Seismic response to natural gas anomalies in crystalline rocks

According to the geological and seismic reflection data of the Chinese Continental Scientific Drilling (CCSD) main-hole (MH), and the anomalies of CH4, CO2, and He are correlated to the three-component seismic reflectors, especially in horizontal component profiles. However, the seismic response is difficult to be explained as the porosity of crystalline rocks is only about 1% in well section where the gas anomalies occur. Seismic velocity measurement of the MH cores indicated that compared with water-saturated rock samples, seismic velocity (especially the S-wave) could be distinctly decreased by gas contained in tiny cracks despite of the low porosity, and then notable seismic response could be induced in gas-filled crystalline rocks. It could be predicated that if the porosity of certain rocks in the middle crust rose due to water-rock interaction and had natural gas filled, then there would be more probability for natural gas in top of the mid-crust to fill in the crystalline rocks with increased porosity. In such case, based on the decrease of S-wave velocity in crystalline rocks, seismic method could be applied in the future to explore natural gas reservoirs in the middle crust.