Progradational reflection from lacustrine Yanchang Formation in Ordos Basin, China
Li Huiqiong1, Pu Renhai1, Wang Daxing2,3, Zhao Yuhua2,3, Zhang Jie2,3, Zhao Deyong2,3
1. Department of Geology, Northwest University, Xi'an, Shaanxi 710069, China;
2. Research Institute of Exploration and Development, Changqing Oilfield Company Ltd., PetroChina, Xi'an, Shaanxi 710021, China;
3. National Engineering Laboratory for Exploration and Development of LowPermeability Oil & Gas Fields, Xi'an, Shaanxi 710018, China
Abstract:Numerous progradational reflections from the lacustrine Yanchang Formation in the Ordos Basin are identified on 2D seismic sections. These reflections suggest deltaic and gravity depositions that flowed into the lake respectively from Northeastern, Southwestern and Southeastern directions during the periods of Chang 7 to Chang 6, Chang 4+5 to Chang 2, Chang 7 to Chang 3 and Chang 6 to Chang 4+5. Each of the Progradational body consists of a few progradational lobes of high velocity sandstones and mudstones. Different progradational lobes are divided by low velocity shale with the thickness of 5 to 20 meters, which causes a high amplitude oblique event within the progradational body. A maximum water depth up to 228 m and the maximum bed form dip of 1.7 degree of the ancient lake can be deducted from the horizontal and vertical distances between the top lap point and down lap point of an oblique event. The southwest slope of the lake was steeper with 0.6-1.3 degrees and the northeast slope of the lake was relatively gentle with 0.5-0.6 degrees. Great amounts of log and core data of wells located on the progradational reflection sections indicate that the progradations occurred in a background of shallow to deep lakes and their interior sand bodies are made mainly from underwater distributary channels, mouth bars, gravity-flow channel sand body and so on. Different micro facies of sand bodies are of different dimensions and width/thickness ratios. The sandstones in the progradational reflections approximately occupy 30% to 60% of total stratigraphic thickness, and tend to be connected laterally each other. An individual sand body often causes a short axis of medium-weak-amplitude seismic facies. The progradational reflections are significant favorable reservoir facies and many large oil fields have been found within them.