--> Fine Characterization of Volcanic Rock in the South of Huanghekou Sag of Bohai Sea

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Fine Characterization of Volcanic Rock in the South of Huanghekou Sag of Bohai Sea

Abstract

In the south of Huanghekou Sag of Bohai basin, the volcanic rock is extremely abundant in Dongying formation. On one side, the effective reservoir space is filled by the volcanic rock, which is widely distributed with great thickness. So it is unfavorable for the research and calculation of reservoir. On the other side, Shahejie formation as one of the most important exploration layers is covered with the thick volcanic rock, which reduces the efficiency of well drilling and increases the cost of exploration in sea. For this reason, there is an influence on the well deployment. Therefore, it is necessary to make a fine characterization of the three dimension distribution of the volcanic rock, which based on the center-fissure eruption model and the different seismic reflection characteristics. From the analysis result, the volcanicity is usually featured with multiphase and regional eruption. The formed volcanic lithofacies are of diversity, which are mainly the effusive facies and volcanic conduit facies. The effusive facies is widely distributed, which usually shows strong amplitude and relatively continuous reflection characteristic. The density and velocity of the effusive facies rock are relatively high, which means that the impedance of the effusive facies rock is more than the surrounding rock. So it results to the strong amplitude in seismic section. According to seismic data and lithologic information, we determine the minimum absolute amplitude (15800) for the threshold value of such facies. On the other hand, the volcanic conduit facies shows columnar shape structure, weak amplitude and disorder or blank reflection in seismic section, which are different form the layered and continuous reflection characteristic of the surrounding formations, which results to differences in variance volume. Based on variance technology, we determine the minimum variance threshold value (0.34) for such facies. Finally, based on the 3D visualization technology of seismic data and the threshold values, we apply the grading technology of amplitude and variance cube to make a fine characterization of the plane distribution and vertical thickness of the volcanic rock, and analyze its development regularity. In recent years, under the guidance of this study we optimize well deployment, discover 35 million tons of reserves in this area, complete offshore exploration with low cost and high efficiency and hope it can contribute to the research of volcanic rock in other areas.