The Apparent Stratigraphic Concordance of Reflux Dolomite: New Insights from Synsedimentary Reactive Transport Models

Garcia-Fresca, Beatriz ¹; Jones, Gareth D.²; Xu, Tianfu ^3
1 Bureau of Economic Geology - Jackson School of Geosciences, The University of Texas at Austin, Austin, TX.
² Carbonate Stratigraphy R&D, Chevron Energy Technology Company, San Ramon, CA.
³ Earth Sciences Division - Hydrogeology, Lawrence Berkeley National Laboratory, Berkeley, CA.

Understanding the distribution of dolomite in a sequence stratigraphic framework is a widespread method for interpreting synsedimentary dolomitization and constraining subsurface reservoir models. Early-formed stratiform dolomite bodies are often coincident with cycle caps and, by association, are interpreted to have been generated by equivalent time/space events. The present work shows that stratigraphic surfaces are not necessarily time-equivalent with the early dolomite bodies associated with them. We investigated stratigraphic dolomite patterns using the reactive transport simulator TOUGHREACT to: 1) evaluate the geological, hydrological and chemical controls on the reflux dolomitization of a high-frequency cycle and 2) explore intra- and inter-cycle episodic brine reflux during the consecutive deposition of three high-frequency cycles. Experiments were based on partially dolomitized cycles of a Cretaceous Glen Rose Formation outcrop in central Texas. Results demonstrate that refluxing bines, with salinities up to gypsum saturation, were capable of dolomitizing a high-frequency cycle to a depth of 1.5 m beneath the cycle top in 450 years; underlying cycles to a depth of 100 m were fully dolomitized in 2,500 years. Simulations of episodic brine reflux during the deposition of three high-frequency cycles revealed the potential for the complex evolution, migration and convergence of multiple dolomite fronts. The incomplete consumption of Mg along a flow path and the variable rate of dolomitization due to the dolomite “seed-effect” are primarily responsible for this phenomenon. These results suggest that the observed relationships between dolomite patterns and sequence stratigraphic surfaces may be casual, which has significant implications for correlating and building reservoir models. When correlating diagenetic geobodies, we should incorporate our understanding of the hydrogeologic and geochemical regime, as well as the stratigraphic framework.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009