GENESIS OF THE DOLOMITE IN THE SHAHEJIE FORMATION OF JZ OILFIELD, BOHAI BASIN

Bioclastic dolomite and sand-bearing oolitic dolomite are widely distributed in the study area. Drilling shows that the lithology and thickness of dolomite vary greatly from well to well. In order to reveal the genesis of the dolomite, the physical and chemical properties of the depositional media, paleoclimate and paleoenvironment of the area are systematically studied by means of sporopollen and micro paleontology, carbon and oxygen isotopes and trace element B. It is found that since the 3^rd Member of the Shahejie Formation, warm subtropical climate had gradually turned to dry, and the water body become salty, which provided basic conditions for the dolomite to form. On the nearshore and shallow lake platform with hard basement, however, water was clean, which was in favor of the growth and reproduction of benthic organisms and the formation of limestone and bioclastic dolomite with huge thickness. As the bioclastic content is high, pores are well developed, and the calcite is also more thoroughly replaced by dolomite. Upon the basis, a seepage reflux model for such kind of dolomite platform is established by the authors. In the uplifting zone of the nearshore and shallow parts of the lake, where fresh and saline water mixed, it is easy to form the silty dolomite with sparry edge and cloudy nucleus and the oolitic dolomite or bioclastic dolomite with concentric structure. However, dolomitization is often incomplete, calcite content is high sometimes, and therefore dolomitic oolitic limestone is common in some wells. For these dolomitic deposits, a genetic model of mixed water is proposed specially for nearshore-shallow lake uplift zones. The genetic models of lacustrine dolomite are of important significance for hydrocarbon exploration in carbonate rocks in Bohai Bay Basin.