Abstract: Based on core and thin-section observations, and analyses including scanning electron microscope, carbon, oxygen and
strontium stable isotopes, X-ray diffraction and trace elements, the geochemical characteristics and genetic mechanism of dolomites in
the carbonate buried hills of the Shaxibei structural belt (Bohai Bay Basin) were studied systematically. Results show that the dolomites
can be divided into four types, including micritic dolomite, granular dolomite, muddy/limy dolomite and saddle dolomite. They vary in
distinguished characteristics. The micritic dolomite and granular dolomite are formed under penecontemporaneous environments, as
implied by δ18O greater than -7‰, 87Sr/86Sr ratio in the range of 0.7090-0.7120 that is close to the seawater value, relatively high Na2O
content with value greater than 0.06%, low degree of order, and gypsums observed under thin section. In contrast, the muddy/limy
dolomite is formed under burial environments, as implied by fine crystalline structure, relatively negative δ18O values of -8‰--10‰,
87Sr/86Sr of 0.7120-0.7140, lower Na2O content, higher degree of order, and bright secondary-enlarged edge observed under
microscope. The saddle dolomite is of hydrothermal origin, as implied by medium and coarse crystalline structure, negative δ18O
values, high 87Sr/86Sr values, and diabase observed under microscope. Therefore, the dolomites in the study area were mainly of
penecontemporaneous and burial origins, and some may be formed under hydrothermal environments along faults. The burial
dolomitization and associated dissolution during the Ordovician increase the effective porosity of dolomite, which plays a postive role
in the formation of high quality reservoirs.

Key words: Bohai Bay Basin, Shaxibei structural belt, Lower Paleozoic, carbonate buried hill, formation mechanism of dolomite