Mineralogy and Element Geochemistry of Oil Shales in the Lower Cretaceous Qingshankou Formation of the Southern Songliao Basin, Northeast China: Implications of Provenance, Tectonic Setting, and Paleoenvironment

Abstract

Songliao Basin is a large Mesozoic oil shale-bearing basin in the northeast of China, where multiple layers of oil shales and mudstones developed in the first member of the Qingshankou Formation (K 2 qn 1 ). Mineralogy and geochemical data were used to reveal the provenance and tectonic setting of the sediments; furthermore, the differences in the paleoenvironment between mudstones and oil shales as well as between different oil shale layers were revealed. Core observation shows that oil shales are light brown with horizontal bedding, while mudstones are dark gray with a massive structure. X-ray diffraction data revealed abundant clay minerals and a certain amount of syngenetic pyrite in the oil shales, indicating that oil shale forms in a reducing water environment. The element ratios revealed that the parent rocks were mostly felsic volcanic rocks, and the discriminant diagrams indicated that the tectonic setting was of a continental island arc. The paleoenvironment and the genetic mechanism of oil shales are revealed by geochemical data, and the oil shales and mudstones were deposited under warmhumid paleoclimate, salty brackish, and strong reducing water environments. The geochemical curves show four high and low change cycles in the column, which means that the oil shale layers have a higher organic matter content than the mudstone layers because of the higher primary productivity, more saline water environments, and stronger reducing conditions. From the lower layer 1 to the upper layer 4 of oil shales, the extent of the warm-humid climate gradually weakened, whereas the salinity of the water environment gradually increased.

Organic Matter Accumulation in the Youganwo Formation (Middle Eocene), Maoming Basin, South China: Constraints from Multiple Geochemical Proxies and Organic Petrology

“…In addition, intense paleoweathering can occur under warm and humid climates and further promote the input of terrigenous detrital minerals. 55 Figure 13f shows that with increasing 10*Ti/ K ratios, the TOC content first increases and then decreases. A low rate of terrigenous detrital input was probably conducive to OM accumulation in the JTOS, which is revealed by the reasonable positive correlations between TOC values and 10*Ti/K ratios (Figure 13f).…”

“…In addition, intense paleoweathering can occur under warm and humid climates and further promote the input of terrigenous detrital minerals Figure f shows that with increasing 10*Ti/K ratios, the TOC content first increases and then decreases.…”

Characteristics, Genesis, and Hydrocarbon Aspects of Lacustrine Siderite-Rich Marl (Early Jurassic): A Case Study in the Dachanggou Basin, Xinjiang, Northwest China

“…Another important factor controlling the development of siderite is the redox environment; generally, a weakly reducing environment is conducive to the formation of siderite, , while the contact zone between siltstone and oil shale represents a transition from strong to weak hydrodynamic forces, and the redox environment has changed from an oxygen-bearing environment of shallow lakes to an oxygen-deficient to a weakly reducing environment of semideep lakes. Ti, Zr, and Rb are often used to indicate terrigenous clastic input. − The siderite content in marl is significantly negatively correlated with these terrigenous clastic input indicators (Figure j–l), showing that low clastic input is conducive to the formation of siderite in marl. U and Mo are often used to determine the redox properties of lakes and oceans. − The U–Mo crossplot shows that the sedimentary environment of all the samples was an oxic–suboxic environment, indicating that the water body was active and lacked stratification (Figure ).…”

Paleoenvironmental Reconstruction and Organic Matter Accumulation of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

“…Identifying tectonic settings is significant as it can influence the elemental transport and ultimate distribution in sedimentary archives. , For example, organic matter-associated metals within a rifted basin tend to accumulate in the gentle-slope side because they have relatively favorable depositional conditions that can support the formation of reducing conditions and facilitate redox metal sequestrations. , Previous studies suggested that the stable elemental associations can be used to infer tectonic settings, owing to their relative stabilities during the transport process. − The ternary diagrams of Co–Th–Zr/10 and Sc–Th–Zr/10 can identify four tectonic settings: oceanic island arc, continental island arc, active continental margin, and passive continental margin Figure shows that the depositional setting in the Qiongzhusi Formation was a continental island arc.…”

“…Identifying tectonic settings is significant as it can influence the elemental transport and ultimate distribution in sedimentary archives. 59,60 For example, organic matter-associated metals within a rifted basin tend to accumulate in the gentleslope side because they have relatively favorable depositional conditions 61 that can support the formation of reducing conditions and facilitate redox metal sequestrations. 62,63 Previous studies suggested that the stable elemental associations can be used to infer tectonic settings, owing to their relative stabilities during the transport process.…”