Alluvial Fan in Paleogene Shahejie Formation, Dongpu Faulted-basin

Key words: 
• alluvial fan /
• sedimentary cycles /
• the No.2 member of Shahejie Formation /
• faulted lacustrine basin /
• Dongpu depression

Abstract: The No.2 member of Shahejie Formation(Paleogene) is one of the major target beds in Dongpu depression. An investigation based on seismic, well-logging and core data indicates that the member consists of alluvial fan sediments in the southern west sag. The sediments are mostly brown-grey brown finer lithic arkose and coarse siltstone, interbedded with brown mudstone,with medium-low compositional and textural maturation.Some sedimentary structures, such as graded bedding, small-scale cross bedding, ripple lamination and horizontal bedding caused by density and traction flow deposition,and such as mud crack,burrow,swash mark and curty bedding caused by exposure and others, are seen in cores,and scatters are distributed along a line parallel to C=M line in CM plot.The features show that the sediments are deposited under the influence of density and traction flow.The study area is combined into a depositional unit with the West Slope because of a decrease of the boundary faults action in the depositional age of the member, resulting in finer-grained front middle-fan deposition in the study area and coarse-grained proximal fan in the West Slope of Dongpu depression. This is why the alluvial fan sediments of Es2 member have finer grain-size in the study area. In seismic profiles and well-logging curves, three sedimentary cycles are recognized in the member, showing an fining-upward trend. The cycles are correlated between wells in the area, indicating the control of tectonics and weather.