CARBONATE CEMENTS AND HYDROGEOLOGY OF THE AL-UKAIR COASTLINES, SOUTHERN SHORELINES OF THE ARABIAN GULF: A SCANNING ELECTRON MICROSCOPIC STUDY

The influence of water chemistry in carbonate cement mineralogy, more specifically, Mg/Ca ratio in crystal (polymorph) habit has been demonstrated by laboratory experiments. Bulk studies have indicated that calcite growth is inhibited through either step-blocking by Mg2+ adsorption and slow dehydration or enhanced mineral solubility associated with Mg2+ incorporation. Based on several studies, meteoric water, generally low in Mg/Ca ratio, would favor formation of equant crystals of calcite. On the other hand, marine waters marked by high Mg/Ca ratios would produce the elongate (c-axis) aragonite crystals. The southern shorelines of the Arabian Gulf stretching from the backdune/supratidal to the subtidal areas are characterized by systematic increase in the Mg/Ca ratio in pore-water. A scanning electron microscopic (SEM equipped with EDX) study of the carbonate sediments from Al-Ukair lagoon systems, south of Dammam, the capital of Eastern Province of Saudi Arabia, shows systematic variations in calcium carbonate mineralogy in response to the pore-water chemistry represented by different shoreline sub-environments. Considering the fact that the composition and crystal habit of the primary carbonate minerals have profound effects on the diagenetic evolution and reservoir quality of carbonate rocks and carbonate-cemented clastic rocks, if preserved in geologic records, such mineralogy should provide a better understanding of reservoirs deposited in similar settings.