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Uncertainty Analysis of Reservoir Quality and Pressure Barriers in a Sub-Salt Carbonate Platform Using Forward Stratigraphic Modeling (Serpukhovian Stage, Karachaganak Field, Kazakhstan)

Abstract

Predicting reservoir quality and the presence or absence of pressure barriers is difficult in carbonate reservoirs. For instance, the Lower Carboniferous Serpukhovian carbonates in Karachaganak Field are characterized by complex facies architectures and reservoir compartmentalization. This carbonate sequence is also poorly imaged by seismic due to the presence of a thick Permian salt cover, adding to subsurface uncertainties. Here, we use diffusion-based forward stratigraphic modeling (DionisosFlow) for the Serpukovian stage at Karachaganak Field coupled with a sensitivity analysis workflow (CougarFlow). The goals of the study were to (1) review seismic horizon picks based on process-modeling constraints, (2) understand the controls on the regional stratigraphic stacking pattern, and (3) to develop a risk analysis map for the presence of reservoir facies, including potential lateral pressure barriers. Results points out the need for revisions of the existing reservoir-scale static models. First, the Serpukovian Stage at Karachaganak comprises two carbonate buildups: the larger Main Buildup in the eastern portion of the field, and the smaller Western Buildup. All simulation results indicate that the current seismic reflector pick for the base of the Serpukovian Stage in the Western Buildup is incompatible with the seismic geometries and facies distribution for this system. Instead, the best modeling estimate for the base of the Sepurkovian stage is about 200 meters higher in the stratigraphy. This result is also supported by the drilling of a recent well that penetrated the top of the Permian on the Western Buildup 200 meters higher than anticipated. Second, modeling shows that fast regional subsidence rates were the primary control for the Early Serpukhovian aggradational stacking pattern, and that slowing subsidence rate during the Late Serpukhovian Uralian orogeny resulted in a switch to progradational sequence patterns. Finally, we demonstrate that wind direction had a major impact on the presence and distribution of reefs and breccia reservoir facies; the presence and distribution of the two reservoir facies, as well as the angle of the clinoforms, are thus accurately captured by forward modeling. Our forward stratigraphic modeling workflow coupled with uncertainty analysis thus provides useful new quantitative constraints to understand the vertical and lateral heterogeneities of the reservoir, including the presence of lateral pressure-barriers.