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Sedimentology and Stratigraphy of the Kashagan Buildup, Carboniferous, Pricaspian Basin, Kazakhstan

Claps, Michele 1; Zempolich, William G.2; Casaglia, Francesca 2; Ronchi, Paola 2
1 AGIPKCO [presently PDO, Muscat, Oman], San Donato Milanese, Italy.
2 AGIPKCO, San Donato Milanese, Italy.

Kashagan field is a large isolated carbonate buildup in the Southern Pricaspian Basin. It is characterised by three stacked reservoir units: Upper Devonian, Tournaisian to Middle Visean and Upper Visean to Lower Bashkirian, the latter (RU1) being the main hydrocarbon-bearing reservoir.

Kashagan RU1 is subdivided into six 3rd order sequences, separated by major subaerial unconformities. These are marked by paleokarst, collapse breccias and pedogenic features resulting from long-term exposures of the entire platform. All these major intervals display an internal layered architecture, recognizable on logs and cores.

The main sedimentological features identified on cores (from depositional to early diagenetic signatures) allowed to build a depositional lithofacies scheme for the reservoir unit. The facies were grouped into main categories, based on similarities in water-depth, hydrodynamic energy and depositional setting. In the inner part of the platform the main facies range from shallow water to relatively deeper platform facies, while the platform flank is commonly dominated by high-energy, bioconstructed and resedimented facies.

The identification of depositional breaks and the lithofacies vertical stacking pattern, associated to their well log interpretations, led to define a finer stratigraphic breakdown at the 4th order cycle scale. These cycles (10’s meters scale) show a distinctive shallowing-up trend and are capped by subaerial unconformities. Depositional cyclicity in the upper sequences (Serpukhovian and Bashkirian) shows higher facies heterogeneity in comparison to the Visean. The resulting layering scheme comprises more than sixty 4th order cycles and is correlatable across the reservoir.

Based on the facies vertical and lateral associations the facies polygons originally defined on seismic were geologically characterized as Kashagan Environments of Deposition (EoD). The depositional facies were clustered for the RU1 into four main EoD: Platform Interior, Transition Zone, Platform Margin and Outer Shelf. Slope and Basin are presently still undrilled. Biostratigraphic and palaeoenvironmental studies have contributed important information to support the EoD conceptual model. Data from outcrops and other analogues have been also considered and incorporated. The EoD model was finally improved using the facies output of the Dionisos depositional forward modeling to generate a high-resolution lithofacies mosaic for each individual 3rd order sequence.

 

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