Understanding and Characterizing Carbonate Slope Heterogeneity: A Subsurface Example from Karachaganak Field, Kazakhstan

Modern and ancient carbonate slopes are depositionally complex due to a spectrum of autochthonous and allochthonous deposit types that exhibit variability along strike, dip, and across stratigraphy. A hierarchy of geological frameworks including sequence stratigraphy, depositional regions and rock types is fundamental to capturing the temporal and spatial variability of carbonate slopes. Efforts to characterize this framework rely on integrating a variety of data with different scales of observation including modern and ancient outcrop analog and application, well logs, core, and seismic. Of particular interest to the hydrocarbon industry are the origins of impedance contrasts in carbonate slopes; resulting reflection patterns are ultimately interpreted as indications of the sequence stratigraphic framework in geological and reservoir models. Without the aid of other tools it is challenging to determine if the reflection patterns from seismic derive from lithostratigraphy, diagenesis, or original bedding. Each of these possibilities affects the confidence of the geological framework. In this study we use an integrative approach to describe the compositional and architectural variability of the geological model from the Karachaganak Field as a workflow example that ultimately reduced reservoir uncertainty and improved our knowledge of carbonate slope variability.

Karachaganak Field, northern Precaspian Basin, Kazakhstan, is a reef-rimmed Permo-Carboniferous carbonate platform with steep foreslopes. Prediction of depositional rock types and associated geometry was achieved through integrated analysis of core, log, and outcrop analog data. Reservoir properties controlled by stratigraphic trends were correlated with the geographic distribution of clinoform styles, sediment wedge types, and secondary features such as platform margin reentrants. Overall, this approach linked seismic-scale architecture, petrophysical behavior, and geographic facies distribution, thereby providing insights for future reservoir development and modeling. For example, production data from the current drilling program shows that the youngest Carboniferous distal slope settings are among the poorest producers due to depositional heterogeneity and compartmentalization, characteristics that the conceptual and base-case reservoir models predict. Recent results from wells with trajectories that target older slopes are a success and validate the conceptual geological and reservoir models.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California