Geochemical Characteristics and Genesis of Crude Oil from DongzhiZhengning Area of Ordos Basin

Key words: 
• Ordos Basin

Abstract: Ordos basin is a large inner continental sedimentary basin in China, with an area of about 37×104 km2. It contains abundant Mesozoic crude oil resources with an reserve estimation of up to 10×108 metric tons of oil from Mesozoic reservoirs. These crude oils are derived nonmarine source rocks, so that it has also been used as a typical model of a nonmarine sedimentary basin generating oil. It contains six structural units. Stratigraphic sequences involving Mesozoic petroleum geology principally are Upper Triassic Yanchang Formation and Lower Jurassic Yanan Formation, which are both hydrocarbon source rocks and reservoirs of Mesozoic oils and gases in Ordos basin. Dongzhi Zhengning area is a new Mesozoic petroleum exploration target in the Ordos basin. Understanding the genesis of oil from this area is very important for revealing the formation law of oil reservoir and guiding petroleum exploration and development in this area. However, the research on geochemistry of crude oils from DongzhiZhengning area has been not performed and genesis of the oil is still known little. In this paper, biomarker compounds in oils from the region and Mesozoic source rock from Ordos basin were systematically analyzed. The genesis of the oil was discussed by studying the geochemical characteristics of these biomarker compounds. Similar geochemical features of oils from DongzhiZhengning area suggest generation from a source with mixed terrgenous and algalbacterial organic matter. The biomarker distributions of the oils show that they were formed under a weak reducing and freshwater environment. Molecular maturity parameters indicate that the crude oils are mature. The results of oilsource rock correlation showed that the oil was mainly derived from Chang7 source rock. 