Diagenesis Research And Pore Evolution In Yinggehai Basin

Yinggehai Basin is global well-known as the large natural diagenetic laboratory for its fast sedimentation rate (800m/Ma), high geothermal gradient (4-6℃/100m), high strata pressure(pressure coefficient 1.0-2.3) and above-normal temperature. The diagenesis research, pore evolution and prediction of high-quality reservoir of the basin were studied by analyzing the measured data and thin sections.Five diagenetic districts are divided according to the varied environment as follows: Dongfang District, Ledong District, Yingdong Slope District, Lingao District and Dongnan District. Based on the homogenization temperatures of fluid inclusions, vitrinite reflectance Ro, sporopollen color index TAI, pyrolysis analysis, organic acids, X-ray diffraction, ordinary sheet, blue epoxy resin-impregnated thin sections and scanning electron microscopy test data according to the oil industry standards (SY/T547-2003) of the division of diagenetic stages in clastic rocks, the diageneses of clastic rocks in Yinggehai Basin are divided into period A of early diagenetic stage, period B of early diagenetic stage, sub–period A1 of middle diagenetic stage, sub–period A2 of middle diagenetic stage, and period B of middle diagenetic stage. Because overpressure retards diagenesis and geothermal fluids promote it, the diagenesis of Dongfang District and Ledong District is the strongest due to geothermal fluid, followed by Yingdong Slope District and Lingao District, Dongnan District is the weakest due to overpressure.As the results show, the dominant influence factors on reservoir physical properties are diagenesis and sedimentary facies. The stronger sedimentary facies hydrodynamic, the better reservoir physical properties. Compaction and cementation variation of reservoir properties, the dissolution can improve reservoir properties. Study has showed the overpressure plays a significant role in the protection of reservoir's porosity by retarded compensation. The meteoric water formed secondary porosity and promoted cementation while flowing underground. The correlation between porosity and their influence factors is analyzed based on Stepwise regression method, the results show effects of sedimentary facies on the porosity of reservoir decreased gradually along with the increase of buried depth and diageneses, on the contrary,effects of diagenesis on porosity of reservoir increased gradually.If the reservoirs are similar with each other on diagenetic stage, sedimentary facies and pressure conditions, that their physical properties and type of the reservoirs should also be similar, so according to this principle, we can know that by means of superposing digenetic facies maps, the sedimentary facies maps and overpressure distribution maps, favorable reservoir developing regions should be predicted. In Yinggehai Basin, Yingdong Slope District and Lingao District in edge of the basin with good reservoir properties where sands bodies development and diagenesis is relatively weak. The reservoir are buried deep at present in Dongnan District, but they were buried slow in early time and fast in later period, coupled with the overpressure plays a significant role in the protection of reservoir's porosity, so effective reservoir can develop in Dongnan District. High-quality reservoirs are mainly distributed in Dongfang District and parts of Ledong District, as well as Yingdong Slope District and Lingao District which buried shallow and diagenesis is relatively weak.