Constraints of Paleoclimate and Paleoenvironment on Organic Matter Enrichment in Lishui Sag,East China Sea Basin:Evidence from Element Geochemistry of Paleocene Mudstones

Lei Chuang; Ye Jiaren; Yin Shiyan; Wu Jingfu; Jing Yuqian

doi:10.3799/dqkx.2023.011

Abstract：In order to fully understand the hydrocarbon generation potential of the Paleocene Yueguifeng,Lingfeng and Mingyuefeng formations in the Lishui Sag,East China Sea Basin,variations in paleoclimate and paleoenvironment and their influence on organic matter enrichment are investigated through analyzing major and trace elements. Results show that the Paleocene sediments are low in terrigenous detrital components and rich in authigenic components,where major and trace elements did not experience diagenetic alteration. Multiple geochemical proxies suggest that paleoclimate during the Yueguifeng,Lingfeng and Mingyuefeng formations depositional periods was humidity,drought and humidity,respectively,while corresponding paleo-salinity was brackish water,saline water and fresh water,and paleowater depth was relatively deep water,relatively shallow water and shallow water,respectively. Water column evolved from reducing condition to oxidizing condition,and then to weak oxidizing-oxidizing condition,respectively. Organic matter productivity and preservation/degradation controlled by co-variations of paleoclimate and paleoenvironment accounted for the difference in organic matter enrichment of the Yueguifeng,Lingfeng and Mingyuefeng formations mudstones. The Yueguifeng Formation mudstone was deposited in semi-deep to deep lake environment with high productivity under warm-humid climate. Considerable planktonic algae were efficiently preserved at a stratified and dysoxic water column,resulting in high organic matter abundance. The Lingfeng Formation mudstone was developed at oxygen-enriched shallow marine environment under arid climate,which was not conducive to the reproduction and preservation of planktonic algae,giving rise to low organic matter abundance. The Mingyuefeng Formation coal-bearing mudstone was deposited in marine-continental transitional environment with low productivity under warm-humid climate,where flat terrain and considerable higher plants contributed to the development of alternated beds of coal and mudstone. The future oil and gas exploration in the Lishui Sag should focus on hydrocarbon generation center of the Yueguifeng Formation with high organic matter abundance and hydrocarbon generation capacity,and select effective structural or lithologic traps.