Factors Controlling the Lower Radioactivity and Its Relation with Higher Organic Matter Content for Middle Jurassic Oil Shale in Yuqia Depression, Northern Qaidam Basin, China: Evidence from Organic and Inorganic Geochemistry

Abstract

Organic-rich oil shale with unusual lower radioactivity (expressed by GR) was found in Member 7 of Dameigou Formation, middle Jurassic (J 2 d 7 ) in Yuqia depression of northern Qaidam Basin, China. In order to systematically and contrastively investigate the factors controlling the lower GR and its relation with higher organic matter (OM) content (expressed by total organic carbon, abbr. TOC), organic and inorganic geochemical analyses were performed on samples consisting of oil shale and the underlying conformable contact dark shale from Well YQ-1Y. Our study shows that GR of J 2 d 7 oil shale is mainly derived from uranium and thorium. Compared with dark shale, oil shale is characterized by higher OY and TOC, lower GR, and clay mineral content. During oil shale deposition, the paleoclimate was relatively arid, indicated by a decreased C value and siderite content as well as an increased carbonate content and Classopollis. Under such paleoclimate conditions, sedimentary water became more anoxic, suggested by higher V/(V + Ni), pyrite content and lower pristane/phytane (Pr/Ph). From oil shale to dark shale deposition, according to analyses of Al 2 O 3 /TiO 2 , TiO 2 versus Zr, La/Sc versus Th/Co, La/Th versus Hf, and La−Th−Sc, the felsic igneous rock could always be deduced as the parent rock of provenance; however, the increasing arid paleoclimate resulted in weakened chemical weathering of provenance, inferred by relatively low chemical index of alteration, chemical index of weathering, and plagioclase index of alteration corresponding to the input degree of radioactive materials and other terrigenous detrital materials (TDMs), evidenced by Ti and Al contents and terrigenous (%). Meanwhile, the relatively high P/Ti and Ba/Al both indicated increased primary paleoproductivity. Together with the maximum flooding stage of oil shale deposition, the relatively low radioactivity tends to be associated with the inhibited input of clay minerals and radioactive materials, largely caused by increasing arid paleoclimate. The accompanying decreased TDM benefited primary paleoproductivity and anoxic conditions; their combined influence could induce sapropelic OM accumulation.

Natural Radioactivity in Soil and Bitumen in Al-Marj Spring and Abu-Jir Village, Anbar, Western Iraq

“…This can illustrate by the influence the oxidation state which yields to chelate of uranium thereafter establish disruption of radioactive equilibrium of U-series then tendency in favor of radium (Al-Atia and Mehdi, 2005). The specific activity of 40 K is attributed to clay minerals content in samples, especially illite which has a strong ability to adsorption with radioactive elements (Guo et al, 2021), that is reflected in the activity of 40 K of sample (4M-S) which associated with high clay mineral abundance comparing with other samples, this is show obviously in Table 3 and Table 5.…”

“…There is a direct relationship between terrestrial gamma radiation and the concentration of radionuclides in soil (Ademola et al, 2014). These radioactive elements are favorably bonded to clay minerals and organic matter (OM) by adsorption, but in varies ability ranging from strong bonded in montmorillonite and illite to weak in kaolinite and chlorite, and less bonded to OM (Guo et al, 2021).…”

Paleoenvironmental Settings of the Soma Coal Basin (Turkey): Insights from Maceral Data, Biomarker, and Carbon Isotopic Composition

“… Ternary plot of steranes showing depositional facies. (adapted with permission from ref ( 89 ) . Copyright 2021 ACS., 89 , 90 ) …”

A Comparative Study of Different Quality Oil Shales Developed in the Middle Jurassic Shimengou Formation, Yuqia Area, Northern Qaidam Basin, China

“…Laminations are highly developed, and large amounts of carbonate stripes, siderite concretions, ostracods, and lamelibranch fossils are present in the oil shales. Based on these findings, a shallow-to-deep lake has been deemed the main depositional environment of the oil shales in the region [23][24][25]27,[29][30][31][32].…”

“…Oil shale is typically defined by a minimum oil yield of 3.5 wt.% and a maximum ash yield of 40% and can be subdivided according to the oil yield into three groups: low-quality oil shale (oil yield of 3.5-5.0%), medium-quality oil shale (5.0-10.0%), and high-quality oil shale (10.0% and above) [11,[19][20][21][22]. Large amounts of oil shales have been found in the Middle Jurassic Mudstone Member of the Shimengou Formation in the northern Qaidam Basin, where they are widely distributed with a large thickness but of different qualities [23][24][25][26]. Their average oil yield is 6.2 wt.%, with a maximum value of 11.5 wt.%.…”