Hydrogeological, hydrochemical and electric logging data of 24 groundwater wells of the D- field of the Shigaya, SW of Kuwait, were used to evaluate the Middle Eocene Dammam carbonate aquifer and to recognize the major geochemical processes that controlling the groundwater quality. The results indicate that the Dammam aquifer has a heterogeneous lithological nature, that composed of limestone, dolomitized at the top, with dispersed argillaceous materials ranges from 0 to 21% and total matrix porosity ranges from 4 to 35 %. The highest matrix porosity is recorded at the top of the formation, that was enhanced by dolomitization, fracturing, and dissolution. Interpretation of the hydrochemical data indicate that the groundwater type is Na2SO4 that enriched with Na+ and Ca++ and strong acids (SO4-- and Cl-) than the weak acids (CO3--+ HCO3-). This indicates a simple dissolution or mixing nature of the groundwater and a reverse ion-exchange between sodium ions of the meteoric water and the magnesium and calcium ions of the groundwater. The sulphate and chloride ranging from 1046 to 1345 ppm and 420 to 647 ppm, respectively. The sodium concentration ranges from 282 to 456 ppm and the calcium ranges from 225 to 387 ppm, while magnesium has the lowest concentration of between 118 and 137 ppm. A rapid decrease of the Dammam water salinity was detected after 1977, as a result of groundwater exchanges that took place between the overlying, relatively lower salinity, Kuwait Group aquifer through the wellbores whenever the wells are not being pumped.