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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
2006. The American Association of Petroleum Geologists. All rights reserved.
DOI:10.1306/04030605194
Geothermal convection in the Tengiz carbonate platform, Kazakhstan: Reactive transport models of diagenesis and
reservoir
quality
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Gareth D. Jones,1 Yitian Xiao2
1ExxonMobil Development Company, 800 Bell St, Houston, Texas 77002; [email protected]
2ExxonMobil Upstream Research Company, 3120 Buffalo Speedway, Houston, Texas 77098; [email protected]
ABSTRACT
A fundamental challenge in carbonate reservoir
characterization is predicting the spatial distribution of diagenesis. We used a reactive transport model to investigate the viability of geothermal convection and associated patterns of diagenetic porosity modification in the Tengiz isolated carbonate platform
reservoir
.
Before burial, forced convection generates significant calcite dissolution (locally up to 45%) toward the platform center, minor calcite cementation (up to 0.4%) in the slope, and moderate calcite dissolution and cementation (up to 1.6%) in Serpukhovian boundstone convective cells. The patterns and rates of diagenesis proved critically sensitive to specified vertical permeability.
After burial with 200 m (660 ft) of salt, modeled subsurface temperature contrasts drive
platform-scale free convection. Flow is hydraulically closed, but significant dissolution, up to 7.3% after 20 m.y., occurred in the Serpukhovian and Visean platform interior, and minor cementation up to 0.7% occurred toward the margin. A shale-filled salt-withdrawal basin, 500 m (1640 ft) deep, significantly modifies the subsurface temperature distribution and free convective flow. Ascending groundwaters beneath the withdrawal basin created a zone of calcite dissolution (up to 24.5% in 20 m.y.), with a mushroom geometry and minor cementation (up to 2.3% in 20 m.y.) in the distal platform interior and margin. Rates of diagenesis are dramatically reduced with increasing overburden as compaction retards convective flow. From a generic perspective, free convection persists if the salt overburden is substituted with shale, although flow is reversed, resulting in a different distribution of diagenesis.
Simulations of geothermal convection provide a physically viable model for the integration of direct diagenetic observations to augment predictions of reservoir
quality in Tengiz and other carbonate platform reservoirs.
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