Biotic precursors are key factors in evaluating hydrocarbon-source rocks. They are normally hard to determine due to the degradation of organisms, and the classification is more challenging since the incompleteness of fossil preservation. In the present study, spherical fossil microalgae are abundant and well-preserved in chert layers of the Fengcheng Formation (Kasimovian-Asselian) from the Well MY1 in the Mahu Sag, northwestern Junggar Basin, Xinjiang, Northwest China. These spheres are so far known to distribute in the northern part of the Mahu Sag, ranging from deep center to marginal area of the Mahu paleo-lake. They are very abundant and considered to be primary producer around the Well MY1 and nearby in the Mahu paleo-lake during the depositional period of the Fengcheng Formation. These spherical microalgae are likely the biotic precursors in the Fengcheng Formation source rock. Due to the absence of isolated specimens, detailed systematic study cannot be made on these spherical fossil microalgae and their classification is still uncertain. Specimens of these spherical fossil microalgae are extracted for the first time from the Well MY1. They show a great similarity to the palmelloid cells of the modern Dunaliella algae in terms of morphology, ecology, physiology, and biomarker compounds. These spherical fossil microalgae are herein described as anew genus and species, i.e., Archaeodunaliella junggarensis n. gen. n. sp. under the family Dunaliellaceae of the order Chlorophyta, which has an affinity with modern Dunaliella. (c) 2025 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS.