Proper analysis of the original intergranular porosity loss of sedimentary strata is an important tool to gain insight into the influence of diagenetic processes on the quality of hydrocarbon reserviors. With emergence and progress of numerical technologies for simulating diagenetic processes in sedimenraty formations, the changes in the original porosity of reserviors attributed to more diagenetic processes (e.g. dissolution processes and the stabilization from one mineral to another one) can also be quantitatively evaluated. This paper proposed a set of formulas for the numerical computation of the loss in the original intergranular porosity of sandstone reserviors by compactional processes (COPL) and by intergranular cemention processes (CEPL), which consider the gain in the intergranular porosity by dissolution processes (DIPL). Theoretical analysis based on the measured data of sandstone samples shows that the real COPL will be obviously underestimated if the COPL computed by the traditional point-count method is far below CEPL and dissolution processes generated the additional porosity. Comparably, the real CEPL will only be slightly over-estimated. The deviation in COPL resulting from a DIPL of 5.0% lies in the range 0.75% ~ 2.10%, whereas the deviation in CEPL is in the range between 0.4 and 1.6%.