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Stratigraphic Framework and Reservoir Quality Distribution, Tengiz Field, Western Kazakhstan

L. J. (Jim) Weber, ExxonMobil Exploration Company, Houston, TX
Paul M. (Mitch) Harris, ChevronTexaco Energy Technology Company, San Ramon, CA

The supergiant Tengiz field of western Kazakhstan produces oil from an isolated carbonate platform (areal extent of 160 km²) of Devonian and Carboniferous age. Seismic and well data clearly show two principle regions within the buildup, platform and slope that directly relate to reservoir quality and production characteristics.

The supersequence-scale stratigraphic framework was developed through an integrated interpretation of seismic, core, log, and biostratigraphic data. An initial broad Late Devonian platform was followed by punctuated backsteps during the Early Carboniferous (Tournaisian and Viséan). The uppermost Early Carboniferous (Serpukhovian) is characterized by several kilometers of platform progradation seaward of the Upper Viséan platform break. The basal Upper Carboniferous (Bashkirian) platform succession was aggradational. Drowning in the Early Bashkirian halted carbonate platform growth. Paleotopographic relief from the top of the Bashkirian platform to the basin floor approaches 1,500 meters within several kilometers lateral distance. See Figure 1.

On the platform, hydrocarbons are produced from Upper Viséan through Bashkirian reservoirs in grainstone and mud-lean packstones. Multiple porosity types are recognized, but matrix permeability is controlled primarily by intergranular porosity. Down dip of the platform margin, in-place upper-slope microbial boundstone and transported lower-slope boundstone debris form thick and areally extensive mappable reservoirs (Late Viséan and Serpukhovian) that have distinctive seismic facies and production/performance characteristics. Fractures contribute to non-matrix permeability in these boundstones.

The coarse stratigraphic architecture was used to further subdivide the platform portion of the reservoir for better reservoir characterization and for reservoir modeling. See Figure 2.

The temporal and spatial variability in reservoir quality of the platform, as shown by cross sections and maps, is directly related to stratigraphy. Time-slice mapping of synchronous depositional facies provides the basis for predicting reservoir distribution and continuity. The reservoir is also partitioned based on geographic position along a platform-to-basin profile. Polygons that define regions of similar depositional environment for each mappable layer define areas of similar reservoir quality. These polygons were input into the 3-D geologic model and helped guide the distribution of porosity and permeability in a geocellular model.

Expansion of plant facilities and well drilling costs at Tengiz will require considerable outlay of capital spending in the near future. Investigation of stratigraphy at Tengiz intends to better constrain geologic risk associated with volume assessments and prediction of reservoir quality. This will be especially critical not only for expansion scenarios that rely on primary depletion, but also in reservoir continuity issues critical to success of gas displacement projects under consideration.

Figure 1

Figure 2