Cases of generalized low-frequency shadows of tight gas reservoirs

The phenomenon that frequency decreases and amplitude increases near the bottom of a gas layer on a seismic profile is called a low-frequency shadow, but this phenomenon may not occur in all gas reservoirs. When the tight gas reservoir is thick enough, spectral decomposition data after Fourier transformation will indicate characteristics similar to those of low-frequency shadows, which we call generalized low-frequency shadows. Compared to the dominant frequency of the nongas-bearing zone spectral, the dominant frequency of a gas zone moves toward the low end of the frequency range and the low-frequency amplitude increases accordingly. By analyzing known gas reservoirs such as the Sulige and Yanchang tight sandstones in the Ordos Basin and tight carbonate rocks in the Tarim Basin, we can see that, with the visual dominant seismic frequency close to 30 Hz, the peak frequency of the gas-bearing tight sandstones and tight dolomite reservoirs will move from approximately 30 to 10–15 Hz. There is a certain correlation among the drop of the dominant frequency of a tight gas reservoir, the attenuation energy difference, and the thickness and productivity of the gas layer. Several cases indicate that nearly all tight gas layers thicker than 15 m exhibit attenuation characteristics of generalized low-frequency shadows.