Abstract: The lower-paleozoic marine carbonate rocks in North China Basin were generally characterized by low abundance of organic matter and high maturity. The low abundance of organic matter was caused by the sedimentary environment. Lithology, rock texture and sedimentary structure proved that the North China Basin in the Lower Paleozoic was mainly in the shallow inland sea and lagoon, where water was unquiet and salty. The evaporation rate was high, and the sedimentary environment was unfavorable for low-grade living beings to grow and be preserved. The high maturity was caused by the deep-seated metamorphism produced in the Mesozoic-Cenozoic period. Especially, when the Mesozoic was covered by the low-middle Triassic Formations with the thickness ranged from 2000 meters to 2500 meters, the buried depth of the Lower Palaeozoic source rocks was over 3000 meters, which resulted in the early-hydrocarbon-generation fully. According to the data of 1500 samples, the organic matter abundance of few samples exceeds 0.5 percent, and their average value is less than 0.2 percent. The content of chloroform bitumen "A" is lower than 0.012 percent. The average concentration of S₁+S₂ is lower than 0.2mg/g. All these indications are inaccessible to the standard of effective oil source rock. The TTI-R_o conversion and strata correlation were used to recover the sedimentary environment of the low-middle Triassic carton basin. The calculated thermal metamorphic grade of the Lower Paleozoic at the end of Triassic is higher than 1.2 percent, which accords well with the present distribution of thermal metamorphic grade in North China Basin.