Influence of Diagenesis on Reservoir Properties of Shahejie Sandstones in Dongying Depression, Bohai Bay Basin, China

Dongying depression in Bohai Bay Basin is a prolific oil-producing Cenozoic basin in China. Palaeogene Shahejie Formation is the most important productive series in Dongying depression with the average porosity and permeability of sandstones reaching 16.49% and 79.94mD respectively. The influence of diagenesis on reservoir properties of Shahejie sandstones in Dongying depression is studied by polarizing microscope, cathode luminescence microscope, stable oxygen isotopic measurement, XRD, EPMA and mercury injection apparatus technologies. Quantitative estimation of the amounts of porosity loss by compaction and cementation shows that compactional porosity loss averages 22.11% (55.28% of the initial porosity), and cementational porosity loss averages 5.11% (12.78% of the initial porosity). In the study area, cements mainly consist of carbonate cement, authigenic quartz and authigenic clay minerals. Carbonate cement is the main cement which could be divided into three types: penecontemporaneous micrite dolomite, calcite and ankerite. The δ18O values of calcite fluctuated from -16.86‰ to -12.29‰ PDB, and the calcite precipitated at 90—120C while the δ18O values of ankerite fluctuated from -12.20‰ to -10.20‰ PDB, and the ankerite precipitated at 110—135C. The calcite (averaging 4.70%) caused 3.66% porosity loss while the ankerite (averaging 4.92%) caused 3.83% porosity loss approximately. In addition, the permeability reduced by 54.63mD because of carbonate cementation. As a reservoir quality enhancement, dissolution developed at the depth of 2500—3500m in Dongying depression. The dissolution can be subdivided into two groups: framework-feldspar dissolution and pore-filling carbonate dissolution. About 9.16% feldspar dissolved and transformed into kaolinite, illite and chlorite. Consistently, clay minerals increased by 7.38% on average at the same depth. The average content of carbonate dissolution is about 2.1%. It can be concluded that the fluctuant porosity from dissolution (averaging 1.36%) may be important in some situations, although it generally represents a minor fraction of total porosity.