Seismic response to natural gas anomalies in crystalline rocks

This research further shows the increased porosity facilitates natural gas concentrations in top of the mid-crust to form some large gas reservoirs, which can be detected using the three-component seismic method. The results are reported in Science in China Ser. D 2008 (No.9 in Chinese and No.12 in English). As drilling machines have reached 12 km-deep now, these gas reservoirs can be practically exploited.

The geological and seismic reflection data of the Chinese continental drilling reveal that the gas anomalies of CH4, CO2, and He occur in the deep crystalline rocks. In the drilling hole, the concentration of CH4 occurs mainly in the interval of 2310 m-3280 m with the peak value of 260 ppm; Notable concentration anomalies of He also occur in the interval ranging between 10 and 46 ppm. The general near-vertical reflection profiles show that strong seismic responses are mainly correlated to ductile shear zones and have rather weak correlation with the location of the gas anomalies. However, after the three-component seismic survey was performed in the drilling site, horizontally converted wave reflectors clearly appear at 2310m-3280m where the main CH4/He anomalies occur. Thus the zone of the gas anomalies could be approximately defined by reflectors of nearly horizontally converted S-wave events.

The measured physical parameters of the cores include porosity, water-permeability, elastic wave velocity, conductivity, and thermal conductivity etc. For the interval with gas anomalies (2310m-3280m), gneiss has the lowest porosity (1% or so) and the lowest permeability in the whole borehole. This result contradicts the conventional wisdom of a positive correlation between fissures and gas anomalies of He and CH4, implying the anomalous gas may exist in tiny invisible fissures of crystalline rocks.

Then, further investigations must be done to see whether the effects of the tiny fissures on seismic wave velocity are different from macroscopic fractures. The contrast of seismic velocity measuring between gas- or water-saturated cores reveals that the increase of porosity and velocity of S-wave do not often correspond to each other as porosity jumps at the intervals of macroscopic fractures. A careful comparison may discover a rather negative correlation, i.e., the correlation between porosity increase and velocity decrease of S-wave. Based on a comparison of the data, it is found that despite the low porosity of 1% or so of the gneiss samples, P-wave velocity with heat dry gas located below 2250m is reduced by 500-900m/s, about 10% less than that in water-saturated rock samples whereas velocity of S-wave is reduced by 500-800m/s (about 15% less). By thoroughly analyzing the wave velocities measuring of cores, the cause of seismic response to gas anomalies in crystalline rocks can be explained as follows: gas contained in tiny fissures of crystalline rocks can cause a significant decrease of seismic wave velocity (especially the velocity of S-wave) and make a series of seismic responses in seismic wavefield relating to the velocity decrease of S-wave.

Seismic velocity measurements explain a notable seismic response could be induced in gas-filled crystalline rocks. It has be predicated previously by geochemical experiments that the porosity of crystalline rocks in the middle crust increases sharply due to water-rock interaction, and then there would be more probability for natural gas to be concentrated in top of the mid-crust, forming some gas reservoirs owing to the Earth degassing. In such cases, seismic method could be used to explore natural gas reservoirs in the middle crust.

Reference: Zhang R H, Zhang X T, Hu S M, et al. Kinetic experiments of water-rock interaction at high temperatures and high pressures corresponding to the middle crust condition (in Chinese). Acta Petrol Sinica, 2007, 23(11): 2933-2942