Relationship between porosity evolution and hydrocarbon charging in tight sandstone reservoirs in Oligocene Huagang Formation, Xihu Sag, East China Sea Basin

The pore evolution of tight sandstone reservoirs in the Oligocene Huagang Formation in the central and northern parts of the central reversal structural belt in the Xihu Sag of the East China Sea Basin was recovered based on the analyses of reservoir properties, burial history and thermal evolution history, and using cast thin section identification, scanning electronic microscope, electron microprobe, fluid inclusion analyses and porosity recovery quantified calculation. The relationship between porosity evolution and hydrocarbon charging was also discussed. The results showed that the porosity evolution in the Huagang Formation can be classified into three stages:14.28% in the stage B of early diagenesis, 10.35% in the stage A of middle diagenesis, and 8.75% in the stage B of middle diagenesis, accompanied by three stages of hydrocarbon charging. The first (19-17 Ma) and second (17-7 Ma) stages of hydrocarbon charging took place before the end of stage A of middle diagenesis. The reservoirs were not tight yet, and the acidic fluid brought by hydrocarbon charging played an important role in reservoir reformation, providing favorable porosity and permeability for later hydrocarbon charging. The third stage (7-0 Ma) of massive hydrocarbon charging happened from the end of stage A to the stage B of middle diagenesis, when reservoirs compacted gradually. The third section of Huagang Formation, buried shallow and was not tight yet, showed relatively better reservoir properties and became a main charging target. It is now the most favorable hydrocarbon exploration target for the Huaguang Formation in the overall densification background.