Determination of Hydrocarbon-bearing Property of Deep-water Sandstone by Fluid Pressure Gradient from Seismic Attributes

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In the same hydrodynamic system, because the densities of oil and gas are smaller than that of water, their pressure gradients are also smaller. Therefore, the pressure gradient can determine fluid properties. Based on seismic data and well logging data, this paper attempts to apply the equivalent medium theory to predict the pressure gradient, and then to identify fluids. Firstly, the upper and lower limits of bulk modulus and shear modulus of rocks can be obtained by using wellbore and well logging interpretation data. Secondly, based on the equivalent medium theory, the fluid velocity (when rock rigidity approaching zero) and the rock matrix velocity (when porosity approaching zero) are predicted. Thirdly, the predicted two types of velocity curves and the original acoustic curves are used for seismic inversion. Finally, according to the inversion results, the formation pressure and pressure gradient can be obtained by using the Fillippone pressure formula, and the hydrocarbon-bearing property of reservoirs can be determined according to the theoretical pressure gradients of different fluids. For offshore deep-water sandstone in an overseas block, when the frequency attributes for hydrocarbon detection cannot reflect hydrocarbons well, the fluid pressure gradient attribute is used to predict hydrocarbons, and the prediction coincidence rate reaches 70%.