Early Cretaceous aquatic algae and algal laminae from the Jiuquan Basin, northwestern China: Evidence of the oil precursors for terrestrial source rocks

Abstract

Aquatic algae are the main biotic source of organics in both marine and terrestrial hydrocarbon source rocks. Unlike marine deposits, fossil evidence of algal remains in terrestrial source rocks is rare, especially in pre-Cenozoic rocks. However, abundant algae-related biomarkers are commonly found in terrestrial oil-bearing basins. We conducted a palynological study of the source rocks in the Lower Cretaceous Xiagou Formation in the Jiuquan Basin, northwestern China. The abundant planktonic algae comprise dinoflagellate cysts (Zhongyuandinium decorosum, Parabohaidina (Conicoidium) tuberculata and Palaeoperidinium sp.) and chlorophyta (Botryococcus sp. and Tetrastrum multifilorum). These algae are extremely transparent, which makes their identification difficult. However, they show high yellow fluorescence under blue light which increases the clarity of surface ornamentation. In addition, the Lower Cretaceous Xiagou Formation is characterized by fine laminated deposits. The lamination consists of light- and dark-colored laminae couplets with thickness of approximately 0.03–0.5 mm. The dark laminae are rich in algae and amorphous organic matter (AOM). Algal remains are densely distributed along the bedding surface and show a strong yellow color under blue fluorescence. The algal remains probably played an important role in oil-generating microlayers in the source rock. In contrast, the organic components in the light-colored laminae are dominated by phytoclasts. Additionally, the biomarkers 4-methylsterane and dinosterane have also been detected in fossil-bearing beds, providing further evidence that dinoflagellate cysts flourished in the Jiuquan Basin during the Early Cretaceous. Thus, these aquatic algae are not only the likely source of oil in the Lower Cretaceous source rocks in the Jiuquan Basin but also suggest that blooms of early dinoflagellate cysts occurred in continental interiors during the Early Cretaceous.