Abstract:

The geochemical study on well Songke-1 in Songliao basin indicates the biological development, varied water environments and paleoproductivity of paleo-Songliao Lake during the Late Cretaceous. A formula used to quantify the paleoproductivity of ocean was applied to restore paleoproductivity during the Late Cretaceous in Songliao basin. The results show that the highest paleoproductivity achieved at 3 000 g/（m²·a）during the deposition of the 1st Member of Qingshankou Formation (K₂qn¹) and the 1st Member of Nenjiang Formation (K₂n¹). Biomarker indicators show that the paleoproductivity was mainly contributed by algae and the paleolake was in hypereutrophic or eutrophic state with stratified water column and anoxic water-sediment interface. It is believed that the eutrophic lake was the result of transgression during the Late Cretaceous and formed high paleoproductivity. During the early age of Qingshankou 2nd-3rd Members’ deposit, the nutrients of the lake decreased to mesotrophic-eutrophic and the bioproductivity was decreasing. In the late age of Qingshankou 2nd-3rd Members’ deposit, the bioproductivity bloomed again and the lake became mesotrophic. The lake was in mesotrophic state with bioproductivity average at 365 g/(m²·a) during the deposit of Yaojia 1st Member. Both in the early and late deposit of Yaojia 2nd[CD*2]3rd  Members, the lake was in oligotrophic state with the bioproductivity lower than 100 g/（m²·a）and in the mid stage of deposit the lake became mesotrophic with increasing bioproductivity. It is concluded that the paleolake was characterized with blooming biomass and vibrating water environment. Further study shows that hypereutrophication and eutrophication are essential for depositing high-quality source rock.

Key words: Songliao basin, well Songke-1, Cretaceous, lake paleoproductivity, petroleum gas