Possible origin of 25-norhopanes in Jurassic organic-poor mudstones from the northern Qaidam Basin (NW China)

25-Norhopanes have been found for the first time in Jurassic organic-poor mudstones in the northern Qaidam Basin (northwestern China). The carbon number distribution in the m/z 177 mass chromatograms does not consist of the full suite from C28 to C34, although this is a well-known feature generally ascribed to 25-norhopanes in source rocks. The ranges of C29 25-norhopane/C29 30-norhopane and C29 25-norhopane/C30 hopane are 0.07-0.08 and 0.03-0.04, respectively. These values are within the recognized global background level for 25-norhopane concentrations in source rocks [Blanc, P.L., Connan, J., 1992. Origin and occurrence of 25-norhopanes: a statistical study. Organic Geochemistry 18, 813-828]. Here, we provide additional evidence for the existence of 25-norhopanes in mudstones/source rocks. They were not detected in organic-rich mudstones, implying that the accumulation or diagenesis of the organic-poor mudstones may have provided the right conditions for the formation of 25-norhopanes. The depositional setting is suggested to be anaerobic and distant from the lacustrine lakeshore, based on biomarker and petrological features and previous sedimentary environment studies. In such a nutrient limited environment with relatively low source organic matter, microbial reworking of biomass occurred - microbial consortia scavenged the existing hopanoids, contributing biomass and releasing nutrients to perform 25-norhopane-forming reactions.