Hydrocarbon potential, organisms, and depositional environments of the Lower Cambrian shales along the southern margin of the Ordos basin, Northern China

Abstract

The dark shales of the Lower Cambrian Dongpo Formation, along the southern margins of the Ordos Basin, were researched based on biomarkers and carbon isotope analyses of outcrop samples to reveal their hydrocarbon potential, organism composition, and depositional environment. Similar to the Yangtze and Tarim blocks, the Lower Cambrian in the southern margin of Ordos Basin, which is a part of North China block, also deposited high abundance marine source rocks. The shales displayed good to excellent total organic carbon (TOC) content (up to 13.75%). The crossplot of phytane/nC18 vs. pristane/nC17 and organic carbon isotopes indicate the kerogen to be primarily type I-II. The low Hydrogen index values, and high pyrobitumen reflectance (>3.0%) indicate that the Dongpo shales are currently in over-mature stage. The Dongpo shales thus exhibit high hydrocarbon generation potential and might have produced and discharged massive amounts of hydrocarbons during their geological history. The G/C30H ratios (0.17–0.29) and extended tricyclic terpanes Ratio (ETR) values (0.54–0.61) suggest sedimentary environments of stratified water and relatively moderate salinity. In addition, the Dongpo shales display low Pr/Ph (<0.63), low C30*/C29Ts (<0.44), high C35 22S/C34 22 S (>0.47), and relatively abundant elemental sulfur. These parameters indicate that Dongpo shales were formed in anoxic waters. The abundant steranes and hopanes in the Dongpo shales suggest the diversity of organisms, including eukaryotes and prokaryotes. The composition of eukaryotes (planktonic algae) is consistent with that of the Yangtze and Tarim blocks, but there are obvious differences with Oman, reflecting the heterogeneity in the global distribution of eukaryotes during the Early Cambrian. In addition, compared with the Yangtze and Tarim, the relatively contribution of prokaryote to Dongpo shale is low, which may also be an important reason for the obvious enriched of 13C in kerogen. The positive relationship between TOC and Pr/Ph and C27 St (%) suggests that high primary productivity in the paleoenvironment probably attributed to red algae. The Pr/Ph, C35/C34 values and others indexes are related to anoxic conditions, which is good for preserving deposited organic matter.