Abstract
Lacustrine shales in the third submember of the Chang7 (Chang73) of the Triassic Yanchang Formation have the highest oil and gas generation potential in the Ordos Basin, North China. To unravel factors governing organic enrichment within this submember, Rock-Eval pyrolysis, major and trace elemental analyses, and molecular composition of extractable organic matter were applied for redox condition, paleosalinity, dilution effect by terrestrial input, paleoproductivity, and paleoclimate condition investigation. The total organic carbon (TOC) contents of the Chang73 organic-rich lacustrine shales show a tripartite feature and can be divided into the upper organic-rich section (UORS, average TOC 6.8 wt %), the middle organic-lean section (MOLS, average TOC 3.5 wt %), and the lower organic-rich section (LORS, average TOC 6.7 wt %). The variation of the productivity-related paleoclimate is likely the main driving force leading to the change of organic richness within the Chang73 submember. The MOLS was deposited under a relatively hot and arid climate (high Sr/Cu but low Rb/Sr values) with lower paleoproductivity (low P org/Ti and P org values). Additionally, clastic dilution may further reduce the TOC content to a certain extent in the MOLS. The UORS and LORS, however, were deposited under a warm and humid climate, which leads to enhancement of chemical weathering (high Ln(Al2O3/Na2O) values), increased nutrient input, and elevated paleoproductivity. Furthermore, paleoproductivity of UORS and LORS was further boosted by additional key nutrients, such as Fe and P2O5, provided by syn-depositional volcanic ash. Both paleoredox (U/Th, C org/P, and Pr/Ph) and paleosalinity (Sr/Ba, gammacerane index) proxies suggest no noteworthy variation of redox and salinity conditions throughout the Chang73 interval.