46 Subsurface Geologic Methods 



the sea traps within its sediments a new assemblage of fossil flora and 

 fauna. Great breaks in the physical records are accompanied by breaks 

 in the biologic record. Fossils are, therefore, the most dependable means 

 of detecting the time break necessary to establish an unconformable 

 break in the geologic column. 



Subdivisions of the Geologic Record: The greatest breaks in the geo- 

 logic record result from widespread continental emergence. These may 

 be accompanied by large-scale mountain building with much deformation 

 of previously formed sediments. These revolutions separate geologic time 

 into eras and are marked by worldwide unconformities. Within each era 

 are periods of crustal disturbance strong enough to cause widespread re- 

 treats of the seas from the continent masses. These produce breaks in the 

 geologic record, which are widespread but not universal. The unconformi- 

 ties produced by those breaks separate rocks into systems, and the corre- 

 sponding time units called periods. Smaller and more local breaks divide 

 periods into epochs and systems into series of rocks. 



Oil Reservoirs. A number of conditions may be responsible for oil 

 accumulation near unconformable contacts. Soluble rocks such as lime- 

 stone when exposed to erosion may become porous by solution. Subse- 

 quent deposition of an impervious shale above the unconformable con- 

 tact may trap oil in large quantities in the limestone. The West Edmond 

 field of Oklahoma produces from the Bois d'Arc limestone member of 

 the Hunton formation. An unconformity overlain by Pennsylvanian 

 shales seals the oil, which is trapped in truncated Bois d'Arc limestone. 

 This limestone was made porous by solution during the weathering of 

 the upturned formations. 



Insoluble residual materials may collect on an old erosional plain or 

 against the slope of an old high. These accumulations of porous mate- 

 rials form potentially good reservoirs. 



The rocks immediately above an unconformity are often shallow- 

 water sands and gravels. These may be highly porous and suitable for 

 reservoir rocks. 



"Bald-Headed" Structures: "Bald-headed" structures are anticlines or 

 domes that have been eroded so that the producing formations are removed 

 from the tops of the structures. Subsequent submergence and deposition 

 have sealed the truncated formations below the unconformity. Oil is 

 found in the flank sands of these structures. The Oklahoma City pool, a 

 cross section of which is shown in figure 18, is an excellent example of 

 this sort of pool. Simpson formations were stripped from the top of the 

 structure by pre-Pennsylvanian erosion. Production from this formation 

 was found on the flanks of the structure. The Billings dome in Noble 

 County, Oklahoma, was barren at the top owing to erosion of Simpson 

 formations. It was not until 17 years later that geologists recommended 

 drilling on the flanks of the structure, where large production was found 



