--> Massive Dolomite on the Upper Jurassic to Lower Cretaceous Carbonate Shelf, Northeastern Saudi Arabia: Insights From Reactive Transport Modeling

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Massive Dolomite on the Upper Jurassic to Lower Cretaceous Carbonate Shelf, Northeastern Saudi Arabia: Insights From Reactive Transport Modeling

Abstract

Recent regional mapping has shown that massive, stratigraphically discordant dolomite occurs within the Upper Jurassic to Lower Cretaceous intervals in the Jaladi area, northeastern Saudi Arabia. Areas where this massive dolomite occurs (over an area of more than 30,000 km2) have been avoided in the past due to severe dolomitization and drilling hazards, but are currently active exploration focus areas. Petrographic and geochemical analyses indicate that dolomitization is complicated with a wide range of associated diagenetic processes, including precipitation of calcite, fluorite, and kaolinite, silicification, dedolomitization, and emplacement of pyrobitumen. Previous studies have proposed two dolomite formation mechanisms: (1) lateral migration of tectonic-induced magnesium-rich brines from the Gotnia Basin to the north, introduced during burial, and (2) upward migration of hot fluids along fractures/faults sourced from sub-Jurassic stratigraphic systems. Reactive transport modeling was conducted to test the feasibility of these hypotheses and reconstruct the diagenetic history. The modeling results indicate that (1) massive dolomitization by hydrothermal fluids requires the presence of dense fracture systems and repetitive injection of hot fluids, and (2) tectonic-loading is likely to be the main mechanism for the large scale dolomitization. Neither of the two mechanisms can, however, explain the complicated diagenetic history alone. A combination of two processes may be required, i.e., tectonic-loading dolomitization accounting for first-generation dolomite, and late stage hydrothermal dolomitization overprinting the early dolomite in the vicinity of fracture systems. This model reproduces the mapped dolomite geometries and properties, and also predicts the reservoir quality modification associated with the hydrothermal dolomite. Results of this study will enhance our understanding of the dolomitization mechanism and associated diagenetic processes, and may provide new insights on exploration strategies for extensively dolomitized carbonate platforms.