Karachaganak Field, a Permo-Carboniferous carbonate platform, is a retrograde gas-condensate-oil reservoir located in the Pricaspian Basin. The field, in production since 1985, holds about 9 Bbls of oil and gas condensate and 48 Tcf of natural gas. Historically, the reservoir properties distribution has been debated because of both the complex depositional setting and the strong diagenetic overprint. These uncertainties have been assessed in the present study by analyzing and integrating the vast amount of geological and production data with the target of building a history matched reservoir model.

Seismic facies analysis, integrated with analogues outcrop and core and log data, reveals different geological contexts that ranges from platform interior bedded deposits to aggrading mounds, prograding clinoforms, slopes and basin sediments. Different Depositional Regions, characterized by specific petrophysical characteristics, as estimated from core, log, Well-Tests, PLT and production data, were defined.

The study, performed through an iterative process between geological and numerical models, allowed the definition of geologically meaningful interpretations of the complex dynamic behaviors. Different critical issues were better addressed by Depositional Region: quantity and magnitude of the enhanced permeability related to micro-fracturing and vugs, rock matrix permeability scale factors from plug to whole core, geological meaning and impact of sealing barriers, dolomite estimation and modeling, mound size range and modeling. A "best estimate" reference model has been thus defined and a very good history match achieved.

The uncertainties of the field have been then investigated considering the achieved history match as the benchmark. Starting from the reference model the petrophysical characteristics of the different Depositional Regions have been changed far from the history matched wells at an estimated threshold distance not perturbing the history match. Low and high scenarios models were thusly defined. These represent possible alternative "end members" consistent with the geological data and still endorsed by the history match.

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