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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