Petrographic and Geochemical Constraints on Regionally Extensive Hydrothermal Dolomites in Jurassic Carbonates of Saudi Arabia

Abstract

This research focuses on the petrographic, geochemical, and reservoir petrophysical aspects of Jurassic carbonates in the subsurface of northeast Saudi Arabia. Dolostones account for ~85% of the reservoir rocks and are composed of equant and baroque dolomites with the two commonly co-occurring. The two dolomite types record nearly identical stable isotopic and fluid inclusion values, characterized by negative δ18O values (means: -7.5‰ and -7.4‰VPDB for equant and baroque dolomite, respectively) and high homogenization temperatures (means of 101.6ºC and 98.3ºC for equant and baroque dolomites, respectively). The 87Sr/86Sr in dolostones ranges from 0.70761 to 0.70787, which is higher than Late Jurassic seawater, indicating formation from marine-derived fluids of Paleogene age. Uranium–lead dating on dolomite yields ages of 58.0 ± 6.5 Ma to 11.9 ± 2.4 Ma. All data indicate that dolomites formed and recrystallized at high temperature after burial. Regional and burial studies show that the dolostones never experienced burial temperatures as high as those recorded by the fluid inclusions, indicating a hydrothermal origin. Integrating the data with previous structural and burial studies, we interpret the extensive dolomitization to have been facilitated by surface-breaching faults, which were active during part of the diagenetic history of the carbonates. The fault-damage zones served as conduits and allowed evaporated seawater to sink during the Paleogene. Sinking fluids charged a deeper aquifer and the bulk of dolomitization was from convective circulation of hot fluids within the fault and fracture network, with some seismic pumping possible. This process also altered some reservoir properties in the dolostones. Samples with porosities 20% and permeabilities 100 mD were only observed in the dolostone and not limestone. Petrographic and geochemical analyses revealed the presence of pore spaces exclusive to the dolostones. These pores are interpreted to have developed during the hydrothermal alteration, through mesogenetic dissolution events that are unrelated to and absent from the limestone. Combined petrographic and geochemical observations indicate dolomites precipitated and recrystallized in a hydrothermal setting. The recrystallization resulted from the pulsed nature of fluid flow, which either caused temperature fluctuations or introduced fluids of variable saturation state.