Geochemical characteristics, depositional environment, and controlling factors of Lower Cretaceous shales in Chagan Sag, Yingen‐Ejinaqi Basin

Abstract

The Lower Cretaceous shales of Chagan Sag, Yingen‐Ejinaqi Basin, are considered as the source rocks of hydrocarbons discovered in the sag. Borehole shales from the Lower Cretaceous were subjected to comprehensive geochemical study to evaluate the quality of source rock, to reconstruct the paleodepositional environment and to determine the controlling factors of organic matter enrichment. Content of present‐day total organic carbon, and hydrogen index, averaging 1.4% and 259.2 mg hydrocarbon/g total organic carbon, respectively reveal poor to fair quality of organic matter. Average value of vitrinite reflectance (Ro) of 1.0% indicates mature organic matter. Maceral composition and biomarker analysis provide evidence for a mixed aquatic and terrigeous organic matter input. Based on Sr/Cu, Sr/Ba, and gammacerane index, we propose that organic matter was deposited in closed hypersaline under hot and arid climate conditions. The ratios of V/(V + Ni), and pristane/phytane (Pr/Ph), as well as the presence of β‐carotene suggest a stratified water column with anoxic bottom water condition. Low to moderate productivity as a result of deficient nutrient supply, the excellent organic matter preservation condition under stratified anoxic water column and high sedimentary rate controlled the quality of source rock. In addition, influence of factors such as content of clay minerals on organic matter enrichment cannot be ignored.