The Origin of Erosional Valley of Southern Mangar Sag in Tarim Basin and Anal ysis about its Petroleum Geol ogy Significance

Key words: 
• Tarim basin /
• Erosional valley /
• Origin /
• Undcr current of deep ocean /
• Petroleum geology

Abstract: Tarim hasin, which is located in the northwest area of China, is the largest intracratonic petroliferous basin in China. There are strong energy seismic waves intercalated between T_g5;T_g5 seismic reflecting layers (the Middle and Upper Ordovician) in the most seismic sections of southern Mangar Sag in the basin. The abnormal reflected waves are present not only in north一south sections,but also in west一east ones. Closed net tracing correlation can be conducted in Mangar area with a size of about 30,000 square kilometers, indicating that these geological interfaces do exist underground.Based on the analysis of seismic reflection characteristics, geodynamic aspects of this basin,and the theory about under current of deoCearl,it is considered that the Lower Paleo-zoic eroxional valley of Mangar Sag in Tarim basin was caused by the erosion truncation of un-der current of deep ocean. Dnder current of deep ocean is another kind of current of deep wa-ier besides turhidity one. "hhc under current may erode most area of continental rise,and trun-Fate deep gullet and even much older sediments. In the large erosional valley,it may be de-posited various kinds of sediments,including a small scale volcanic rock. Wells Tazh ong 31and 33,which take anomaly body as the drilling object,penetrated the formation (T_g5 wave group) of a deep water sediment of dark gray shale with interhedded thin fine sand and silt,and thin (only 10 miters)andesite. Mangar sag is one oI the most important hydrocarbon一bearing depressions in the basin,and recent一year一exploration has proved that Cambrian and Ordovician are the most important source rocks in cast Tarim basin. So the crosional valley is located at perspective position near the oil source. It is possible to form lithologic deposits. Bi-tuminous sand and oil tract have been shown from siliirian to Llpper Ordovician in wells Tazhong 31 and 33.and a little dense oil has been got in well Tazhong 31.Above all,petroleum geological conditions of the erosional valley are extremely supprior and it may become an important ob.lect of oil and gas exploration in Tarim basin.