2.2.1 Geophysical Surveying 



The initial step in searching for potential petroleum deposits is 

 to analyze data about geologic characteristics of an area, derived 

 through a geophysical survey. The prime objective of that analysis is 

 to identify and locate reservoir rocks and structures (traps) in which 

 oil and gas could have accumulated. A knowledge of the subsurface is 

 also helpful in detecting near-surface conditions such as fault zones 

 (prevalent off California and Alaska) which pose possible hazards to 

 exploration and subsequent production operations. 



Description 



The seismic survey is the principal geophysical technique employed 

 by oil companies or their contractors for identification of potential 

 lease tracts that hold the most promise. Figure 9 schematically 

 illustrates the operation of a marine seismic system. During seismic 

 surveying operations, a ship with a crew of six to ten travels along a 

 predetermined path or grid towing signal -generating and recording 

 equipment. The signal generated by the energy source (usually air or 

 gas guns are used), results in a series of sonic pulses or seismic 

 waves, that travel through thfe water and are reflected and refracted by 

 the underlying rock formations. The returning sonic waves are detected 

 by hydrophones towed by the vessels and are recorded in digital format 

 on magnetic tape. The data is translated into vertical cross-sections 

 of each traverse. The cross-sections are then interpreted to determine 

 the presence of possible structural and stratigraphic traps. Subsurface 

 structure contour maps are prepared for selected formations which appear 

 promising. 



One method for analyzing seismic data covering selected geologic 

 formations that has received wide industry acceptance is the "bright 

 spot" technique. This technique has been credited with the direct 

 determination of oil and gas prior to drilling in young sediments with a 

 relatively simple geologic structure. This method is based upon locating 

 large variations in seismic reflections, the greater the difference in 

 velocity between two formations, the greater the amplitude of the re- 

 flected energy. As the velocity in a petroleum-bearing sandstone 

 (reservoir rock) is lower than either a water-bearing or non-porous 

 sandstone, the presence of petroleum-bearing sandstone will cause a two 

 to five fold increase in the amplitude of the reflected energy. By 

 processing the seismic data to highlight the true amplitudes of the 

 reflections, it is possible to directly identify petroleum-bearing 

 formations. The data displays "strong events" or "bright spots" when 

 abnormally broad contrasts in velocity are present. While the technique 

 has been successfully employed in certain areas, it is not applicable in 



all cases. 



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