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.