Abstract

Types of organic matter and mineral associations and microstructures of shales can reflect the depositional mechanism and sedimentary environment. Therefore, analysis of organic matter and mineral associations is a prerequisite for research on fine-grained sedimentary rocks. Shales from the Eocene Shahejie Formation in the Dongying Sag of China were selected to classify their lithofacies and to investigate the characteristics of their organic matter and mineral associations. This analysis identified six lithofacies (e.g., laminated shales and massive mudstones); in all the lithofacies, clay minerals exhibit a positive correlation with detrital minerals, thus indicating that they were derived from the same source. The comprehensive analysis of mineral and organic matter associations reveals that detrital minerals were deposited with low-hydrogen index (HI) OM. The deposition of detrital minerals was mainly a physical process. Clay minerals can undergo deposition in one of two ways due to their surface charge: they can either aggregate with high-HI OM via chemical deposition, thus forming organic-rich laminae, or they can be deposited together with low-HI OM via physical deposition, thus forming clay-rich laminae or a massive matrix. Carbonate minerals, which often coexist with high-HI OM, are biological sediments. The analysis of the sedimentary characteristics of these organic matter and mineral associations indicates that the sedimentary processes differ between various lithofacies: e.g., the discontinuous laminated shale represents the product of biophysical processes. Differences in depositional mechanisms are also present in each sub-member. Therefore, it is important to analyze the properties of minerals and organic matter, as well as their associations, to more deeply understand the classification of lithofacies and the depositional processes of shales and mudstones.

Highlights

  • Shales and mudstones are typical fine-grained sedimentary rocks

  • This study aims to clarify the types of organic matter and mineral associations and their deposition mechanisms in the Dongying Sag

  • Microscopic analysis reveals that the shales and mudstones in the Shahejie Formation in the Dongying Sag exhibit various types of microstructures

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Summary

Introduction

Shales and mudstones are typical fine-grained sedimentary rocks. Studies of the origin of shales and mudstones have become the foundation of research on reconstructing the depositional models for lacustrine sedimentary rocks [1]. Minerals 2018, 8, 222 include carbonates, clay minerals and other detrital minerals (mainly quartz and feldspar), and the OM includes structureless OM and structured OM. These inorganic materials and OM vary in their chemical and physical properties, which are influenced by the physical, chemical and biological processes that occur during sedimentation; during these processes, a series of organic-mineral association types are formed [9]. The compositions of clay minerals and OM are more complex than those of detrital minerals and terrestrial plant debris All of these data indicate that minerals and OM have different relationships

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