Guo Wang, Chen Gang, Li Yuhong, Li Yonghong, Zhang Yunpeng, Zhou Junlin, Han Wei, Xu Xuemin, Ma Yao, Dang Hongliang
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China.
Xi'an Center of China Geological Survey, Xi'an 710054, China.
ACS Omega. 2021 Mar 8;6(11):7360-7373. doi: 10.1021/acsomega.0c05618. eCollection 2021 Mar 23.
Organic-rich oil shale with unusual lower radioactivity (expressed by GR) was found in Member 7 of Dameigou Formation, middle Jurassic (Jd) 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 Jd 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 . 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 AlO/TiO, TiO 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.
在中国柴达木盆地北部鱼卡凹陷中侏罗统大煤沟组第7段（Jd）发现了具有异常低放射性（由自然伽马GR表示）的富有机质油页岩。为了系统地、对比性地研究控制低GR的因素及其与高有机质（OM）含量（由总有机碳表示，缩写为TOC）的关系，对YQ - 1Y井的油页岩及其下伏整合接触的暗色页岩样品进行了有机和无机地球化学分析。我们的研究表明，Jd油页岩的GR主要源自铀和钍。与暗色页岩相比，油页岩具有较高的OY和TOC、较低的GR以及粘土矿物含量。在油页岩沉积期间，古气候相对干旱，表现为C值和菱铁矿含量降低，碳酸盐含量增加以及......。在这种古气候条件下，沉积水变得更加缺氧，表现为较高的V/（V + Ni）、黄铁矿含量以及较低的姥鲛烷/植烷（Pr/Ph）。从油页岩到暗色页岩沉积，根据Al₂O₃/TiO₂、TiO₂与Zr、La/Sc与Th/Co、La/Th与Hf以及La - Th - Sc的分析，总是可以推断出长英质火成岩为物源母岩；然而，古气候干旱加剧导致物源化学风化减弱，这可由相对较低的蚀变化学指数、风化化学指数以及斜长石蚀变指数推断得出，这些指数与放射性物质和其他陆源碎屑物质（TDM）的输入程度相对应，由Ti和Al含量以及陆源物质（%）证明。同时，相对较高的P/Ti和Ba/Al均表明初级古生产力增加。连同油页岩沉积的最大洪泛期，相对较低的放射性往往与粘土矿物和放射性物质的输入受抑制有关，这主要是由古气候干旱加剧导致的。随之而来的TDM减少有利于初级古生产力和缺氧条件；它们的共同影响可促使腐泥型OM堆积。
