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 δCeN and δEuN, 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 Ceanom 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.
Highlights
This paper presents a systematic research on major elements, rare earth elements characteristics, grain size analysis, and the geological implications of an oil shale section from the Chang 7 oil layer in southern Ordos Basin
Based on the above experimental results, the following conclusions can be obtained: (1) Al/Si and Si/(Si + Al + Fe) of oil shale samples from Chang 7 oil layer in southern Ordos Basin indicate that quartz is dominated in minerals and the oil shale deposits near terrigenous provenance
ΣLREE/ΣHREE of the shale samples varies in the range of 8.72–13.41, suggesting the enrichment of
Summary
With the decreasing conventional petroleum, oil shale is gradually becoming one of the most significant unconventional resources and has received unprecedented attention [1,2,3,4]. Studied the rare earth elements of fine grand sediments of every sub-unit of Yanchang Formation and found that the provenance of the whole Ordos Basin changed from the Chang 7 period [16]. Oil yield and total sulfur of the oil shale from Chang 7 Formation in southern Ordos Basin showed industrial quality of middle-grade (5% < oil yield ≤ 10%) with extra-low sulfur criterion (total sulfur ≤ 1.0%) [4,17,18,19]. In order to have a better understanding of oil shale from the Chang 7 oil layer, systematic major elements and rare earth elements characteristics, grain size analyses, and their geological implications of oil shale outcrops of the Yishicun Profile in Tongchuan City are discussed to reconstruct sedimentary background and tectonic setting for the further development of unconventional resources in China
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