Paleobiological Characteristics and Its Reservoir Significance of Bioclastic Migmatite in First Member of Shahejie Formation in Bohai Sea

Lacustrine migmatite is widely distributed in the first member of Shahejie Formation of Paleogene in Bohai Sea. The bioclastic migmatite is the high-quality reservoir with good physical properties and high productivity. In this paper, a systematic paleontological study of bioclastic components in bioclastic migmatites is carried out. It is confirmed that gastropods are the most important bioclastic types in the first member of the Shahejie Formation in Bohai Sea. 21 genus and 27 species of gastropods and 19 genus and 74 species of ostracoda are identified. They are mainly miniaturized types suitable for living in a certain salinity and shallow water environment. It is found that the gastropods are easily to accumulate in high energy environment, while the content of ostracods is relatively high in low energy environment combined with paleoenvironmental studies. The analyses of casting thin section, SEM and EPMA show that bioclasts play an important role in the evolution of migmatite reservoir. The reservoir space is directly provided by the cavity hole of the bioclastic body, the mold hole of the shell, the shielding hole and the differential contraction joint. Early dissolution of bioclasts can effectively improve the permeability of the reservoir, promote dolomitization of the bioclastic enrichment section, and form a high-quality reservoir of bioclastic dolomite. Bioclasts form a stable mud crystal set with the action of microorganism, which plays a supporting role of pore lattice. The physical properties of the bioclastic migmatite mainly composed of gastropods are better than the others, as influenced by the differences of shell composition and various kinds of biological sedimentary environment.