Diagenetic fabrics and genetic significance of the dolostones from the Angdaerco area in the Qiangtang Basin

Six types of diagenetic fabrics have been distinguished for the dolostones from the Angdaerco fossil oil reservoirs in the Qiangtang Basin according to thin section, cathodoluminescence, back scattered electron images and scanning electron microscopy analysis, in integration with crystal size, crystal shape, crystal surface, contact relationship and relevant genetic information. They include power to fine-crystalline nonplanar anhedral fabric; power to fine-crystalline nonplanar fogged center-clear margin fabric;fine-to medium-crystalline planar subhedral-euhedral fabric;fine-to medium-crystalline planar fogged center-clear margin fabric;fine-to medium-crystalline planar subhedral-euhedral zonal fabric, and medium-to coarse-crystalline nonplanar anhedral fabric. The diagenetic fabrics outlined above suggest that the dolomite in this area should be the result of the replacement of calcite by evaporated salted seawater, and be transformed by multistage diagenetic fluids within different genetic models, including evaporated salted seawater genetic model in the penecontemporaneous stage;seepage reflux genetic model in the shallow burial stage;burial dolomitization genetic model in the medium to deep burial stage, and hydrothermal metasomatism genetic model associated with tectonic movement in deep burial stage. Residual granular structure and algal-laminated structure in the dolostones have disclosed that original high-energy sedimentary environments such as the shoal facies, tidal flat facies and bioclastic shoal facies are more conductive to the formation of the dolostones in the study area, and that the development of dolomitization and spatial distribution of dolostones are generally controlled by the original sedimentary microfacies and rock types.