Geothermal Convection in the Tengiz Carbonate Platform, Kazakhstan: Reactive Transport Models of Diagenesis and Reservoir Quality

Gareth D. Jones and Yitian Xiao
ExxonMobil Production Company, Houston, TX

A fundamental challenge in carbonate reservoir characterization is predicting the spatial distribution of diagenesis. We used “Basin2” a 2D 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 %) towards the platform center, minor cementation (up to 0.4 %) in the slope and moderate 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 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 Sepukhovian and Visean platform interior and minor cementation, up to 0.7 % towards the margin. A shale filled salt withdrawal basin, 500 m 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 integration of direct diagenetic observations to augment predictions of reservoir quality in Tengiz and other carbonate platform reservoirs.