Use of Outcrop Analogs to Interpret Seismic Facies in the Karachaganak Field
David Katz, Ted Playton, Paul (Mitch) Harris, 2010. "Use of Outcrop Analogs to Interpret Seismic Facies in the Karachaganak Field", Seismic Imaging of Depositional and Geomorphic Systems, Lesli J. Wood, Toni T. Simo, Norman C. Rosen
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Slopes of carbonate platforms are composed of a spectrum of deposit types, many of which exhibit complex vertical and lateral facies relationships due to partitioned contributions from multiple sediment factories via a variety of resedimentation processes. This depositional heterogeneity is often in part below the resolution of seismic imaging and poorly constrained through well log and core correlation. Outcrop exposures provide continuous and high-resolution facies, stratal, geometrical, and juxtaposition information that is generally not available from subsurface data but critical for characterizing carbonate slope systems as the framework for reservoir and simulation models.
Karachaganak Field, northern Precaspian basin, Kazakhstan, is a high-rising, Permo-Carboniferous isolated carbonate platform in which the oil leg of a gas condensate reservoir resides primarily within steep, progradational slope strata. Seismic mapping of the slopes at Karachaganak identifies a range of clinoform characteristics (continuity, curvature, declivity), margin configurations (accretionary, escarpment), and stratal terminations (onlap wedges, internal downlap). A narrow corridor of high-quality seismic data and poor core coverage to-date warrants the use of outcrops to develop further play concepts and validate and characterize reservoir attributes. Of the several seismic and geological models proposed for Karachaganak, the Chevron observations described in this paper divide the seismic facies model into five primary seismic facies. These facies have been determined by their amplitude character and include reflector continuity, strength, and geometry. The primary seismic facies are: (1) planar clinoforms, (2) exponential clinoforms, (3) chaotic wedge, (4) proximal sheet-like wedge, and (5) distal sheet-like wedge. These facies exhibit temporal and spatial organization throughout the Karachaganak seismic data. Planar clinoforms and chaotic and sheet-like wedges dominate the eastern slopes of the platform. Exponential clinoforms dominate the west. Chaotic wedges are present in the western and eastern foreslopes; they are associated with a second order turnaround from aggradation to progradation and suggest this was a critical period of margin instability.
Extensively studied outcrop exposures of carbonate foreslopes, including the Upper Devonian (Frasnian and Famennian) of Western Australia and the Lower and Upper Permian (Wolfcampian, Leonardian, and Guadalupian) of West Texas, display similar ranges of clinoform, margin, and internal geometries to those observed at Karachaganak. Application of the outcrop information provides a linkage between seismically determined stratal patterns and sediment type, as well as helps to constrain the timing and distribution of significant processes such as margin collapse, megabreccia emplacement, and bypass of grain-dominated accumulations into the basin. Integration of available subsurface data and insights from outcrop analogs ultimately enables enhanced prediction of reservoir quality, connectivity, and architecture in steep carbonate slopes, as well as other heterogeneous carbonate settings.