The origin and formation mechanism of N2-rich Lower Cambrian shale gas remain ambiguous, posing serious exploration risks in the Lower Cambrian shale gas in South China. In this study, high-vacuum-pulsed electromagnetic and helium-filled ball-mill rock-crushing methods were developed to measure the released gas contents, compositions and nitrogen isotopes of N2 in shale-crushed gas from Lower Cambrian organic-rich shales in South China. Key factors in N2 enrichment in shale gas and its origins were investigated. Results showed that mineral compositions and thermal maturity affected the contents and compositions of shale-crushed gas. Carbonate cementation, and feldspar dissolution and reprecipitation, formed isolated pores filled with migrated bitumen and thus generated high gas contents with larger CH4/N2 ratios of shale-crushed gas. The thermal maturation of Lower Cambrian organic-rich shale can be further divided into the early and late stages of overmaturation with a boundary of 3.4–3.5%EqVRo. The contents and CH4/N2 ratios of shale-crushed gas continued to increase at the early stage of overmaturation, and then a rapid decrease at the late stage of overmaturation (>3.4–3.5%EqVRo), reflecting the release of large amounts of N2 by intensified aromatization of organic matter. An empirical equation of TONb = 0.0138 × TOC with R2>0.98 was used to estimate the relative contribution of inorganic nitrogen (TINb) and organic nitrogen (TONb) to the bulk of nitrogen content in the overmature Cambrian shale. The EqVRo-δ15NN2 plot allows identification of N2 derived from thermal decomposition of ammonium-bearing clay minerals or residual kerogen. The N2 generated at the early stage of overmaturation (including at wells LX03, EYY1, SNY1 and W001-4) in Lower Cambrian shale gas is characterized by δ15NN2 values ranging from −2 to +2‰, mainly reflecting thermal deammoniation and decomposition of ammonium-bearing clay minerals. In contrast, the N2 generated at the late stage of overmaturation (DQ outcrop) in Lower Cambrian shale gas is characterized by δ15NN2 values greater than +4‰, indicating thermal cracking of pyridinic nitrogen and pyrrolic nitrogen. The N2 generated from thermal transformation of ammonium-bearing clay minerals and pyridinic nitrogen and pyrrolic nitrogen is the main source of N2-rich shale gas in the Lower Cambrian shale of South China.