GEOCHEMICAL CHARACTERISTICS OF CRUDE OIL FROM YANCHANG AND YAN’AN FORMATIONS IN PENGYANG AREA OF ORDOS BASIN AND THEIR IMPLICATIONS FOR OIL-SOURCE

Abstract: Researches have been devoted to the uncertainties of sources of oil from the Yan’an and Yanchang Formations in the Pengyang area of the Ordos Basin. Using the methods of GC-MS, biomarkers are analyzed, upon which correlation of oil-sources is made for the crude oil samples from the Yanchang and Yan’an Formations with those from the Chang-7 high quality source rock in both the Pengyang area and the area within the basin. Results show that the crude oils produced from the Yanchang Formation and Yan’an Formation of the Pengyang area came from the west Tianhuan Depression and was formed under a deoxidized lake environment of freshwater and mildly brackish water. Biomarker characteristics suggest that the crude oils of Yanchang Formation and Yan’an Formation come from the same oil source, of which the parent materials are mainly from aquatic organisms and higher plant, and the crude oil is mature. Facts show that the maturity, parent materials and sedimentary environment of crude oil from the Pengyang area are similar with the Chang-7 source rock formed within the basin center. It is thus concluded that the crude oil is coming from the Chang-7 mudstone in the basin center, but not the local Chang-7 deposits in the Pengyang area.