Characteristic and Geological Implications of Major Elements and Rare Earth Elements of Triassic Chang 7 Oil Shale in Tongchuan City, Southern Ordos Basin (China)

Abstract

The study of the oil shale from Triassic Chang 7 oil layer in Ordos Basin is of great importance to oil and gas resources investigation. Based on systematic analyses of major elements and rare earth elements of twelve oil shale samples and grain size analyses of four sandstone samples from the Yishicun Profile in southern Ordos Basin, the elements characteristics and corresponding geological implications are discussed. The Al/Si and Si/(Si + Al + Fe) of oil shale samples are in small range, with the averages of 0.29 and 0.67, indicating that quartz is the main mineral and the oil shale deposits near terrigenous provenance. Rare earth elements of the oil shale illustrate the enrichment of light rare earth elements (LREEs) and deficit of heavy rare earth elements (HREEs). The Chondrite-and North American shale composite (NASC)-normalized distributing patterns manifested that the oil shale have been derived from the same terrigenous source and controlled by a similar sedimentary environment. The vertical variation of major elements and REEs show that the heterogeneity of oil shale samples is relatively low. By grain size analysis, the interlayer sandstone is the typical turbidite. The Chemical index of alteration (CIA) and (Fe + Mn)/Ti of the oil shale samples ranges in 65.02-78.09 and 6.60-25.82, respectively, indicating that during oil shale sedimentation, the paloeclimate is warm and humid with moderate chemical weathering and that there are obviously hydrothermal fliud activities. The correlation between δCe N and δEu N , total rare earth elements (ΣREE) and (Dy/Sm) N , implying that the diagenesis of oil shale is relatively low with middle diagenetic stage A period. The Ce anom of oil shale samples ranges from −0.094 to −0.049, suggesting that the redox condition of oil shale sedimentation is dominated by strong reducing condition, and the (La/Yb)n is from 1.3 to 2.1, manifesting the whole sedimentary rate of oil shale is relatively low. By the comparison with REEs distribution characteristics from surrounding potential provenance, the main provenances of Chang 7 sedimentation in southern Ordos Basin are from Yinshan Mountain and Qinling-Dabie Mountain.

The petrological characteristics and significance of organic-rich shale in the Chang 7 member of the Yanchang Formation, south margin of the Ordos basin, central China

“…This possibly indicates that the water depth of this region was Fig. 3 Occurrence of jarosite in the organic-rich shale in Chang 7 member a stripe jarosite in sample YSC-51-108 (plane polarized light), b stripe jarosite in sample YSC-51-108 (cross-polarized light), c ash interval in shale in sample YSC-22-1 (cross-polarized light), d ash interval in shale in sample YSC-22-1 (reflected light), e stripe and sphere jarosite (in light gray) in sample YSC-51-48 (backscattered electron), f cubic jarosite in crack in sample YSC-33-5 (secondary electrons) (backscattered electron), g cubic jarosite in the crack (secondary electrons), h cubic jarosite covers the feldspar (secondary electrons), i flaky-shaped jarosite in the pore (secondary electrons), j framboidal pyrite pseudocrystal remnant in sample YSC-51-42 (backscattered electron), k jarosite coat of organic matter in sample YSC-51-82 (backscattered electron), l framboidal pyrite pseudocrystal in sample YSC-29A-19 (backscattered electron) relatively shallow and the shale deposits were near a terrigenous province, which was also proved by the ratios of Al/Si and Si/(Si + Al + Fe) (Li et al 2018). Some authigenic feldspars (Fig.…”

“…The contents of phytane (Ph) and pristine (Pr) are usually related to the redox conditions during sedimentation, and the Pr/Ph ratio is a sensitive indicator of the redox conditions which range from 0.39 to 0.60, indicating reducing conditions (Li et al 2017). Additionally, the discrimination diagram of redox conditions (V/Cr and V/(V + Ni)) suggested a dysoxic-toanoxic bottom water environment (Li et al 2018). Accordingly, it is clear that the redox conditions in the water body have a positive impact on the enrichment of organic matter (Fig.…”

A Novel Static Correction Approach for Eliminating the Effect of Geophones—A Case Study in Coal Reservoirs, Ordos Basin, China

“…In recent years, many papers [17,32,[35][36][37][38][39] have been published about the Ordos Basin, China, and this area has become a hotspot. In this paper, field data are derived from the Daliuta Coal Mine (Figure 2) at Yulin city, Shanxi Province, China, which is located at 39 • 13 53"-39 • 21 32" N, 110 • 12 23"-110 • 22 54" E. The Daliuta Coal Mine lies in the north of the Loess Plateau and southeast of the Maowusu Desert, Ordos Basin, China.…”

Sedimentation Models and Development Mechanisms of Organic-Rich Shales of the Lower Carboniferous Dawuba Formation: A Case Study in the Yaziluo Rift Trough, South of Guizhou Province, Southern China

“… 5 , 13 , 14 Due to the different environments of sedimentation and tectonic evolution, the development mechanism of organic-rich shales in southern Guizhou is different from that of the Sichuan Basin, and therefore, the later exploration and exploitation patterns are also different. 15 , 16 In the past few decades, many scholars have demonstrated the applicability of using major elements, trace elements, and rare earth elements as effective indicators for studying sedimentary environments 17 − 19 and to determine mechanisms controlling the organic matter enrichment in black shales. 5 , 20 , 21 The sedimentary environments and lithological assemblage changed very rapidly vertically and horizontally, which caused disagreement about the sedimentary environments and patterns in the study area.…”