地质科技通报 (May 2022)
Discussion on characteristics of mudstone and shale
Abstract
The types of fine-grained sedimentary rocks are complicated because their definition is based on the concept of texture grain size, which lacks effective constraints on mineral composition; In addition, different scholars have described the fabric phenomena at different scales, which altogether mades fine-grained sedimentary rocks lack of scientific and systematic classification. To analyse distinguish the characteristics of mudstones and shales, a large number of investigations and surveys were conducted in the textbooks of sedimentology and related literature at home and abroad. It is found showing that since the conception of fine-grained sediments (size less than 62.5 μm) appeared in the 1930s, it was accepted widely by the researchers, and based on particle size, most scholars generally appreciated it that the fine-grained sediments could further be divided into clay grains (size less than 3.9 μm) and silt grains (size between 3.9 μm and 62.5 μm). However, domestically and internationally, the particle size of fine-grained sediment "mud" (mud grains) was divided differently. Generally, scholars in Europe and America defined the mud as a particle size less than 62.5μm, including clay and silt grains. In our country, the particle size of mud corresponded to that of clay grains, which was less than 3.9μm, mainly following the former Soviet Union′s sedimentary scheme, since the 1950s. This divergence is also the fundamental reason for the confusion of concepts. In general, the consolidated mudstone has the same grain size structure and composition as shale, but does not have the "lamination" or "fissile" structural characteristics of shale. Dialectically speaking, "lamination" emphasizes the stratigraphic vertical differences formed by sedimentary processes, while "fissile" refers to the mechanical weaknesses formed by weathering during diagenetic processes. In terms of the mineral composition of mudstone and shale, there are fine-grained mixed sedimentation effects of quartz and feldspar siliceous minerals, clay minerals and carbonate minerals. Therefore, it is necessary to comprehensively classify fine-grained sediments into lithologies and lithofacies based on sedimentary genesis, combined with structural characteristics, mineral types, abundance of organic matter, and particle sources comprehensively. Practice has shown that the development and utilization of shale oil and gas require the integration of geology and engineering, and distinguishes the difference in characteristics of mudstone and shale is of great significance to the optimization of pay zones.
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