Electromagnetic surveying for mapping the content of subterranean reservoirs

1. A method for recovering a volume of hydrocarbon from a submarine or subterranean reservoir comprising:

conducting an electromagnetic survey to determine a content of the submarine or subterranean reservoir having an electromagnetic characteristic and having an approximate geometry and location that are known, comprising:

applying a time varying electromagnetic field in the form of a wave to the strata containing the reservoir;

detecting the electromagnetic wave field response;

and analyzing the effects on the characteristics of the detected field that have been caused by the reservoir,

thereby determining the content of the reservoir, based on the analysis, wherein the analysis comprises comparing the electromagnetic characteristic theoretically predicted for the reservoir based on the approximate geometry of the reservoir and based on the reservoir being a water-bearing or hydrocarbon-bearing reservoir to the electromagnetic characteristic for the reservoir that is determined from the detected electromagnetic wave field response of the reservoir, wherein a distance between a transmitter and a receiver is given by the formula 0.5λ≦L≦10λ; where λ is the wavelength of the transmission through an overburden and L is the distance between the transmitter and the receiver;

inferring from the electromagnetic survey that the volume of hydrocarbon is present in the reservoir; and

producing the volume of hydrocarbon from a well that penetrates the reservoir.

2. The method of claim 1 , wherein the volume of hydrocarbon comprises oil.

3. The method of claim 1 , wherein the volume of hydrocarbon comprises natural gas.

4. The method of claim 1 , wherein the field is applied using at least one stationary transmitter located on the earth's surface.

5. The method of claim 1 , wherein the at least one transmitter is located proximate a bed of a body of water, the bed including a seabed.

6. The method of claim 1 , wherein the field is transmitted for a period of time of from 30 seconds to 60 minutes.

7. The method of claim 1 , wherein the at least one stationary receiver is arranged to detect a direct wave and a wave reflected from the reservoir, and the analysis includes extracting phase and amplitude data of the reflected wave from corresponding data from the direct wave.

8. The method of claim 1 , wherein the wavelength of the applied time varying electromagnetic field wave is given by the formula

0.1s≦λ10s;

where λ is the wavelength of the transmission through an overburden overlying a reservoir and s is the distance from a seabed to the reservoir.

9. The method of claim 1 , wherein, substantially

1=2s=2λ, wherein s is a distance from a seabed located over the reservoir to the reservoir.

10. The method of claim 1 , wherein the transmission frequency of the time varying electromagnetic field is from 0.1 Hz to 1 kHz.

11. The method of claim 1 , wherein the analysis includes comparing the results of measurements taken with results of a mathematical simulation model based on known properties of the reservoir and conditions of an overburden.

12. The method of claim 1 , including suppressing a direct wave, thereby reducing the required dynamic range of receivers receiving a reflected wave and increasing resolution of the reflected wave.

13. The method of claim 1 , including as preliminary steps; performing a seismic survey to determine the geological structure of a region and analyzing the survey to reveal the presence of a subterranean reservoir.

14. The method of claim 1 , wherein the electromagnetic characteristic theoretically predicted for the reservoir is a member of the group consisting of resistivity and permittivity.

15. The method of claim 1 , further comprising equipping a vessel with the transmitter.

16. The method of claim 15 wherein the vessel is a ship.

17. The method of claim 15 wherein the transmitter is attachable to a cable that is attachable to the vessel whereby the transmitter may be deployed by the vessel.

18. The method of claim 15 wherein the transmitter comprises a dipole antenna.

19. The method of claim 1 further comprising drilling the well that penetrates the reservoir after inferring from the electromagnetic survey that the volume of hydrocarbon is present in the reservoir.

20. A method for determining a content of a submarine or subterranean reservoir having an electromagnetic characteristic and having an approximate geometry and location that are known, comprising:

conducting an electromagnetic survey comprising applying a time varying electromagnetic field in the form of a wave to the strata containing the reservoir;

detecting the electromagnetic wave field response; and

analyzing the effects on the characteristics of the detected field that have been caused by the reservoir,

thereby determining the content of the reservoir, based on the analysis, wherein the analysis comprises comparing the electromagnetic characteristic theoretically predicted for the reservoir based on the approximate geometry of the reservoir and based on the reservoir being a water-bearing or hydrocarbon-bearing reservoir to the electromagnetic characteristic for the reservoir that is determined from the detected electromagnetic wave field response of the reservoir, wherein a distance between a transmitter and a receiver is given by the formula 0.5λ≦L≦10λ; where λ is the wavelength of the transmission through an overburden and L is the distance between the transmitter and the receiver; and

preparing a map that comprises a depiction of at least a portion of the content of the reservoir as derived from the electromagnetic survey.

21. The method of claim 20 , wherein the content of the reservoir comprises oil.

22. The method of claim 20 , wherein the content of the reservoir comprises natural gas.

23. The method of claim 20 , wherein the field is applied using at least one stationary transmitter located on the earth's surface.

24. The method of claim 20 , wherein the at least one transmitter is located proximate a bed of a body of water, the bed including a seabed.

25. The method of claim 20 , wherein the field is transmitted for a period of time of from 30 seconds to 60 minutes.

26. The method of claim 20 , wherein the at least one stationary receiver is arranged to detect a direct wave and a wave reflected from the reservoir, and the analysis includes extracting phase and amplitude data of the reflected wave from corresponding data from the direct wave.

27. The method of claim 20 , wherein the wavelength of the applied time varying electromagnetic field wave is given by the formula

0.1s≦λ≦10s;

where λ is the wavelength of the transmission through an overburden overlying a reservoir and s is the distance from a seabed to the reservoir.

28. The method of claim 20 , wherein, substantially

1=2s=2λ, wherein s is a distance from a seabed located over the reservoir to the reservoir.

29. The method of claim 20 , wherein the transmission frequency of the time varying electromagnetic field is from 0.1 Hz to 1 kHz.

30. The method of claim 20 , wherein the analysis includes comparing the results of measurements taken with results of a mathematical simulation model based on known properties of the reservoir and conditions of an overburden.

31. The method of claim 20 , including suppressing a direct wave, thereby reducing the required dynamic range of receivers receiving a reflected wave and increasing resolution of the reflected wave.

32. The method of claim 20 , including as preliminary steps; performing a seismic survey to determine the geological structure of a region and analyzing the survey to reveal the presence of a subterranean reservoir.

33. The method of claim 20 , wherein the electromagnetic characteristic theoretically predicted for the reservoir is a member of the group consisting of resistivity and permittivity.

34. The method of claim 20 , further comprising equipping a vessel with the transmitter.

35. The method of claim 34 wherein the vessel is a ship.

36. The method of claim 34 wherein the transmitter is attachable to a cable that is attachable to the vessel whereby the transmitter may be deployed by the vessel.

37. The method of claim 34 wherein the transmitter comprises a dipole antenna.

38. The method of claim 34 further comprising drilling a well that penetrates the reservoir after inferring from the electromagnetic survey that the content of the reservoir comprises hydrocarbon.