Wavelet transform coupled with Fischer plots for sequence stratigraphy: A case study in the Linxing area, Ordos Basin, China

Abstract

Accurately dividing stratigraphic sequence using traditional data such as core and logging data is challenging due to the insufficient availability of paleontological fossils and rock data, particularly in Carboniferous-Permian strata of the Linxing area of Ordos Basin, dominated by lagoons, barrier islands, and tidal flat sedimentation. This drawback hinders the exploration and exploitation of hydrocarbon reservoirs. To address this issue, this study presents a combination of continuous wavelet transform (CWT) and Fischer plots, enabling the accurate identification of each level of sequence boundary and high-resolution sequence stratigraphic correlation. The optimal scale factors, 120, 60, and 20, are determined by applying different scale factors to gamma ray (GR) curves corresponding to long-term cycle, medium-term cycle, and short-term cycle divisions, respectively. The analysis of sequence characteristics of numerous core segments and the corresponding CWT results (i.e., wavelet coefficient curve and time-frequency energy spectrum) led to the identification of three main types of medium-term cycles (i.e., progradational, regressive, and aggradational) that were then applied to the entire well section of the study area. Moreover, the Fischer plots method was used to identify the long-term cycle based on the determination of medium-term cycles and the division of short-term cycles. This method provided intuitive identification of the relative changes in sea level position. Additionally, the characteristics of sea level changes were consistent with the results of the sea level analysis of rock and logging curves. Therefore, the combination of CWT and Fischer plots can effectively and comprehensively establish an isochronous stratigraphic framework for clastic rock hydrocarbon reservoirs, similar to this study.