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传统观点认为由于奥陶系没有陆生高等植物出现，故而不会出现腐殖型有机质。首次在塔里木盆地塔中北斜坡中、上奥陶统泥灰质烃源中发现了类似于腐殖型有机质的海相偏腐殖型烃源岩。生物学、有机岩石学及核磁共振分析表明，奥陶系偏腐殖型烃源岩以宏观藻为主要生烃母质，其产物主要为凝析油和天然气。该类烃源岩主要是发育在灰泥丘沉积模式的丘间洼地和层状生物灰泥丘沉积亚相中的泥质灰岩类。该类烃源岩及其产物具有独特的生物标志化合物特征：较高的姥植比值、三环萜烷中低碳数部分含量较高及丰富的重排甾烷等。由该类烃源岩所生成的天然气的地化特征也较为特殊：较高的氮气含量、偏轻的甲烷碳同位素值和较高的干燥系数等。由于具较高的有机质丰度和生烃潜力，故该类烃源岩是塔中地区重要的气源岩。
The traditional view that due to Ordovician terrestrial plants does not appear, so there will be no humus organic matter. For the first time, marine humus-type source rocks similar to humus-type organic matter were found in the upper Ordovician marl source in the Tazhong north slope of the Tarim Basin. Biology, organic petrology and nuclear magnetic resonance analysis show that the Ordovician partial humic hydrocarbon source rock macroscopic algae as the main hydrocarbon generating parent material, and its products are mainly condensate and natural gas. The source rocks of this type are mainly mud-limestone species developed in the inter-mound depression of the mud-mound depositional model and the sedimentary subfacies of the stratified biological mud-mound. The source rock and its products have unique characteristics of biomarker compounds: a higher ratio of basking plants, tricyclic terpanes in the lower part of the higher content of carbon and rich rearrangement sterane and so on. The geochemical characteristics of natural gas generated from these source rocks are also quite special: higher nitrogen content, lighter carbon isotope values and higher drying coefficient. Due to its high organic matter abundance and hydrocarbon generation potential, this type of source rock is an important gas source rock in the Tazhong area.