Sedimentary Evolution Characteristics of Fine-Grained Lithofacies under the High-Resolution Isochronous Shelf System: Insights from the Wufeng-Longmaxi Shales in the Sichuan Basin

Guanping Wang,Wei Du,Zhijun Jin,Hongliang Wang,Jing Wu,Tong Zhu,Zongquan Hu,Guangxiang Liu,Zhongwei Wu

doi:10.2113/2021/6628867

Abstract

Abstract The deposition and evolution of fine-grained sediments is a hot topic in fine-grained sedimentary rock studies and is important for accurately evaluating shale gas sweet spots. In this paper, the fine-grained deposition and evolution characteristics of the Wufeng-Longmaxi shales, major targets for Chinese shale gas exploration, were studied by using core observations, thin section analyses, scanning electron microscopy, geochemical analysis, and fossil identification. This work accurately identified six typical lithofacies; among them, the organic matter-rich siliceous shale facies (OMRSSF), the high-organic matter siliceous argillaceous shale facies (HOMSASF), and the medium-high organic matter low calcareous siliceous shale facies (M-HOMLCSASF) are favorable facies for shale gas exploration. The high-resolution isochronous unit in the shelf fine-grained sedimentary system was established, and the differential evolution of lithofacies in the system tract was discussed. The lithofacies deposition and differentiation in the transgressive system tract were controlled by the transgressive scale and tectonics under increasingly shallow water conditions. The lithofacies deposition and differentiation in the regressive system tract were controlled by tectonics and the preexisting lithofacies. The lithofacies in the regressive system tract had more frequent facies transitions and greater differentiation than those in the transgressive system tract, and they exhibited significant spatiotemporal inheritance. Sequential differential sedimentary sequences and symmetric differential sedimentary sequences were distinguished in the continental shelf sedimentary system. The lithofacies depocenters and subsidence centers were consistent in the transgressive system tract, while the tectonically active paleocontinent was important in the regression system tract. This study is of great significance for further high-resolution exploration of marine shale and improvement of the theory of shelf fine-grained sedimentary systems.

Highlights

With the successful exploitation of gas in shale units in North America, such as the Barnet, Marcellus, and Woodford shales, and in China, including the Wufeng-Longmaxi shales [1,2,3], marine shale has attracted the attention of international sedimentologists
A total of 6 typical lithofacies were identified in the study area (Figure 2): organic matter (OM)-rich siliceous shale facies (OMRSSF), high-OM siliceous argillaceous shale facies (HOMSASF), medium-high-OM low calcareous siliceous shale facies (M-HOMLCSASF), low-medium-OM argillaceous shale facies (L-MOMASF), low-OM silty clay shale facies (LOMSCSF), and low-OM shelly facies (LOMSF)
Macro-microscale features show that the organic matter-rich siliceous shale facies (OMRSSF), high-organic matter siliceous argillaceous shale facies (HOMSASF), and MHOMLCSASF are favorable facies for shale gas exploration

Summary

Introduction

With the successful exploitation of gas in shale units in North America, such as the Barnet, Marcellus, and Woodford shales, and in China, including the Wufeng-Longmaxi shales [1,2,3], marine shale has attracted the attention of international sedimentologists. Marine shale is a type of fine-grained sediment with small-scale cyclic characteristics. Lithosphere and strong heterogeneity, and accurately understanding and predicting the lateral and vertical sedimentary characteristics of organic-rich marine shale changes have become key scientific issues [4,5,6,7,8]. An increasing number of reports indicate that the establishment of a sequence stratigraphic framework and the evolution of sedimentary facies are valuable tools for the evaluation of these successions [9,10,11]. Sequence stratigraphic concepts are valuable tools for establishing the highresolution temporal framework of strata, but accurately identifying sequence boundaries is a difficult topic, especially in fine-grained sediments [12]. Due to the lack of a systematic classification basis in fine-grained sediments, the identification of sequence boundaries is still controversial [9, 13,14,15]

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Conclusion