Geobiological processes of the formation of lacustrine source rock in Paleogene

Abstract

Gypsum-salt rocks and coccolith calcareous shale are widely deposited in the lower part of the Paleogene Shahejie Formation in the Dongying Sag, Shandong Province. The gypsum-salt rock is believed to be formed during the earlier deposition in salt lake, while the overlying coccolith shale in saline lake with relatively low salinity. By comparing the lake environment and lacustrine microbial communities between ancient and recent lakes, cyanobacteria are regarded as the main representative of productivity during the formation of gypsum-salt strata series, with the annual productivity of 1500–2000 gC m−2 yr−1. Based on the research of ultramicrofossils in the calcareous shale, coccolith is considered as the main contributor to the productivity during the formation of calcareous shale. On the basis of statistic data of sedimentary rates, shale laminations, and coccolith fossils in each lamination, the quantitative value of productivity is calculated, with the annual productivity of 2250–3810 gC m−2 yr−1 (averagely 3120 gC m−2 yr−1). Statistic data of large amount of pyrite framboids indicate that the lower water column was persistently in sulfidic or anoxic conditions during the deposition of gypsum-salt strata series, but it changed to be dysoxic when the coccolith calcareous shale was deposited. Both of these water conditions are favorable for the preservation of organic matter. It is estimated that the organic carbon burial efficiency of the Lower Paleogene salt lakes and saline lakes of Dongying Depression is about 10%–15%, which is calculated and analyzed using the multi-parameter geobiological model.