In order to determine a direct hydrocarbon indicator in an oil field formed by low- to medium-porosity fast sandstone, we examine wireline data from four wells. Fluid substitution indicates that the sensitivity of the acoustic impedance and Poisson's ratio to oil-to-brine changes is very small. It appears, however, that due to diagenetic processes, the porosity in the brine-filled strata is noticeably smaller than that in the oil-saturated intervals. This porosity difference makes the impedance in the presence of oil noticeably smaller than that where brine is present. The respective impedance cutoff can serve as a discriminator for fluid detection in the seismically derived acoustic impedance volumes. The lesson learned is that merely relying on a rock-physics tool, such as fluid substitution, may not necessarily provide a fluid-detection recipe. Instead, we need to examine a plethora of natural events that may affect rock properties and then translate these effects into seismically detectable variables.