Description: Passive seismic low-frequency from approximately 1–6 Hz data have been acquired at several locations around the world. Spectra calculated from these data, acquired over fields with known hydrocarbon accumulations, show common spectral anomalies. Verification of whether these anomalies are common to only a few, many, or all hydrocarbon reservoirs can be provid-ed only if more and detailed results are reported. An extensive survey was carried out above a tight gas reservoir and an adjacent exploration area in Mexico. Data from several hundred stations with three-component broadband seismometers distributed over approximately 200 km 2 were used for the analysis. Several hydrocarbon reservoir-related microtremor attributes were cal-culated, and mapped attributes were compared with known gas intervals, with good agreement. Wells drilled after the survey confirm a predicted high hydrocarbon potential in the explora-tion area. A preliminary model was developed to explain the source mechanism of those microtremors. Poroelastic effects caused by wave-induced fluid flow and oscillations of different fluid phases are significant processes in the low-frequency range that can modify the omnipresent seismic background spectrum. These processes only occur in partially saturated rocks. We as-sume that hydrocarbon reservoirs are partially saturated, where-as the surrounding rocks are fully saturated. Our real data obser-vations are consistent with this conceptual model.