Electromagnetic surveying for hydrocarbon reservoirs

Abstract

An electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir, comprising obtaining first and second survey data sets with an electromagnetic source aligned end-on and broadside relative to the same or different receivers. The invention also relates to planning a survey using this method, and to analysis of survey data taken in combination allow the galvanic contribution to the signals collected at the receiver to be contrasted with the inductive effects, and the effects of signal attenuation, which are highly dependent on local properties of the rock formation, overlying water and air at the survey area. This is very important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes of structure.

1 . An electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir, comprising: 
 providing an electromagnetic source having a dipole axis and first and second detectors;    moving the electromagnetic source with the dipole axis of the electromagnetic source aligned end-on relative to the first detector so that a first survey data set is sensitive to resistive hydrocarbon layers exploiting largely galvanic effects; 
 moving the electromagnetic source with the dipole axis of the electromagnetic source aligned with the dipole axis of the electromagnetic source aligned broadside relative to the second detector so that a second survey data set is relatively insensitive to resistive hydrocarbon layers exploiting dominantly inductive effects;  
 moving the electromagnetic source relative to each detector to collect data over a range of source-to-detector distances; and  
 processing the first survey data set and the second survey data set.  
   
   
   
       2 . An electromagnetic survey method according to  claim 1 , comprising obtaining the first and second survey data sets concurrently.  
   
   
       3 . An electromagnetic survey method according to  claim 1 , wherein a right angle is formed between a first line leading from the first detector to the electromagnetic source and a second line leading from the second detector to the electromagnetic source.  
   
   
       4 . An electromagnetic survey method according to  claim 1 , further comprising selecting the electromagnetic source to be a mobile horizontal electric dipole source equipped with a streamed antenna which is towed above the seafloor to obtain the first and second survey data sets.  
   
   
       5 . An electromagnetic survey method according to  claim 1 , wherein each detector is static while the first and second survey data sets are obtained.  
   
   
       6 . An electromagnetic survey method according to  claim 5 , further comprising placing each detector on the seafloor while the first and second survey data sets are obtained.