Application of seismic sedimentology to prediction of beach and bar sandbodies in gentle slope of lacustrine basin: A case study of the Lower Cretaceous in Chepaizi area, Junggar Basin, NW China

Abstract

The exploration of beach and bar sandstone reservoirs is a tricky issue. In this study, under the guidance of seismic sedimentary theory, the 90° phase shift technique, stratal slicing, and quantitative seismic sedimentology were used to identify and characterize the distribution of beach and bar sandbodies and reflect the evolutionary history in the Chepaizi area, Junggar Basin. Four third-order sequences in the Lower-Cretaceous and relevant systems tracts in the study area were studied using seismic sedimentology, and with the assistance of the comprehensive analysis of core, lithology and well-logging data, stratal slices were interpreted and calibrated, and the distribution and evolution of beach bar sandbodies were predicted. Based on the detailed and extensive analyses on sedimentary characteristics and seismic sedimentary quantitative results, this study established two typical beach and bar sedimentary models in the Chepaizi area, namely normal bedrock-beach bar and fan delta-beach bar. In early sedimentary period of SQ I, beach and bar sandbodies were reformed by coastal, wave and onshore currents, and developed around the lateral margin of the fan delta front. During sedimentary periods of SQ II to SQ IV, the northern provenance stopped supplying sediments, and the wave and coastal currents eroded the bedrock of the western Chepaizi uplift and transported sediments to the study area, forming beach and bar sandbodies parallel to or obliquely intersecting with coastal line. Key words: seismic sedimentology, phase shift, stratal slice, beach and bar, Chepaizi area, Junggar Basin