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.

Key words: low-middle Triassic, North China Basin, source rocks, organic matter abundance, thermal metamorphic grade, hydrocarbon-generation history

摘要： 华北盆地下古生界海相碳酸盐岩普遍具有有机质丰度低和成熟度高的特点.有机质丰度低是沉积造成的,岩性、岩石结构和沉积构造均证明华北盆地早古生代以浅水陆表海、浅水泻湖环境为主,水体动荡、咸化、蒸发作用强烈,不利于低等生物发育和保存.有机质成熟度高是中新生界的叠加深成变质作用造成的,特别是中下三叠统沉积厚度达2000~2500m,区域性地覆盖在古生界之上,使下古生界埋深超过3000m,发生了较充分的早期生烃.对下古生界1500余个有机质丰度数据进行的分析表明,下古生界碳酸盐岩有机碳丰度大多都低于0.5%,平均在0.2%以下;氯仿沥青"A"含量低于120×10^-6,S₁+S₂低于0.2mg/g,达不到生油岩标准.利用TTI—R_o和地层对比恢复了中下三叠统克拉通盆地和上三叠统的沉积面貌,并计算出中三叠世末下古生界热变质程度R_o>1.2%,与现今古生界热变质程度的分布相符.

关键词: 中下三叠统, 华北盆地, 烃源岩, 有机质丰度, 热演化程度, 生烃史