Three-dimensional seismic analysis of Late Paleozoic coal-bearing series reflections in the Hangjinqi, North Ordos Basin, China

The study of sand bodies and coalbeds that formed during strong events is conducive to understand the relationship between source rocks and reservoirs. Two sets of Late Paleozoic coal-bearing sequences including the Late Carboniferous Taiyuan Formation and Early Permian Shanxi Formation, with an accumulated thickness of more than a half wavelength (90 m), were deposited in the Hangjinqi region on the northern margin of the Ordos Basin. These strata appear as two peaks and two troughs, three peaks and three troughs in the 3D seismic data. We analyzed this coal-bearing seismic response through a combination of well-data statistical analysis, the establishment of forward modeling, and the creation of crossplots. The coal-seam thicknesses of the Taiyuan and Shanxi Formations and the spacing between the two sets of coal indicate the major roles in the merging and bifurcation of events. The formation of three peaks and three troughs requires that the thicknesses of the two sets of coal are greater than 2 m. The spacing between the two sets of coal must also exceed 10 m with a detectable lateral extension, which would otherwise generate two peaks and two troughs. The two peaks and two troughs indicate a combined relationship between the thin sandstones at the bottom of the Shanxi Formation and the thick coalbeds in the Taiyuan and Shanxi Formations. The three peaks and three troughs suggest a combined relationship between the thick sandstones at the bottom of the Shanxi Formation and the thin coalbeds in the Taiyuan and Shanxi Formations. The drilling results clarified that the well situated at the two peaks and two troughs regions is accompanied by higher gas productivity, revealing that the coal-seam thickness has a significant influence on gas accumulation. The presence of two peaks and two troughs indicates favorability for natural gas exploration.