Discovery and identification of clay-sized tuffaceous rocks in Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, China

Abstract

This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, which are rich in black organic matter (including oil and asphaltene), in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks (including tuff, sedimentary tuff and tuffaceous sedimentary rock) with high hydrocarbon generation capacities. Thus, these rocks can be defined as clay-sized tuffaceous source rocks. Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member. Through the macroscopic observation of drill cores and outcrop profiles, microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis (including major, trace and rare earth elements), this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks, especially clay-sized tuffs, have the following characteristics. First, the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white, yellowish brown and purplish grey, and mixed colors occur in areas with strong bentonite lithification. Second, the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization, forming abundant flaky aluminosilicate minerals with directional arrangement. In thin sections under a polarizing microscope, the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green. Third, the rock samples plot in the igneous rock field in the TiO₂-SiO₂ cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams, indicating that the samples are derived from the same source. The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin, China.

Keywords

Ordos Basin / Chang 7 Member / clay-sized mudstone / clay-sized tuffaceous rock / lithologic identification

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