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Understanding the Development of an Early, Diagenetically-Controlled Template for Reservoir Quality Distribution Using Forward Sediment/Diagenetic Models of the Tengiz Platform, Kazakhstan


The pore network established during the early development of a carbonate platform by depositional and early diagenetic processes can have a strong impact upon subsequent fluid flow processes, including any later phases of diagenesis and/or hydrocarbon emplacement. Sequence stratigraphy provides a framework within which we can consider the spatial distribution of depositional facies and their overprint by sequential episodes of meteoric diagenesis. However, particularly within long-lived platforms, multiple overprints by meteoric diagenesis can lead to complex reservoir quality distributions that are hard to relate back to the underlying cyclicity within the sedimentary succession. Our investigation uses a process-based forward sediment model, CARB3D+, to reproduce the platform architecture, sediment distributions and meteoric diagenetic overprinting of the Tengiz platform in western Kazakhstan. The main hydrocarbon-bearing interval spans a period of Earth history that transitions from global greenhouse to icehouse conditions, with increasing amplitude sea-level variations and changes in dominant depositional mineralogy. Meteoric diagenesis during this time is modelled as a function of a semi-arid climate and thin soil development during subaerial exposure, similar to the modern northern Bahamas. Our results reproduce the gross-platform architecture and facies patterns of Tengiz, as well as many important aspects of the porosity distribution seen in log and core measurements. The extent of final diagenetic modification is controlled largely by subsidence rate and the amplitude of high-frequency sea level variations. Within the platform interior this produces intervals of cementation within the Visean platform, intense cementation in the Serpukhovian and intervals of cementation and dissolution in the Bashkirian, where primary aragonitic sediments is more abundant. Towards the platform margin our models predict cavernous porosity, due to mixing zone dissolution, coincident with areas where bit drops often occur. The distribution of this is strongly controlled by progradation rates. Slow progradation appears to lead to well-developed cavernous porosity, whilst faster progradation generates wide-spread, but less developed cavernous porosity. Simulations strongly suggest that the template for the distribution of late stage diagenesis, which is significant across within the field, was established early on in the history of the platform.