Paleoenvironment Reconstruction and Organic Matter Enrichment Mechanisms of the Middle to Late Triassic YanChang Formation at the Southern Edge of the Ordos Basin

Key words: 
• Carnian Pluvial Episode (CPE) /
• paleoenvironment /
• organic matter enrichment /
• biomarkers /
• Yanchang Formation /
• Ordos Basin

Abstract: [Objective] The Carnian Pluvial Episode ( CPE ) had a significant impact on the climate of the Late Triassic, and the warm and humid climate was conducive to the formation of fine-grained sediments and the enrichment of organic matter. Based on the study of CPE event during the sedimentary period of Yanchang Formation in Ordos Basin, the response characteristics of CPE event to the enrichment of thick organic-rich black shale in Chang 7 member are revealed. [Methods] In this paper, based on the comprehensive analysis of ZK903 drilling data, the characteristics of paleoclimate, paleoredox, paleosalinity and the source of organic matter in ancient lakes from Chang 7 to Chang 8 members of Yanchang Formation were discussed in depth with the data of biomarkers as the core, combined with various analytical methods such as clay minerals, trace elements and organic carbon isotopes. The organic matter enrichment model under the influence of CPE event in the study area was established. [Results] The vertical difference of sedimentary environment in Ordos Basin is obvious. The lithology of the Chang 8 to Chang 73 members in the study area changes from sandstone to black shale. The sedimentary period is significantly affected by CPE event, and the enrichment of organic matter gradually increases. The average TOC of Chang 73 member is 9.6 %. The changes of Sr / Cu and Sr / Rb values and clay mineral content indicate that the climate type in the study area changed from hot and dry to warm and humid under the influence of this event. The values of V/Cr, U/Th, Pr/Ph and Ts/(Ts+Tm) in terpane indicated that the redox environment changed significantly during the sedimentary period of Chang 73 member in the study area. Combined with Sr / Ba and gammacerane index (GI), it shows that under the influence of CPE events, the temperature and precipitation in the study area increased, the water surface of the lake increased, and the stability of the water body increased, which aggravated the stratification of the water body. The lake showed a fresh water reduction environment. The values of ∑C21-/∑C22+ and C27/C29(aaa)R in biomarkers increased significantly in Chang 73 member, indicating that the source of organic matter was dominated by low aquatic organisms such as algae. [Conclusions] The CPE event significantly affects the paleoclimate background of the study area and the sedimentary environment of the ancient lake water body. The development of fine-grained sediments in a stable stratified environment of water body also provides good productivity and preservation conditions for the enrichment of organic matter in this area, and promotes the deposition of organic-rich black shale, that is, oil shale.