462 



over the water, and the natural gas at the very top, all in the pores and 

 cavities of the host rock. 



The salt domes of the Gulf of Mexico are cylindrical bodies of salt 

 that have been squeezed from their parent formations, piercing upward 

 through a succession of overlying sedimentary layers (see sketch B, 

 Fig. 2). Against the walls of a salt dome the oil-bearing layers are 

 closed off, providing a reservoir and a seal for trapping the oil. Such 

 diapiric structures have been known to occur farther offshore and in 

 deeper waters in the Sigsbee Abyssal Plain beneath the floor of the 

 Gulf of Mexico. Some structures have protruded above the flat surface 

 of the Plain and are termed "knolls." Those without surface expres- 

 sion are "domes." Recent seismic discoveries have revealed the pres- 

 ence of diapiric structures in numerous offshore locations around the 

 world. 34 The Sigsbee Knolls were discovered in 1954 by the Vema, the 

 research vessel of Columbia University's Lamont-Doherty Geological 

 Observatory. In 1968, Lamont scientists on board the Glomar Chal- 

 lenger drilled through the cap rock of the Challenger Knoll in 11,720 

 feet of water and recovered a core 472 feet long. The sediments in the 

 core contained oil and were similar in composition to other salt domes 

 on shore and offshore. 35 



Except for this type of reservoir, almost all production areas off- 

 shore are geologically related to fields on shore. Since the continental 

 margins are essentially submerged edges of the continents, knowledge 

 of petroleum habitats on land can be applied offshore with considerable 

 certainty. 



Beyond these near-shore areas, the petroleum potential of the outer 

 continental shelf and slope has been little investigated. But there are 

 indications that the presence of petroleum source beds is very likely in 

 the continental slope, and progressively less beyond the slope into' the 

 abyssal plains and oceanic deeps. Exceptions of course can be found 

 in depositional ocean basins with great sedimentary accumulations and 

 salt-dome structures like those described above. However, as with 

 other ocean resources, the exploration and exploitation of petroleum 

 resources of the seabed depends on to the technological capability 

 and economic feasibility for future development. 



OTHER SUBSURFACE DEPOSITS 



Deposits of sulphur, coal, salt, potash, and oil shale are known to 

 occur on the continental shelves of many seas. Sulphur has been mined 

 from salt-dome structures in the Gulf of Mexico, and similar structures 

 are known in the Arabian Gulf, the Red Sea, and the Caspian Sea. 



In the Gulf of Mexico, sulphur occurs in considerable quantities in 

 the rock formations capping the salt domes. Several of these domes 

 are now being mined on land, and an elaborate mining operation is 



3 * E. D. Schneider and G. L. Johnson. "Deep-ocean dlapir occurrences." American Asso- 

 ciation of Petroleum Geologists Bulletin (November 1!)70), pages 2151. 2169. See also: 

 H. K. Wong, et al. "Newly discovered group of diapiric Structures In Western Medlter- 

 ^"^nn'' American Association of Petroleum Geologists Bulletin (November 1970), pages 



88 M. Ewing, et al. Site 2." In "Initial Reports of the Deep Sea Drilling Project," Vol. I. 

 i; Iul i e o„ 1!)(59)l I ja K es 84-111. (Lamont Doherty Geological Observatory Contribution 

 No. 1364.) 



