Geochemical characterization and depositional environment of source rocks of small fault basin in Erlian Basin, northern China

• Source rocks from small fault basins were analyzed with geochemical techniques.
• A fresh water condition and diverse conditions of small fault basins are indicated.
• Productivity has an obvious control on source rock formation in small fault basins.
• The differences in tectonic setting cause variations in depositional environment.

A combined investigation of molecular geochemistry, elements and carbon isotopic composition of carbonate distribution was carried out on the Early Cretaceous mudstones of the Tengger Formation in the Erlian Basin, northern China, to define source rock geochemical characteristics, depositional environment and tectonic setting of small faulted basin. The biomarkers of mudstone extracts are characterized by short to middle chain n-alkanes, high CPI values (>1.12), a wide range of Pr/Ph (0.4–1.94), relatively high concentrations of C29 sterane and the presence of relatively low tricyclic terpanes and low gammacerane index values (0.01–0.25), which is consistent with a relatively oxic to anoxic depositional environment of fresh to brackish water conditions, with a major contribution of terrigenous organic matter input and a low contribution of aquatic algal-bacterial organic matter. The oxic to anoxic depositional environment of small fault basins and a fresh and brackish water conditions are also confirmed by reduced sulphur/organic carbon ratio and element distribution. In these small fault basins, the cross-plots of δ13Ccarb and TOC show that productivity has an obvious control on source rock development. The differences in tectonic settings cause variations in productivity, redox conditions and terrigenous organic matter input, which account for the distinctly different geochemical characteristics and depositional environment of source rocks in small fault basins.