Diagenetic controls on the quality of shallow marine sandstones: An example from the Cambro-Ordovician Saq Formation, central Saudi Arabia

Abstract

The Saq Formation is the most important Paleozoic aquifer in central and northern Saudi Arabia. However, owing to the continued depletion of the aquifers and the great depths to which the aquifers are buried in undrilled/underexplored regions, the aquifers porosity and permeability are increasingly becoming an issue of concern. Here, we employed thin-section petrography, petrophysical measurements, X-ray diffraction and scanning electron microscopic analyses, and fluid inclusion microthermometry to investigate the composition and diagenetic evolution of the Saq sandstones. The results of this study indicate that the sandstones are mainly quartz arenite in composition, mostly well sorted, and ranged in grain size from silt to coarse-grained sandstones. We observed three diagenetic stages (early, middle, and late) that impacted the porosity evolution of the Saq sandstones. Mechanical compaction, formation of authigenic kaolinite, and infiltration of smectitic clays occurred during early shallow burial diagenesis. The middle (deep-burial) diagenesis was dominated by mechanical and chemical compaction, formation of authigenic quartz overgrowths, transformation of kaolinite and smectite into illite, and precipitation of late calcite. The late (uplift-related) diagenesis resulted in the formation of authigenic kaolinite, dissolution of the late calcite, and the formation of predominantly pore-filling Fe-oxide (goethite) cement. Fluid inclusion results of this study show that quartz cement mostly formed at temperatures ranging between 151 °C and 175 °C and reached up to 272 °C, with, at least, two phases of overgrowth development. The overall high amounts of rigid grain contents (>65 %), low detrital matrix (<10 %), and coarser grain size have maintained the Saq sandstone quality (average permeability = 2100 mD) despite having been buried to great depths. The findings of this study provide useful insights into the diagenetic evolution of the Saq Formation, which has provided important input data for forward diagenetic modelling of their subsurface equivalent in Saudi Arabia and sandstones of similar depositional settings elsewhere.