Forming mechanisms of reservoir “sweet spots” in tight sandstones of Chang8 Formation, Honghe oil field, Ordos Basin

Reservoir "sweet spots" (that is, relatively high quality reservoirs) exist locally in the low-permeability, tight sandstone reservoirs of Chang8 Formation in the Honghe oil field, controlling hydrocarbon enrichment and production. This reservoir provides evidence and direction for prediction of tight sandstones to study their forming mechanisms. The impacts of sedimentation, diagenesis, fracturing and oil emplacement on reservoir quality of the tight sandstones during the whole reservoir evolution process from sedimentary construction to diagenetic reformation were systematically analyzed using rock sections, casting thin sections, cathode-luminescence, scanning electron microscopy, micro fluorescence observations and statistics, combined with the analyses of mineral composition, volume of cementation minerals and reservoir physical tests. The results indicate that sedimentation is the most important factor controlling tight sandstone reservoir properties by providing a material basis and controlling the original properties of tight sandstones. Diagenesis is another factor which controls the porosity evolution of tight sandstones, and construction diagenesis controls the formation of reservoir "sweet spots" in tight sandstones and the distribution of "sweet spots" in pore-type tight sandstone reservoirs. The superimposed reformation of structural fractures improves the reservoir properties of tight sandstones, especially the permeability, resulting in the distribution of "sweet spots" in fractured tight sandstone reservoirs. Oil emplacement inhibits cementation as one of the important mechanisms of "sweet spot" preservation in the tight sandstone reservoirs.