鄂尔多斯奥陶系碳酸盐岩碳氧同位素特征及其意义*

鄂尔多斯古生代海相地层沉积厚度巨大。鄂尔多斯奥陶系碳酸盐岩的碳氧同位素组成受后期成岩作用影响较小，基本保留了原始海洋的同位素组成： δ¹³C值分布于-7.30‰～2.26‰之间，均值-0.30‰；δ¹⁸O值分布于-13.14‰～-1.94‰之间，均值-6.38‰，碳氧同位素组成与全球基本一致。区域上，鄂尔多斯西缘具有相对较高的δ¹³C值，南缘次之，而东缘最低。纵向上，碳同位素组成逐渐增重，并在中晚奥陶世发生明显的正向偏移，δ¹³C均值由马家沟组的-0.36‰增加到平凉组的0.15‰，至背锅山组增加至0.68‰。碳同位素的区域分布差异表明鄂尔多斯西缘水体相对较深，南缘次之，东缘相对较浅，由早奥陶世至晚奥陶世水体逐渐加深，碳同位素组成反映的海平面变化趋势与沉积相演化一致。鄂尔多斯西南缘中晚奥陶世碳同位素组成的正向偏移，标志着较高的生产力和有机碳埋藏率，具有重要的石油地质学意义，西南缘的平凉组/乌拉力克组和背锅山组是下古生界最重要和有效的烃源岩层。

Characteristics of carbon and oxygen isotopes of the Ordovician carbonate rocks in Ordos and their implication

The Ordos has gigantically thick Paleozoic marine strata.The carbon and oxygen isotopes of the Ordovician carbonate rocks in the Ordos are less modified by post-depositional diagenesis,and thus provide helpful insights into the original ocean.The δ13C values of the Ordovician carbonate rocks in the Ordos range from -7。30‰ to 2。26‰ with a mean value of -0。30‰ and the δ18O varies between -13.14‰ and -1.94‰,averaging at -6.38‰,which indicate the carbon and oxygen isotopes of the Ordovician in the Ordos are comparable to those of the global Ordovician.Spatially,the δ13C shows a decreasing trend,indicating shoaling in water-depth from the western margin of the Ordos,to the southern margin and then to the eastern margin.Temporally,the δ13C gradually increases,showing that sea level rose from the Early to the Late Ordovician,with a significant positive excursion between the Middle and Late Ordovician that is indicated by increase in mean δ13C value from -0。36‰ in the Majiagou Formation to 0.15‰ in the Pingliang Formation and then to 0.68‰ in the Beiguoshan Formation.Sea level change suggested by carbon isotope distribution is consistent with the evolution of sedimentary facies.The positive excursion of carbon isotopes in the Middle and Late Ordovician in the Ordos indicates relative high productivity and organic carbon burial,which is of important petroleum geological significance,reflecting the Pingliang Formation,Wulalike Formation and Beiguoshan Formation in the southwestern margin as the most important and efficient hydrocarbon source rocks of the Lower Paleozoic in the Ordos.