Electromagnetic surveying for hydrocarbon reservoirs

#The invention relates to seafloor electromagnetic surveying for oil and other hydrocarbon reserves.

#Determining the response of subterranean strata within the earth's crust to electromagnetic fields is a valuable tool in the field of geophysical research. The geological structures associated with thermally, hydrothermally, tectonically or magmatically active regions can be studied. In addition, electromagnetic surveying, or sounding, techniques can provide valuable insights into the nature, and particularly the likely hydrocarbon content, of subterranean reservoirs in the context of subterranean oil exploration and surveying.

#Seismic techniques are often used during oil exploration to identify the existence, location and extent of reservoirs in subterranean rock strata. Whilst seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them, especially for pore fluids which have similar mechanical properties. In the field of oil exploration, it is necessary to determine whether a previously identified reservoir contains hydrocarbons or just aqueous pore fluids. To do this, an exploratory well is drilled to determine the contents of the reservoir. However, this is an expensive process, and one which provides no guarantee of reward.

#For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings, in which:

#FIG. 1 shows in schematic vertical section a surface vessel undertaking an EM survey in deep water according to a previously proposed method;

#FIG. 2 is a graph plotting detector signals calculated from two model surveys made according to the previously proposed method, one performed in deep water (dotted line) and one performed in shallow water (solid line);

#FIG. 5 of the accompanying drawings schematically shows a surface vessel 14 undertaking electromagnetic (EM) surveying of a subterranean strata configuration (or rock formation) according to an embodiment of the invention. Features shown in FIG. 5 which are functionally similar to features shown in FIG. 1 are given the same reference numeral. The subterranean strata configuration includes an overburden layer 8, an underburden layer 9 and a hydrocarbon reservoir 12. The surface vessel 14 floats on the surface 2 of the sea 4. A submersible vehicle 19 is attached to the surface vessel 14 by an umbilical cable 16 which provides an electrical, optical and mechanical connection between the submersible vehicle 19 and the surface vessel 14. The submersible vehicle 19 is towed by the surface vessel 14 such that it remains consistently close to the seafloor 6. This is facilitated by an echo-location package 20 which relays information about the height of the towed vehicle 19 above the seafloor 6 to the surface vessel 14. The towed vehicle 19 receives electrical power from the ship's on-board power supply via the umbilical cable 16.

#A waveform generator in the form of a cycloconverter unit 30 generates a selected waveform for an alternating current (AC) drive current. The AC drive current is supplied to a horizontal electric dipole (HED) transmitter 22 which is towed by the submersible vehicle 19. Whilst in this example a cycloconverter is used, any waveform generator capable of generating a suitable output signal may be employed. Furthermore, although in this example the waveform generator is aboard the ship, in other examples the waveform generator may be on the towed vehicle. In these cases it may be supplied with electrical power from the surface vessel's power supply. The supply of the AC drive current to the HED transmitter causes it to broadcast a HED EM signal into the sea 4.

#One or more remote receivers 35 are located on the seafloor 6. The receivers 35 comprise an instrument package 36, an antenna 34, a floatation device 38 and a ballast weight (not shown). The antenna 34 comprises a vertical electric dipole (VED) detector and two mutually orthogonal horizontal electric dipole (HED) detectors. A standard marine compass may be included in the instrument package to record the orientation of the HED detectors. In other examples, only the VED detector is included (i.e. the HED detector pair is omitted). The VED detector is sensitive to vertical electric components of the EM fields induced by the HED transmitter in the vicinity of the receiver 35 and may be a simple VED antenna. The VED dipole produces detector signals forming VED response data. The instrument package 36 records the VED response data for later analysis. At the end of a survey, a remotely operable release system allows the instrument package 36 to be detached from the ballast weight so that the floatation device 38 can carry the instrument package to the surface for recovery and retrieval of the VED response data for analysis.