Geochemical characteristics and the organic matter enrichment of the Upper Ordovician Tanjianshan Group, Qaidam Basin, China

Organic matter enrichment and preservation are significant in the formation of source rocks. The Lower Paleozoic source rocks of the Qaidam Basin have not been extensively studied. We collected outcrop samples in order to examine the sedimentary environment, evolution, and abundance of the organic matter in the “a" member of the Upper Ordovician Tanjianshan Group from the northern margin of the Qaidam Basin. We applied detailed geochemical and mineralogical techniques (including X-ray diffraction, X-ray fluorescence spectrometry, Rock-Eval pyrolysis and gas chromatography-mass spectrometry) to assess its source rock potential, paleoenvironmental setting, analyze the main controlling factors associated with enrichment of organic matter and present an organic matter enrichment model. The results show that the total organic carbon content in the “a" member is elevated, with an average value of 1.75 wt%. Additionally, the organic matter has reached a mature-overmature stage (Requ>1.8%), and the kerogen was mainly type Ⅱ1. The ratios of Sr/Cu, Rb/Sr, Cvalue and CIA reflect hot-arid paleoclimatic conditions during the deposition of the mudstone. Paleoredox indicators V/(V + Ni), δU, Ceanomaly, and Ce/La reveal the mudstones formed in an anoxic environment. Paleowater depth indicators Mn/Fe and Mn/Ti show that the depth of the sedimentary water was shallower than 250 m. Meanwhile, the Mo/TOC value and the U–Mo co-variation model indicate that it was deposited under strongly restricted conditions. The apparent hot-arid climatic conditions were not conducive for the reproduction of algae, which limited paleoproductivity. The formation of strong restricted water conditions and a stratified water column were caused by the change of paleowater depth that positively affected the organic matter accumulation. The TOC content has a good correlation with paleoredox conditions, but does not correlate with the paleo-bioproductivity index P/Ti and Ba/Al. Therefore, the present research suggests the formation of these rocks was mainly based on preservation conditions, resulting from the restricted water setting, and paleoredox conditions, which played a significant role in the organic matter enrichment.