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/nCis vs. pristane/rtCi7 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/C_(30)H 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 C_(30)~*/C_(29)Ts (<0.44), high C_(35) 22S/C_(34) 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 pro-karyotes. 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 ~(13)C in kerogen. The positive relationship between TOC and Pr/Ph and C_(27) St (%) suggests that high primary productivity in the paleoenvironment probably attributed to red algae. The Pr/Ph, C_(35)/C_(34) values and others indexes are related to anoxic conditions, which is good for preserving deposited organic matter.
NSTL