The determination of the original organic matter type and content of source rock in over-mature petroleum systems is critical for accurately evaluating the original hydrocarbon-generation capacity, but has challenged researchers for decades. Here we present a new method for such analyses using inorganic geochemistry, which was applied to a case study of over-mature shales of the Lower Cambrian Niutitang Formation (Upper Yangtze region, South China). Multiple redox proxies (e.g., V_EF, U_EF, Mo_EF, and Th/U ratio) indicate that the Lower and Upper members were deposited under anoxic bottom water and suboxic conditions, respectively. This implies different proxies suitable for recovering the original hydrogen index (HI_o) values, i.e., Cu_XS and P_org for the Lower (anoxic) and Upper (suboxic) members, respectively. Then, the original total organic carbon (TOC_o) content of the shales were calculated by using the formula TOC_o = (TOC_pd – (S₁+S₂) ×0.085 – HI_o × 0.0004)/(1 – HI_o/1177), of which TOC_pd refers to the measured present-day TOC data and S₁ and S₂ are the Rock-Eval pyrolysis hydrocarbons. Excellent original hydrocarbon generation capacity was revealed in that HI_o and TOC_o are 600–893.4 mg hydrocarbon (HC)/g TOC (average = 758.4) and 0.2–32.2 wt% (average = 10.2), respectively. Combined with six lithofacies identified from mineralogy and petrology, heterogeneities of the hydrocarbon-generation potential in different lithofacies of the Niutitang Formation shales of this study were revealed. The laminated siliceous shale and laminated carbonaceous shale facies had higher HI_o and TOC_o than the other facies, and can thus be regarded as the lithofacies with the highest hydrocarbon-generation potential. Our study provides a new method using inorganic geochemistry to restore the original kerogen type and TOC content of over-mature shales with a high degree of accuracy, which can be used to robustly evaluate the hydrocarbon generation potential of conventional and unconventional hydrocarbon resources in over-mature source rocks.