Depositional and Diagenetic Controls on Reservoir Quality of Deep-Lake Turbidite Sandstones: Examples From Palaeogene Sha 3 Sandstones in Bonan Oilfield, Bohai Gulf Basin, China

The third member of Shahejie Formation (Es3), the major oil resrvoir units in Bonan oilfield, is typical turbidite sandstone reservoir. The sandstones are characterized by middle-low porosity, low permeability and strong microscopic heterogeneity, which are mainly controlled by both primary deposition and diagenesis. Using core description, conventional thin sections, casting thin sections, scanning electron microscopy, mercury injection capillary pressure analyses and X-ray diffraction to study the lithogology characteristics (lithology, minerals, grain structures), diagenesis and their influences on the reservoir quality (porosity, pore types, pore size, shape and distribution), revealing the formation mechanism of low permeability characteristics of turbidite sandstones. Research shows that the third member of Shahejie Formation are mainly litharenite arkose, the clastic rock components are quartz(38.8% on average), feldspar(34.7% on average), lithic fragments (26.5% on average), and there is a small amount of flint and mica. The ratio of feldspar and lithic fragments, the relatively flexible components, is high. When buried to a certain depth, the fieldspars and clastic fragments get deformed by compaction, and both porosity and permeability decrease quickly. The grain size is dominated by fine sand, the average sorting coefficient is 1.851, which indicates a poor sorting. There are three types of pore throat structures: middle pore with tiny throat, middle-small pore with tiny throat and small pore with extremely narrow throat, which is, overall, characterized by small pore throats, high capillary pressure and a tiny pore throat radius. Compaction, pressure solution and cementation, destructive diagenesis, further damage the reservoir quality, while the solution improves the reservoir at the same time. The third member of Shahejie Formation is buried about at 3500m, the study of sorts and proportion of clay mineral shows that it is in A-substage of middle diageneiss. Grains are concavo- convex contacts, further reduce the pore throat size. Cements contain silica cement and carbonate cement, reduce the intergranular pore in microscope, while the top and bottom of the sandstone is cemented to form impermeable layer in that prevent the flow of fluids in macroscope.