Abstract:

Turbidite reservoirs in the 3^rd member of the Palaeogene Shahejie Formation （hereinafter referred to as Es³） act as an important contributor for the increase of reserves in the Dongying Sag， Bohai Bay Basin. However， their marked richness in lime in the water body during deposition， has severely affected the precision of geophysical prediction and reservoir quality. Based on the genetic link of sedimentary water conditions and mixed lithofacies assemblages rich in lime， we classify the lithofacies assemblages and depict related water conditions， as well as discuss the control mechanism of water conditions on mixed lithofacies assemblages， following the restoration of paleolacustrine water conditions （e.g. salinity， depth） via multiple means. An integration of drilling data， seismic data and core analyses such as paleontology， trace elements， clay minerals， carbon and oxygen isotopes， is applied in the study. The results show that the lithofacies assemblages of the Es³ in the study area can be divided into 7 types， 5 of which are mixed lithofacies assemblages containing lime. Paleoclimate， paleo-provenance and deep-sourced fluids serve to jointly control the paleowater conditions of continental lacustrine basins， while the carbonate content is under the control of water salinity therein. The lime-rich areas are mainly located in the Tuo-Sheng fault zone， northern Caoqiao slope， central anticlinal and slope zones， while the lime-lean areas mainly in freshwater input regions and deep depression zones. Therefore， key turbidite exploration is supposed to be directed towards freshwater-affected areas adjacent to prodelta and freshwater-diluted areas in central depression.

Key words: mixed lithofacies assemblage, paleowater condition restoration, turbidite reservoir, the 3^rd member of Shahejie Formation （Es³）, Paleogene, Dongying Sag, Bohai Bay Basin