The present invention relates to geophysical mapping and more specifically to the prospecting for hydrocarbon reservoirs.

Typically, the electrical resistivity of hydrocarbon-filled sediments is 10-1000 higher than water-filled sediments. This resistivity contrast can be used to discriminate between hydrocarbon-filled and water-filled sediments as described by US patents 4,617,518; 4,633,182; 6,603,313; 6,739,165.

A method based on the use of refracted waves detected at far offsets is proposed by Eidesmo et al. (US patent 6,628,119). Proper use of this method requires to some extent knowledge of approximate reservoir location and geometry from seismic data. The presence of other resistivity contrasts in the survey area will complicate the interpretation. The electrical resistivity of rock types such as salt, volcanic rocks, carbonates and some other lithologies is high (10-10000 ohm-m) and causes the above method to be ambiguous.

US 5886526 discloses a method and an apparatus for determining the anisotropy of a formation comprising multiple layers with different resistivity. The system further comprises deployment of an electromagnetic source and multiple receivers for detecting vertical and horizontal conductivities in the formation.

GB Pat.app. 2390904 discloses an apparatus and a method for electromagnetic surveying of an area of seafloor that potentially contains a subterranean hydrocarbon reservoir by using a vertical electric dipole (VED-source) and a horizontal magnetic dipole (VMD-source) for determining the resistivity in both directions.

Controlled source electromagnetic (CSEM) sounding is a well known geophysical method for mapping resistivity of the subsurface (Kaufman A. and Keller G., 1983, Frequency and transient soundings.). Due to the highly conductive nature of sediments (e.g. 1 ohm-m) high frequency electromagnetic waves are rapidly attenuated. This significantly lowers the resolution with depth and makes detection of thin resistive layers at depth a challenging task.

The concept of the present invention is based on inversion for anisotropic conductivity of the earth formations related to hydrocarbon exploration and production and can use

any survey configuration, including the methods described in GB 2390904 and US 6628119. As such, the method according to the present invention is not survey specific, but a method for detecting thin resistive layers using any type of EM data and survey configurations. The present invention relates to surface resistivity mapping, not 5 mapping within or between wells using well equipment.

We propose a method for detecting the presence of thin, high-resistivity layers (including hydrocarbon-filled reservoirs) based on an estimation of anisotropy in the electrical conductivity of a formation. Information about possible location and depth of IQ the reservoir is not necessary, although it can be used if available.

Although any remote resistivity mapping technique can be used to obtain anisotropic resistivity models with varying success, we propose a survey method optimized for anisotropic resistivity mapping.

I ₅

The method for geophysical prospecting for thin hydrocarbon-filled reservoirs is based on analysis of anisotropy in electrical resistivity of the target formation. A remote resistivity mapping survey preferably optimized for anisotropic mapping is performed. High values in electrical anisotropy along with high vertical resistivity of a formation 20 correspond to presence of thin resistive layers. In addition, the presence of conductive layers such as shales will decrease horizontal resistivity and further increase the value of anisotropy. In contrast, thick resistive rock bodies such as salt diapirs and volcanic rocks will have high values of both horizontal and vertical resistivity, but low anisotropy values.

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The object of the present invention is to address and alleviate the above mentioned problems and shortcomings of the prior art.

The invention will now be described in detail with reference to the enclosed figures ₃ o where:

Figure 1 shows an example of a vertical profile through a cell. Figure 2 shows a recommended optimized survey acquisition.

A typical example of a setting suitable of the proposed method is shown in Figure 1. 35 The figure shows a vertical profile through a cell comprising background formation 1, a shale layer 2 and a hydrocarbon-filled layer 3. An anisotropic electrical resistivity tensor is constructed based on sub-wavelength isotropic resistivity values and geometry. A cell

filled with background formation 1 is intersected by a horizontal layer 2 of shale and a horizontal hydrocarbon-filled layer 3.

Figure 2 shows a recommended optimized survey acquisition in which reference number 4 represents receivers, reference number 5 represents a vertical electric dipole source and reference number 6 represents a vertical magnetic dipole source.

hi a marine environment, we propose an optimized survey acquisition according to the following steps (Figure 2):

• Receivers 4 capable of recording the electromagnetic field are deployed on the sea floor at N predefined positions above the target zone.

• A vertical electric dipole (VED) source 5 is deployed at M specified positions.

• An electromagnetic signal is emitted from each VED source position using a specified waveform.

• The signal is recorded at the receivers.

• A vertical magnetic dipole (VMD) source 6 (horizontal loop) is deployed at M specified positions.

• Emission of electromagnetic signal from each VMD source position is performed using a specified waveform, preferentially similar to that used for the VED source.

• The signal is recorded at the receivers.

• Joint inversion of VED and VMD datasets are performed in order to obtain an anisotropic resistivity model of the subsurface.

• VED and VMD sources can be used and/or deployed simultaneously at a source position.

• The sources and/or receivers can be towed at any speed.

It is also possible to use a VMD source at M specified positions first, and then use the VED source at M specified positions. Also, the VMD and VED sources may be used simultaneously. The source(s) may be stationary or towed and can emit energy constantly or at certain time intervals. The advantage of the above method is that a VED source generates currents which are mostly oriented in a vertical direction. As a result, the sounding will be most sensitive to the vertical resistivity of the formations. On the other hand, a VMD source generates electrical currents which flow mostly in the horizontal planes. As a result, the sounding will be most sensitive to horizontal resistivity of the formations. The combination of these will provide an advantage with respect to anisotropic resistivity imaging.

Examples

In the examples below, a controlled source survey is performed in a marine environment, on land, or in the air. The source used in the example is a dipole, including horizontal electric dipole, vertical electric dipole, horizontal magnetic dipole, vertical magnetic dipole, or any combination of these. Receivers are deployed on the ground or at the sea floor at specified positions and record variations from six components of the electromagnetic field with time. The source(s) is activated at specified positions. Short or/and long offset recording maybe used.

Alternatively, or in addition to, passive source electromagnetic survey can be performed. In addition, other geophysical surveys can be performed, including galvanic resistivity surveys, seismic surveys, magnetic surveys and gravity surveys, hi a marine environment the water depth must be sufficient to deploy both sources and receivers.

In the examples, the source emits a square waveform with 0.1 Hz emitting frequency. However, any emitting frequencies may be used. Several frequencies preferably should be used. Alternatively or in addition time domain analysis of the data can be performed. The examples contain a possible reservoir located 2000 meters below the sea floor. The reservoir has a thickness of 20 meters, and with a resistivity of 100 ohm-m. This reservoir is covered by a shale layer with a resistivity of 0.1 ohm-m and a thickness of 20 meters. The formation rock resistivity is 1 ohm-m. The lateral extent of the reservoir is 200 meters. The method proposed is suitable for a variety of different settings and includes both deep and shallow reservoirs.

hi the case of a deep reservoir, at the target depth, high frequency harmonics are attenuated so the effective resolution is low. A low resolution anisotropic resistivity model is obtained by the process of mathematical inversion (Tikhonov A.N. and Arsenin V.A., 1977, Solutions of Ill-posed Problems. Winston & Sons, Washington). The cell size is chosen to correspond to wavelengths present at the relevant depth (the cell size increases with depth). Li the presented examples, the cell size at target depth is 200x200x200 meters.

Each model grid cell represents a specific volume in the subsurface. Any geometrical features in the volume will be significantly smaller than the wavelength and thus of sub- wavelength resolution. Fluids like hydrocarbons such as oil, gas can be sealed by shale

layers which are relatively conductive. These layers are commonly thin and thus also of sub-wavelength resolution.

Figure 1 shows a vertical profile of a cell where an anisotropic electrical resistivity tensor is constructed based on sub-wavelength isotropic resistivity values and geometry. A cell filled with background formation 1 is intersected by a horizontal layer 2 of shale and a horizontal hydrocarbon-filled layer 3. Shale, hydrocarbon, and the background formation are assumed to have isotropic resistivity values, identified respectively by rs, rh and rb. The cell size is 200x200x200 meters, with a shale thickness of 20 meters and a hydrocarbon reservoir thickness of 20 meters.

The horizontal and vertical macroscopic resistivity of the cell can be computed according to the following rules:

Rh =l/< l/r_i > (1) Rv = < r_i > (2)

Where Rh is the horizontal cell resistivity, Rv is the vertical cell resistivity, < . > denotes averaging, and r_i is the resistivity of the i ^th layer where all layers have the same thickness. Vertical and horizontal resistivity of cells filled with complex sub- wavelength structures can be computed numerically or analytically.

Anisotropy is defined as:

A =RvZRh, (3)

where A is the anisotropy.

This definition is not limiting, and any other components of resistivity tensors can be used in order to estimate various parameters of a sub-wavelength model (e.g. dip, strike etc.)

The presence of a thin horizontal resistive layer will significantly increase the vertical resistivity of the cell while horizontal resistivity will be less affected. This will cause an increase of the anisotropy value.

The presence of a thin horizontal conductive layer will significantly decrease the horizontal resistivity of the cell, while the vertical resistivity will be less affected. This will also cause an increase of the anisotropy value, but the values of Rh and Rv will be low.

Example 1

The cell is intersected by a single hydrocarbon filled layer with a thickness that is 10% of the cell size. The resistivity of the hydrocarbon- filled reservoir layer is 100 ohm-m, and the background resistivity is 1 ohm-m. This leads to the following parameters: Rv=I 0.9 ohm-m Rh=1.12 ohm-m According to equations 1,2, and 3, the anisotropy value will be A=9.8.

Example 2 The cell is intersected by a single shale layer with a thickness that is 10% of the cell size. The resistivity of the shale layer is 0.1 ohm-m, and the background resistivity is 1 ohm-m. This leads to the following parameters:

Rv=0.91 ohm-m

Rh=0.52 ohm-m According to equations 1,2, and 3 , the anisotropy value will be A=I.73

Example 3

The cell is intersected by a hydrocarbon filled layer and a shale layer, both with a thickness that is 10% of the cell size. The resistivity of the hydrocarbon- filled reservoir layer is 100 ohm-m, the resistivity of the shale layer is 0.1 ohm-m, and the background resistivity is 1 ohm-m. This leads to the following parameters: Rv=10.81 ohm-m Rh=0.55 ohm-m According to equations 1,2, and 3 , the anisotropy value will be A=19.47

Example 4

A thick salt body will have high values of both vertical and horizontal resistivity but the anisotropy values will be close to unit.

Example 5

A thick volcanic rock body will have high values of vertical and horizontal resistivity but the anisotropy values will be close to unit.

Example 6

A thick, non permeable carbonate rock body will have high values of vertical and horizontal resistivity but the anisotropy values will be close to unit.

To summarize from the above examples, a sequence of relatively thin shale and hydrocarbon-filled reservoir layers will have high values of anisotropy. This scenario is typical for many petroleum reservoir settings. Anisotropy is a parameter which can discriminate between thin and thick resistive rock bodies.