On a global scale, the total continental margin (shelf and rise) 

 covers more than 20 percent of the world's sea floor and comprises an 

 area half as large as the total land area of the world (Figure 3). The 

 total worldwide area of the continental shelf available for petroleum 

 development is estimated to amount to 1.4 million square miles (3,6 

 million km ). The width of this shelf varies from one coastal area to 

 another in the United States. The shelf can be several hundred miles 

 wide in the Bering Sea off Alaska; in the Gulf of Flexico, it is about 

 60 miles wide; it extends off the Atlantic coast for approximately 40 

 miles and narrows to 20 miles or less off the Pacific coast [7]. 



1.2.2 Exploration and Discovery 



The potential of an offshore basin for reserves is estimated by a 

 sequential process involving geological investigation and geophysical 

 and seismic surveys. The potential of a frontier area can be approximated 

 once the following data are known in order of importance: (1) the areal 

 extent and thickness (volume) of closed oil-bearing geological structures 

 (Figure 4); (2) the number of such structures; (3) the history of previous 

 oil or gas production; (4) the geological age of the rocks in the 

 structure; and (5) the depth to the potential reservoir (oil-bearing) 

 rocks [3]. 



An area in a known petroleum producing basin with large closed 

 structures has a significant potential for hydrocarbons and, if there is 

 an abundance of these structures, the area will continue to attract 

 exploration even after some of the structures are drilled and proved 

 dry. Frontier areas where no previous production has been recorded, 

 however, may support a significant initial exploratory effort, but if no 

 reserves are found, interest may decline rapidly. This is because the 

 investments demanded for geophysical surveys and exploratory drilling 

 are highly speculative--sometimes they pay off but more often there is 

 no return. 



Once exploratory drilling occurs, the speculative nature of a new 

 area is rapidly decreased. If paying quantities of oil are found, as 

 defined by a flow rate test (a "drill stem test") of the exploratory 

 well, the area's potential will be sharply upgraded in industry's view 

 and exploratory efforts may accelerate. 



If the flow rate test gives uninspiring results, other factors may 

 still indicate promise for the area. These factors, determined by 

 sampling cores in the oil-bearing formations, are the rock's porosity and 

 permeability. If both porosities and permeabilities are high, this 

 indicates that hydrocarbons can be easily extracted if they are present. 



After an exploratory hole has been drilled, it will be possible 

 to determine whether marketable oil or gas will be found in commercial 

 amounts. For instance, low viscosity and low sulphur content are more 



15 



