Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Seismic response to natural gas anomalies in crystalline rocks

07.11.2008
The research done at the Chinese Academy of Geological Sciences has shown that after geochemical experiments, the porosity of crystalline rocks in the middle crust increases sharply due to water-rock interaction (see ref.).

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

Yang Wencai | EurekAlert!
Further information:
http://www.ccsd.cn

More articles from Earth Sciences:

nachricht A new dead zone in the Indian Ocean could impact future marine nutrient balance
06.12.2016 | Max-Planck-Institut für marine Mikrobiologie

nachricht NASA's AIM observes early noctilucent ice clouds over Antarctica
05.12.2016 | NASA/Goddard Space Flight Center

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Significantly more productivity in USP lasers

In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.

Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...

Im Focus: Shape matters when light meets atom

Mapping the interaction of a single atom with a single photon may inform design of quantum devices

Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

Simple processing technique could cut cost of organic PV and wearable electronics

06.12.2016 | Materials Sciences

3-D printed kidney phantoms aid nuclear medicine dosing calibration

06.12.2016 | Medical Engineering

Robot on demand: Mobile machining of aircraft components with high precision

06.12.2016 | Power and Electrical Engineering

VideoLinks
B2B-VideoLinks
More VideoLinks >>>