GEOLOGIC AND ECONOMIC BACKGROUND 33 



faulting, of a combination of the two, of differential compaction of sedi- 

 ments over buried hills, of the intrusion of igneous rocks, or of the forma- 

 tion of salt domes. If of stratigraphic origin, it may be the result of the 

 lensing out or shaling out of sands, of old shore lines, of the deposition of 

 sands against old land masses, of the removal of sand by erosion and sub- 

 sequent roofing by deposition of relatively impervious formations or of 

 various combinations of stratigraphic processes. If of chemical origin, the 

 trap may be the result of solution, of cementation, or of metamorphic alter- 

 ation which may involve both processes." 



The more common forms of oil-bearing structures are listed in Table 2. 



TABLE 2 



CLASSIFICATION OF OIL STRUCTURES WITH RESPECT TO 

 APPLICATION OF GEOPHYSICAL METHODS 



I. STRUCTURAL TRAPS 



A. Folded Structures 



1. Anticlines and domes 



2. Salt domes 



3. Domes or anticlines over igneous intrusions 



4. Domes or anticlines over buried ridges 



a. Folds due to uplift 



b. Folds due to differential compaction of sedimentary beds 



5. Noses 



B. Homoclinal Structures 



\. Homoclines cut by igneous dikes 



2. Outcropping homoclines sealed by paraffin or asphalt 



3. Homoclines sealed by local cementation of reservoir rock 



C. Faulted Structures 



\. Faulted anticlines 

 2. Faulted homoclines 



II. STRATIGRAPHIC TRAPS 



A. Unconformities 



B. Lateral variations in lithology (sand lenses) 



C. Lateral variations in porosity and permeability 



in. MISCELLANEOUS 



A. Combinations of two or more of the above structural features 



B. Differential porosity on the flanks of folded structure 



C. Accumulation in faults, fissures, joints, etc. 



Throughout the world there are certain areas or regions (generally 

 sedimentary basins) in which major accumulations of oil occur and in 

 which have been developed the major producing oil fields of the world. 

 The petroleum accumulations (oil fields or districts) in each region are 

 usually related genetically, and structural conditions in the various districts 

 are often similar and typical of the region as a whole. Such regions are 

 known as petroliferous provinces. Examples of petroliferous provinces 

 in the United States are the Permian basin of southeastern New Mexico 

 and the Gulf Coast province of Texas and Louisiana. f 



t Walter A. Ver Wiebe, OiV Fields in the United States (McGraw-Hill, 1930). 



