Geothermal
Convection at Tengiz: Reactive Transport Models of Predictive Diagenesis and
Evidence from the Rocks
Jones, Gareth D.1, Joel F.
Collins2, Yitian Xiao1, Jeroen, A.M. Kenter3,
Paul Harris3, G. Kuanysheva4 (1) ExxonMobil Upstream
Research Company, Houston, TX (2) ExxonMobil Development Company, (3) Chevron
Energy Technology Company, (4) TengizChevroil,
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California