A new method for restoration of sedimentary paleogeomorphology based on lithofacies and geochemistry: A case study of the Qixia Formation in central Sichuan

Abstract

Objective The sedimentary palaeogeomorphology of the Permian Qixia Formation in Sichuan Basin has key influence over the distribution and evolution of carbonate reservoirs. Restoring and characterizing the sedimentary palaeogeomorphology of Gaoshiti area can gain further understanding of the distribution of favorable sedimentary facies and predict the distribution of high-quality reservoirs. Methods Based on the division of the third-order sequence, the high-resolution sequence stratigraphic framework was established by using geochemical proxies, petrological markers and logging markers. Two kinds of sedimentary microfacies, i.e. beach core and beach wing, were also identified. In addition, this study has calculated the difference of layer thicknesses in the sequence, and has identified different types of sedimentary sequences. A number of values including the beach core/beach wing value, Fe/Mn and MgO/Al2O3 ratios and other geochemical parameters were also calculated. The characteristics of the above parameters on the plane are analyzed and re-evaluated, which are all used to restore the palaeogeomorphic characteristics of the sedimentary period of the Qixia Formation in the study area. Results The result of research shows that the palaeogeomorphic characteristics of the 1st Member of Qixia are higher in the northwest and south, lower in the middle, and the palaeogeomorphic drop is obvious. Our new results show that the palaeogeomorphic highlands of the 2nd Member of Qixia are distributed in the middle of the study area, that the western palaeogeomorphology is low, and that the overall difference of palaeogeomorphology is small. Conclusion According to the results of ancient landform restoration, Gaoshiti-Moxi area forms a shallow water ring zone in the northeast-southwest direction where granular shoals are developed. Local micro-positive structures and granular shoals are developed in the south. However, the distribution of micro-positive structures is so limit that the extension range of granular shoals is limit. This study highlights that future exploration and development should focus on the development area of granular shoals in the northeast-southwest direction.

Keywords

• sedimentary paleogeomorphology
• qixia formation
• sedimentary sequences
• lithofacies
• geochemistry