Seismic characteristics of erosion valley of the bottom of Yan'an Formation and the relationship between erosion valley and coal seam, sand body in Zhenjing area

After the deposition of Yanchang Formation in Ordos basin, the eroded valley has been widely developed. In 3D seismic isochronal(t₀) profiles of the bottom of Yan 10 in Zhenjing area in the southwestern margin of the basin, the main valley system extending northeast and northwest, the sub-valley system extending norteast and northwest can be recognized. The main valley is of asymmetrical "U" type, the sub-valley is of "U" and "V" type in the seismic profiles. Through statistical comparison of sandstone percentage in the strata of sandstone thickness, coal thickness, coal seam ratio in the strata in wells Yan 9 and Yan 10, in combination with the location relationship between wells and eroded valley, it was found that the coal seams in the eroded valley tends to thicken, but there is no significant change in the thickness of the sandstone. The coal thickness of Yan 9 and Yan 10 in the area was positively correlated with the strong amplitude of the corresponding layer, but there was no significant relationship between the plane form of the amplitude and eroded valley. It is indicated that the shape and scope of the marsh environment in the development of the Yan'an Formation was not completely limited within the paleovalley. The relationship between the thickness of the sandstone in eroded valley and the weak amplitude is not obvious, the southwest and northeast paleocurrent directions derived by predecessors and sand body distribution during Yan 10 period are basically consistent with the direction of the main valley in the region.