Despite the increasing production of renewable energy, the demand for fossil energy still increases, which vectorised the search of crude oil into deeper horizons of depths below 6,000ft.  At these depths, during the drilling and cementing of wellbore, to exploit crude oil, this activity encounters higher temperatures and pressures, adverse formations, and other related in-situ subsurface hash conditions.  Hence, the need to prevent these antagonistic factors militating against the drilling and cementing of a successful, usable, economic, and safe oilwell became imperative.  Therefore, this research article, investigated the effect of ferrous ion concentration in mix-water on the compressive strength of oilwell cement sheath, using Kolo Creek as a case study.  The research, focused on the application of response surface methodology in Minitab 16, to identify the optimal, comfort, and adverse zones of high ferrous ion concentrations in mix-water on the response surface, which is the compressive strength of oilwell cement sheath.  As a result, secondary data were collected and subjected to Minitab 16 design of experiment using the principles of Box-Behnken design of experiment in response surface methodology.  Also, the response surface was customised, optimised, and analysed, to yield the best responses of the compressive strengths.  Consequently, contour and surface plots were generated as the results, to identify the optimal, comfort and adverse zones of compressive strength in the ferrous cement sheath systems.  The results obtained were bench-marked with the 1500psi compressive strength minimum API Specification for cementing oilwells.  These results revealed that, the optimal, and comfort zones of the investigated compressive strength exist in mix-water with very low ferrous ion concentration (0.00 to 1.26mg/L) used in the formulation of cement slurry; while adverse zone of compressive strength exists, when the mixwater used to formulate the cement slurry had high ferrous ion concentration of 1.27 mg/L and above. Therefore, mix-water with high ferrous ion concentration should be reticulated, to make the mix-water potable, before using it to formulate cement slurries