460 



to a depth of 1,000 feet, nearly two million square miles of the shelf 

 areas are geologically favorable for petroleum occurrence. It could 

 be safely assumed that nearly every coastal nation has offshore areas 

 that are favorable for petroleum accumulation. The imprecision in the 

 estimates is largely due to the lack of adequate data, and the lack of 

 knowledge necessary for projecting and predicting these estimates. 

 This knowledge depends basically on understanding the origin of 

 petroleum and the factors required for its accumulation. Several the- 

 ories exist on the origin of petroleum, but the one that has been most 

 widely accepted holds that the origin of hydrocarbons is organic. The 

 hydrogen and carbon originated from the remains of plant and animal 

 life that existed millions of years ago in former seas or swampy 

 environments. Such life forms were presumed to have been very small, 

 probably microscopic. Support for this theory is derived from inter- 

 pretation of the geological records, and studies of oil fields and oil- 

 bearing formations that have already been explored and developed 

 throughout the world. 



After having been formed, oil accumulates in reservoirs formed by 

 sedimentary layers called "formations." The mountains and the sur- 

 face of the earth are slowly broken down into smaller fragments 

 and particles. These eroded sediments are carried by the rivers 

 and deposited into the seas. As the millions of years pass, bodies 

 of sand, silt, and mud gradually build up in the coastal areas border- 

 ing the continents. The weight of these sediments forces the ocean 

 floor downward, warping it into a trough in which more sediments are 

 deposited. 



This pressure results in two distinct processes : One is deformation of 

 these layers; the other is their transformation into hard rock. 



As the layers are compressed, the oil accumulated in the sediments is 

 forced to migrate into pervious sand bodies with pore spaces between 

 the particles that facilitate the mobility of the oil. Meanwhile, com- 

 paction and particle cementation have turned the loose sediments into 

 rocks, the sand becoming sandstone, the silt siltstone, and the mud 

 mudstone or shale. While sandstone is the ideal medium to contain the 

 oil, limestone, and other porous rocks also are often oil-bearing. The 

 shale or mudstone is the ideal rock to seat it. So, in order for the oil to 

 stay in the pervious sandstone to which it has migrated, it has to have 

 an impervious layer over it to check its migration. 



The earth's crust is mobile and dynamic, forever on the move. These 

 movements result in deformation which manifests itself in the form 

 of uplifted mountains, dovvnwarped valleys, and twisted and contorted 

 sedimentary strata. The beneficial part of this upheaval is the creation 

 of structural forms which provide the traps and reservoirs necessary 

 for the containment of oil. The types of t raps where oil has l>een found 

 are numerous; however, the most ideal structure is a dome called "anti- 

 cline" (see sketch A. Fig. 2) . 



In an ideal situation, a reservoir would be a closed sequence, of 

 sedimentary rocks, including a layer of oil-bearing sandstone capped 

 with a layer of shale or mudstone. The contents of this reservoir in- 

 clude some water left over from the former seas, the oil body floating 



