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PSGeothermal Convection at Tengiz: Reactive Transport Models of Predictive Diagenesis and Evidence from the Rocks*

By

G.D. Jones1, J.F. Collins2, Y. Xiao1, J.A.M. Kenter3, P.M. Harris3, and G. Kuanysheva4

 

Search and Discovery Article #20050 (2008)

Posted May 20, 2008

 

*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007.

Click to view list of articles adapted from presentations by P.M. (Mitch) Harris or by his co-workers and him at AAPG meetings from 2000 to 2008.

 

1 ExxonMobil Upstream Research Company, Houston, Texas, U.S.A.

2 ExxonMobil Development Company, Houston Texas, U.S.A.

3 Chevron Energy Technology Company, San Ramon, California, U.S.A. ([email protected]) ([email protected])

4 TengizChevroil, Atyrau, Kazakhstan

 

Abstract

Reactive Transport Models that couple fluid flow and chemical reactions were used to test the viability of pre and post burial geothermal convection in the Tengiz carbonate platform reservoir. Simulations demonstrate that geothermal convection can drive diagenetic reactions capable of modifying reservoir quality. Specific model predictions include: 1) Concurrent dissolution and cementation in a mixed-convective system prior to burial in the platform rim, 2) Dissolution by forced convection prior to burial towards the platform center, 3) Perpetuation of early diagenetic patterns, but at lower rates after burial, 4) Dissolution beneath salt-withdrawal basins and cementation in the platform interior due to free convective flow modified by halokinetics and 5) Minor to no dolomite.

Ongoing Tengiz reservoir characterization studies were used to evaluate model predictions. Core and petrographic data support or at least do not rule out model predictions 2), 3) and 5). Enhanced porosity that is stratigraphically discordant, vertically oriented and platform-centric supports model predictions 2) and 3). Dolomite is present in the Carboniferous section but is generally volumetrically insignificant supporting prediction 5). Model prediction 1) is possible, but has been overprinted by later cementation and dissolution. A zone of enhanced porosity beneath a salt dome and not the adjacent withdrawal basin suggests model prediction 4) is either invalid or has been overprinted by later diagenesis.

This case study demonstrates the potential of Reactive Transport Models to develop viable and testable hypotheses that if integrated with observations from the rock record results in improved process-based predictions of carbonate reservoir quality.

 

uAbstract

uFigures

uGeologic setting

uDiagenesis

uGeothermal convection

uPredictive Concepts

uRTMs

uSummary

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uGeologic setting

uDiagenesis

uGeothermal convection

uPredictive Concepts

uRTMs

uSummary

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uGeologic setting

uDiagenesis

uGeothermal convection

uPredictive Concepts

uRTMs

uSummary

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uGeologic setting

uDiagenesis

uGeothermal convection

uPredictive Concepts

uRTMs

uSummary

uAcknowledgements

uReferences

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

uAbstract

uFigures

uGeologic setting

uDiagenesis

uGeothermal convection

uPredictive Concepts

uRTMs

uSummary

uAcknowledgements

uReferences

 

 

Geologic Setting

  • Tengiz is a world class “super giant” oil field located in Kazakhstan.
  • Reservoir is a Devonian to Carboniferous age isolated carbonate platform.
  • Sediments are predominantly grainy in the platform interior.
  • The rim and flank (highest rate wells) is composed of fractured microbial boundstones.
  • The seal is provided by a thin shale and a thick salt section.

 

Diagenesis

  • Diagenesis at Tengiz is complex spanning pre to post burial environments.
  • Reservoir quality modification by diagenesis is more significant than previous studies suggest.

 

Geothermal Convection

‘Geothermal convection describes groundwater flow in response to temperature derived variations in fluid density’

  • Geothermal convection has the potential to drive diagenetic processes.
  • Commonly observed in rimmed shelf carbonate platform margins (e.g., Florida; Enewetak Atoll).
  • Invoked to explain calcite cementation and seawater dolomitization.
  • Never directly observed in nature and conflicting conclusions on diagenetic potential.
  • Invoked to explain calcite cementation (Jurassic Smackover Fm) and dolomitization (Nisku Formation).

 

Predictive Concepts

  • Limited potential for seawater dolomitization (rim and platform interior).
  • Limited burial diagenetic modification of Units 2 and 3.
  • Burial dissolution in the central platform.
  • Dissolution beneath salt withdrawal basins.
  • Alternating vertical dissolution and cementation in boundstone slope.

 

Reactive Transport Models (RTMs)

  • Simulate groundwater flow, heat and solute transport (use Basin2 code).
  • Track calcite mineral reactions (cementation/dissolution). Incorporate porosity/permeability feedbacks due to porosity evolution.
  • Calcite maintains local equilibrium with flow along pressure & temperature gradients.
  • Reaction kinetics were not simulated.
  • Calcite has retrograde solubility (warming=cementation; cooling=dissolution).
  • Initial fluid specified as seawater.

 

Executive Summary

  • Reactive Transport Models (that couple groundwater flow and chemical reactions) support the hypotheses that geothermal convection can drive groundwater flow in Tengiz, both before and after burial.
  • The associated spatial distribution of diagenesis is a derivative of this evolving flow system.
  • Results show 5 specific predictive diagenetic concepts:
    • Limited potential for seawater dolomitization.
    • Limited burial diagenetic modification of Units 2&3.
    • Burial dissolution in the central platform.
    • Dissolution associated with salt withdrawal basins.
    • Vertical dissolution and cementation in boundstone slope.
  • Evidence from the rocks supports several of the model prediction.

 

Acknowledgements

We thank ExxonMobil, Chevron, TengizChevroil, and our Tengiz field business partners for permission to share the results of this study. Many colleagues have improved our understanding of Tengiz and helped provide the information necessary to constrain this reactive transport study and comparison to the rocks.

 

References

Collins, J.F., J.A.M. Kenter, P.M. Harris, G. Kuanysheva, D. J. Fischer, and K. L. Steffen, 2006, Facies and reservoir-quality variations in the Late Visean to Bashkirian outer platform, rim, and flank of the Tengiz buildup, Precaspian Basin, Kazakhstan; in P. M. Harris and L.J. Weber, eds., Giant Hydrocarbon Reservoirs of the World: AAPG Memoir 88, p. 55-95.

Heydari, Ezat, 2000, Porosity loss, fluid flow, and mass transfer in limestone reservoirs: Application to the Upper Jurassic Smackover Formation, Mississippi: AAPG Bulletin, v. 84, p. 100-118.

Jones, G.D., and Y. Xiao, 2006, Geothermal convection in the Tengiz carbonate platform Kazakhstan: Reactive transport models of diagenesis and reservoir quality: AAPG Bulletin, v. 90, p. 1251-1272.

Kenter, J.A.M., P.M. Harris, J.F. Collins, L.J. Weber, G. Kuanysheva, and D.J. Fischer, 2006, Late Visean to Bashkirian platform cyclicity in the central Tengiz buildup, Precaspian Basin, Kazakhstan: Depositional evolution and reservoir development, in P.M. Harris and L.J. Weber, eds., Giant Hydrocarbon Reservoirs of the World: AAPG Memoir 88, p. 7-54.

Kohout, F.A., 1967, Ground-water flow and the geothermal regime of the Floridian Plateau (1): GCAGS Transactions, v. 17, p. 339-354.

Machel, Hans G., and James H. Anderson, 1989, Pervasive subsurface dolomitization of the Nisku Formation in Central Alberta: Journal of Sedimentary Petrology, v. 59, p. 891-911.

Saller, Arthur H., 1986, Radiaxial calcite in lower Miocene strata, subsurface Enewetak Atoll: Journal of Sedimentary Petrology, v. 56, p. 743-762.

Sanford, W.E., F.F. Whitaker, P.L. Smart, and G.D. Jones, 1998, Numerical analysis of seawater circulation in carbonate platforms: I. Geothermal circulation: American Journal of Science, v. 298, p. 801-828.

Weber, L.J., B.P. Francis, P.M. Harris, and M. Clark, 2003, Stratigraphy, lithofacies and reservoir distribution, Tengiz field Kazakhstan, in W.M. Ahr, P.M. Harris, W.A. Morgan, and I.D. Sommerville, eds., Permo-Carboniferous Carbonate Platform and Reefs: SEPM Special Publication 78 and AAPG Memoir 83, p. 351-394.

 

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