, high Th and low salinity fluid inclusions. Due to this diagenetic overprint, the platform margin is characterized by a heterogeneous porosity with larger pores and fractures but with lower matrix porosity values.

The understanding of the links between the sedimentological setting, the different diagenetic events and the pore system modifications allowed to construct several semi-quantitative reservoir quality maps that are used as a base for reservoir modelling.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009