Miscellaneous Subsurface Methods 621 



that the recovery amount and the formation depth can be recorded. By 

 visual examination and measurement, a detailed description of the core 

 is made, the formation composition, the texture, the probable geologic age, 

 the dip of formation beds, and the probable fluid content being observed. 

 If indications are that the core may contain oil or gas, further field and 

 laboratory tests are usually made. Should a core contain oil or gas, the 

 visual examination can usually determine only what section may be a 

 potential producing zone. A special coring method must be used to de- 

 termine the direction of the dip of formation beds, and obviously such a 

 determination may be very valuable in locating the probable direction in 

 which a structure may be located. 



Field or proved-area geologic problems usually resemble those en- 

 countered in exploratory work to some extent. The principal difference 

 is that proved-area geologic work generally is directed toward obtaining 

 data that may be used to evaluate a known pay zone or locate a formation 

 marker. When an exploratory well encounters an oil- or gas-bearing zone, 

 the geologic problems then resemble proved-area problems. In proved 

 areas, the geologist may use coring and core data to aid in (1) determining 

 the amount of pay section present and (2) estimating the amount of oil 

 and gas in place. 



Coring in Relation to Production Work 



Frequently coring and core data can be used to an appreciable ad- 

 vantage in production operations and petroleum-engineering work, in ad- 

 dition to helping the geologist with his various problems. Probably the 

 greatest use of coring in production work is for determining zones that 

 should be formation- or production-tested and for determining, if possible, 

 the gas-oil and water-oil contacts in those zones. Also, at times in pro- 

 duction operations, it may be desirable to make an open-hole completion; 

 that is, one in which the oil-string casing is set above the pay section to 

 be tested and produced. Coring can be used to aid in determining the 

 presence of undesirable upper sloughing shales and water-bearing zones 

 immediately above the pay zone. The casing seat may then be picked at 

 a point that would shut off" the undesirable formations and yet allow the 

 entire desired section to be tested. Information about the texture of the 

 formation in a prospective pay zone usually aids in picking the most de- 

 sirable type of completion method and helps determine the possibility of 

 sand production problems. At times diamond coring can aid in opera- 

 tions in areas where very hard and abrasive formations are encountered, 

 because it may be found that coring is more economical than drilling. 



Certain petroleum-engineering work is aided greatly by core-analysis 

 data. An understanding of the formation characteristics and composition 

 may lead to the location of (1) oil zones that subsequently might be over- 

 looked and (2) impermeable zones that may aid greatly in workover 

 operations to shut off" undesirable water or free gas. Core data almost 



