Abstract: A special type of lacustrine dolomite reservoir was discovered in the Member 3 of Paleogene Shahejie Formation in the
Beitang sag, Bohai Bay Basin. The main rock types and mineral characteristics of the dolomites have been studied in detail by a vast
amount of data from core drilling, paleontology, rock thin section, scanning electron microscope, and electron microprobe. The
generation environments of the dolomites were debated by the X-ray diffraction data, major and trace element geochemistry. The
dolomites in the Beitang sag are usually bedded or laminated with mud or analcime, and the mineral components are mainly dolomite,
analcime, and clay minerals. The paleosalinity, paleotemperature, paleo-water depth, and redox conditions of the depositional
environments of the lacustrine dolomites were discussed carefully. We inferred the dolomites were deposited in the lacustrine bay with
a relatively high salinity, high temperature and high PH of the water column. The salinity of the lake increased during the seawater
intrusion, providing favorable physical and chemical conditions, subsequently promoted the dolomite formation.

Key words: Bohai Bay Basin, Shahejie Formation, lacustrine dolomite, geochemistry, sedimentary environment