Slope Heterogeneity and Production Results in a Steep-sided Upper Paleozoic Isolated Carbonate Platform Reservoir, Karachaganak Field, Kazakhstan

Abstract

Abstract Slopes that flank steep, reef-rimmed carbonate platforms are depositionally complex due to a variety of autochthonous and allochthonous deposit types that exhibit vertical and lateral variability. Karachaganak Field, northern Pricaspian Basin, Kazakhstan, is a Permo-Carboniferous isolated carbonate platform with a hydrocarbon column that resides primarily within steep foreslope strata. Historically, the distribution of reservoir properties at Karachaganak has been poorly understood owing to the depositional heterogeneity common in slope environments. These uncertainties were assessed by integrating seismic facies analysis, relevant high-quality outcrop analogues, cores, and log data to produce a detailed depositional model of Karachaganak slopes that predicts compositional and architectural variability. Recent well lookbacks were also performed to assess the impact of carbonate foreslope characterization on well location and design in the current drilling program. Seismic facies analysis with support from outcrop analogues and forward models reveals depositional regions within stratigraphic intervals that can be further divided into multiple styles of clinoforms, slope and basin sediment wedges, margin configurations, and secondary features (i.e. reentrants). Each of the seismic facies is in turn defined by reflector characteristics, such as continuity, intensity, and geometry. Seismic mapping of the abovementioned elements with corroboration from other subsurface data identifies heterogeneity at well, flow unit, and field scales, including 1) bedset-scale variations internal to clinoforms and wedges, 2) along strike and temporal variations in clinoform or wedge style, and 3) platform-scale asymmetry. Integration of core and log data, combined with insights from outcrop analogues, allows for prediction of the depositional rock types and depositional regions within the stratigraphy mapped at Karachaganak. Spatial and stratigraphic trends regarding reservoir properties and connectivity are extractable based on the mapped geographic distribution of different clinoform styles, wedge types, and secondary features. Overall, this approach links seismic-scale architecture, petrophysical behavior, and geographic distribution, thereby providing insights for future reservoir development and modeling at Karachaganak. For example, recent production data from the current drilling program shows that wells in the youngest Carboniferous distal slope settings are among the poorest producers due to depositional heterogeneity and disconnected compartments - characteristics that the conceptual model and base case reservoir model predicts. Recent results from wells with trajectories that tatget older slopes are a success and validate both the conceptual geological and reservoir models.