Testing Carbonate Diagenetic Paradigms Using Reactive Transport Models

Yitian Xiao and Gareth Jones
ExxonMobil Upstream Research Company, Houston, TX

Early diagenesis is one of the primary controls on carbonate reservoir quality. We applied a reactive transport model to investigate early diagenesis in four hydrological zones (vadose, freshwater, mixing, and saline) in an isolated carbonate platform. We examined how climate, sea level, fluid composition, and depositional heterogeneity control early diagenesis. The modeling results significantly improved our understanding of the style, rates, and spatial distribution of early diagenetic reactions and provided unique insights allowing us to question some carbonate diagenetic paradigms. The modeling results demonstrate:

• In the absence of fractures, large-scale sea level lowstands may not have a thick active meteoric zone (< 10 meters) because rainwater quickly reaches saturation upon entering the vadose zone.
• The presence of fractures in the vadose zone leads to faster/deeper dissolution, potentially generating a karst system.
• Freshwater lens diagenesis is dominated by aragonite to calcite transformation with minor calcite cementation due to mass distribution from the vadose zone.
• Mixing zone dissolution occurs only when freshwater has elevated P _CO2 due to dissolved organic matter/microbial oxidation in the vadose zone.
• Mixing zone dissolution is focused near the platform coast area. Current mixing zone models are based on observations in coastal mixing zones and do not apply to inland locations.
• Due to the lack of Mg ²⁺ and slow reaction rate, mixing zone dolomitization is not a viable mechanism for large scale dolomitization. Circulation of seawater in the sub-mixing zone, however, could result in more dolomitization than previous thought.