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Reactive Transport Modeling of Reflux Dolomitization on an Upper Jurassic Homoclinic Carbonate Ramp

Abstract

It has long been recognized that Upper Jurassic carbonate reservoirs in the Eastern Province of Saudi Arabia have been significantly dolomitized and that the distribution of dolomite is highly heterogeneous in these reservoirs. Usually dolomitization strongly influences the redistribution of pore structure and connectivity, permeability pathways, and reservoir producibility. During the late Jurassic, the Eastern Province of Saudi Arabia was located on a vast homoclinal carbonate ramp, and this study used reaction transport modeling (RTM) to predict reflux dolomitization on this ramp. For this study, we modified the classic Saller and Henderson model to hypothesize that an updip evaporative lagoon (as the brine source) existed and migrated progressively toward the downdip, leaving behind a series of linear distributed dolomite bodies along the temporary coastlines. We then conducted 2D RTM to test this hypothesis. Complex strata in the RTM were built, based on a decompacted well log and core description in this region. The model deciphered the complex interplay of calcite dissolution, dolomite and anhydrite precipitation and their feedback on porosity and water chemistry. The results quantitatively demonstrated the spatial and temporal distributions of secondary porosity, mineral abundance, and chemistry of pore water, as well as the paragenetic sequence. The predicted spatial distribution of dolomite bodies agrees well with the observed distribution of non-stratigraphic dolomite in this interval, and the model provides a plausible explanation for the observed linear trend of dolomite bodies. Consistent with observations, both dolomitization and anhydrite precipitation occurred preferentially within the more reactive but less permeable muddy layers, although the grainy layers served as conduits for high flux brine transport. This conclusion highlights the importance of metastable precursor, reactive surface area and microbial mediation on dolomitization.