Thickness Variation Characteristics of Tidal Rhythmites—An Example From the Pinghu Formation, Xihu Sag, East China Sea Shelf Basin

Siqi Liu,Rui Zhu,Changmin Zhang,Jinchi Li,Zeyu Wang

doi:10.3389/feart.2022.698061

Abstract

The Pinghu Formation of the Xihu Sag in the East China Sea shelf basin is influenced by tidal processes, but few studies have focused on its tidal rhythmites. Through detailed observation and description of cores, this article studies the periodicity of the tidal rhythmites of well A-11 by using the grayscale measurement of digital images based on spectral analysis and wavelet transform. According to the statistical data series of millimeter lamination thickness, the sandy lamina thickness, the argillaceous lamina thickness, and the couplet thickness were quantitatively compared and analyzed, to support the interpretation of the main controlling factors of different thickness cycles. The periodicity of sandy laminae, argillaceous laminae, and couplet thickness have distinct differences, which are interpreted to be due to differences in the nature and magnitude of tidal current cycles at the time of deposition. The high-frequency signal represents event deposition, the middle-frequency signal represents tidal current deposition, and the low-frequency signal highlights changes in sedimentary facies. Therefore, the change in the couplet thickness is controlled by event deposition, tidal current deposition, and change of the depositional environment. Our approach to the study of thickness variations in tidal rhythmites supports the reconstruction of the tidal sedimentary environment in the paleostratigraphic sequence.

Highlights

Tidal depositional systems are the product of sedimentary environments dominated or influenced by the effects of tides
Through frequency division processing on the data on variation in the thickness of sandy laminae, argillaceous laminae, and couplets, the frequency division signal was extracted and compared with the sedimentary facies divided by core observations, and the corresponding relationship between the frequency signal and sedimentary facies was summarized
The lamina thickness presents a change cycle. This cycle is consistent with the maximum cycle of the argillaceous lamina thickness in well A-11 (Figure 7B), indicating that the formation of this peak is influenced by the tidal current cycle

Summary

INTRODUCTION

Tidal depositional systems are the product of sedimentary environments dominated or influenced by the effects of tides. The East China Sea shelf basin is an important petroliferous basin in China, and the Eocene Pinghu Formation is one of the main target strata for oil and gas exploration in Xihu Sag. Before and after the deposition of the Pinghu Formation, the Yandang, Oujiang, and Yuquan tectonic movements occurred, resulting in a complicated paleogeomorphology and major changes in sedimentary environments. Zhao Lina et al (Zhao, 2007) proposed that the Pinghu Formation in the Pinghu structural zone of the Xihu Sag could be divided into two main sedimentary facies types: the tidal sedimentary facies and tidal delta sedimentary facies. Based on the detailed observation and description of the cores of well A-11 of the Pinghu Formation in the Xihu Sag, East China Sea Shelf Basin, a large number of tidal sedimentary structures have been identified. The analysis of the lamina’s thickness in the core section of well A-11 in the Pinghu Formation (3,856.6~3,865.89 m), which included the measurement of the thickness of the layer, frequency spectrum analysis, and wavelet analysis, was conducted in the Yangtze University laboratory

Methodology

RESULT

DISCUSSION

CONCLUSION

DATA AVAILABILITY STATEMENT