Sequence stratigraphic and sedimentary evolution of the Cambrian Xiannüdong Formation in the north central Sichuan Basin, SW China

Abstract

The high-frequency cycles seen in the carbonates of the Cambrian Xiannüdong Formation in the Sichuan Basin commonly exhibit a certain coupling relationship with the development of grain shoals; this influences the accuracy of reservoir predictions and the selection of favorable zones for hydrocarbon. MATLAB-based wavelet transform technology is employed to analyze the characteristics of the high-frequency sequences in the Xiannüdong Formation, establish a sequence stratigraphic framework, and clarify their vertical and horizontal relationships with the development of grain shoals. The results indicate that using the Dmey wavelet for continuous wavelet transform of gamma ray (GR) curves effectively reflects regional sedimentary cycles. In the Xiannüdong Formation, we identified two third-order sequences, five fourth-order sequences, and ten fifth-order sequences, all of which exhibit a strong correlation with the one-dimensional wavelet curves derived from wavelet transformations. In the sequence stratigraphic framework, early deposition of the Xiannüdong Formation briefly inherited transgressive processes from the Qiongzhusi Formation, and subsequently underwent a long and frequently fluctuating regressive phase. This study elucidates the development characteristics of grain shoals during marine regressions, and identifies lithology primarily as oolitic limestone, oolitic dolostone, doloarenite, silty oolitic limestone, and silty oolitic dolostone. Longitudinally, grain shoals are primarily distributed in the SQ1², SQ2¹, and SQ2² intervals, and are characterized by the interbedded development of thin and thick layers. They form predominantly during the regressive phase of fourth-order sequences. Planarly, they exhibit a belt-like distribution in the southwest–northeast direction. These findings provide novel insights for conducting high-frequency sequence stratigraphy studies utilizing logging data. They also possess practical implications for constructing high-precision sequence stratigraphic frameworks as well as for predicting the distribution of grain shoals within the study area.