The dinoflagellate cyst Subtilisphaera from the Eocene of the Qaidam Basin, northwest China, and its implications for hydrocarbon exploration

Abstract

Although an abundance of 4-methyl-sterane and dinosterane thought to derive from dinoflagellates, has been detected in the Palaeogene source rocks and crude oils in the Qaidam Basin, China, no unequivocal dinoflagellate cysts have been found previously in Cretaceous–Palaeogene terrestrial deposits in this basin, including the Xiaganchaigou Formation. This study reports on a new discovery using detailed analysis of a dinoflagellate cyst assemblage, composed solely of Subtilisphaera, from the Xiaganchaigou Formation on the north margin of the Qaidam Basin. The biomarkers 4-methylsterane and dinosterane in the corresponding fossil-bearing beds have also been detected, further validating the dependability of dinoflagellate cysts. These fossils provide direct evidence of the dinoflagellate contribution to Palaeogene crude oil in the Qaidam Basin and fill a long-existing gap in oil-source correlation.