Carbon isotope evidence for Ordovician marine hydrocarbon source rocks in Ordos Basin, North China

Ordos Basin is the second largest sedimentary basin in China with gigantically thick Paleozoic marine sedimentary strata. Geochemical analyses show that the carbon isotope compositions of the Ordovician are less influenced by postdepositional diagenesis and basically accordant with that of the global Ordovician. PC values range from -7.30 parts per thousand to 2.26 parts per thousand with a mean value of -0.30 parts per thousand. Temporally, delta(13)C values are gradually increased from the Early to Late Ordovician with a significant positive excursion between the Middle and Late Ordovician, indicated by increase of the mean value from -0.36 parts per thousand in the Majiagou Formation to 0.15 parts per thousand in the Pingliang Formation and up to 0.68 parts per thousand in the Beiguoshan Formation. There is a good relationship between carbon isotopes and organic carbon contents in the Ordos Basin. The positive carbon isotope excursion indicates higher productivity and organic carbon burial, which can consider as a geochemical index for generation of hydrocarbon source rocks. Based on analysis of carbon isotope and sedimentary environment, it is suggested that the central-eastern part of Ordos Basin is unlikely to form a large-scale efficient hydrocarbon source rocks. The Upper Ordovician Pingliang Formation, Wulalike Formation and Beiguoshan Formation in the southwestern margin are the most important and efficient hydrocarbon source rocks in the Lower Paleozoic in the Ordos Basin. In addition, the Middle Ordovician Kelimoli Formation in the western margin is another important source rocks, and the Fengfeng Formation in the southern margin also has a certain account of hydrocarbon generation potential. Viewing from the development of source-reservoir-cap assemblage, the southwestern margin of Ordos Basin has great exploration potential. Dingbian - Etuokeqi area and Linyou - Tongchuan area, located on the west and south side of Central Paleouplift, respectively, is favorable areas for gas exploration in the Lower Paleozoic in the Ordos Basin.