Hematite and hausmannite Ore ground are the most common material for weighting cement, it most adequately fulfills all the requirements and achieves the highest effective specific gravity, they usually dry blend with oil well cement to prepare high-density cement slurries. Cement sheath made of Portland cement and high-density additive of metal oxide such as Hematite and or hausmannite Ore ground decomposes much faster than cement sheath made of net or low Portland cement when it exposes to hydrogen sulfide (H2S) and or wet carbon dioxide (CO2) during the Well-Life. Hematite and hausmannite Ore ground's pre-densified cement sheath breaks down into many compounds through series of chemical reactions often involve an energy source that breaks apart the bonds of compounds. It decomposes into metal sulfide and or carbonate and calcium carbonate with a small concentration of iron sulfide as a result of the metal oxide and or Portland cement content sulfidation and carbonation reactions. Cement sheaths made of net, high, or low-Portland cement without high-density additive of metal oxide decomposition rate is less than cement sheath made of Portland cement with a high-density additive of metal oxide when it exposes to H2S and or CO2. H2S alone does not drastically decompose cement sheaths made of net, high, or low-Portland cement without high-density additive of metal oxide. H2S has limited impact on these cement sheath type as it reacts with iron(III) hydroxide [Fe(OH)3] in which is a part of 10% of the cement hydration products. Carbonic acid reacts with 90 % of the cement hydration products (calcium silicate hydrate and lime) and degrades cement sheath thoroughly. Despite that net cement-sheath does not drastically decompose when it exposed to H2S alone, it does when expose to CO2 alone and the decomposition rate increase in the as H2S concentration increase; unfortunately, it is not a case of if, it is a case of when cement sheaths with or without a high-density additive of metal oxide decompose due to the Carbonation and or Sulfidation reaction under wet CO2 and or sour environments. The cement sheath deterioration not only prevents well production or injection rate by reducing the inner diameter of the tubing and restrict access to the well with surveillance equipment for data acquisition but also damage the well-integrity and allow hydrocarbons along with H2S and CO2 to break through to the surface eventually. Immiscible gas injection is the worst case scenario ever, Portland cement sheath whether it contain a heavyweight additive of metal oxide or not, low-Portland or high Portland cannot sustain the acid gas, and it will lose its integrity. These wells have to fail in a short period due to the cement sheath deterioration. This paper discusses the durability of different oil well cement formulations under H2S / wet CO2 environments and demonstrates why some metal oxide containing cement more stable than others under sour CO2 reservoir downhole conditions.