854 Subsurface Geologic Methods 



3. Modifying structural features (including faults and buried hills and 

 ridges). Nature, size, and amount of closure (if any). Trends. 



4. Changes in structure with depth. Times of subsidiary folding and 

 faulting and modification of earlier structures by later deformation. 

 Isopach maps. 



5. Brief statement regarding relation of production to regional and local 

 structures. 



V. Paleogeology. 



1. Geologic history of area. Influence on sedimentation. Evolution of 

 present structure. Subsurface areal and structure maps below uncon- 

 formities contrasted with surface-geology maps. 



2. Times of igneous activity. Types of intrusion and extrusion and their 

 distribution. 



3. Degree of metamorphism of various horizons. 



VI. Occurrence of oil and gas. (If several subprovinces, consider each separately.) 



1. Types of traps. Describe each and illustrate by producing fields. Rela- 

 tive importance of each. Relations to structure. 



2. Methods of prospecting. 



3. Barren structures and possible reasons therefor. 



4. Reservoir horizons. Number, age, relative importance, character, thick- 

 ness, continuity, porosity, permeability, kinds of cement, thickness, and 

 degree of saturation of each and amount and character of occluded 

 water present. 



5. Water horizons. 



6. Reservoir pressures. 



7. Composition of edge water. 



8. Possible sources of oil and gas and probable time or times of ac- 

 cumulation. 



9. Possibility of applying carbon-ratio theory. 



10. Possibility of extending producing areas or discovering new produc- 

 ing horizons. Indicate most promising areas and ages of prospective 

 horizons. 

 VII. Description of several typical pools illustrating each important mode of oc- 

 currence. 



1. Location, date of discovery, exploration methods employed, etc. 



2. Geography and physiography. 



3. Surface and subsurface geology. Contour maps and cross sections. 



4. Type of trap. 



5. Producing horizons. Character, extent, and depths of each. 



6. Grade of oil. 



7. Methods of drilling. 



8. Depth of drilling. 



9. Completion and production techniques. 



10. Water drive, gas-cap drive or dissolved-gas drive. 



11. Secondary-recovery operations. 



12. Conservation practices. 



13. Ultimate recovery per acre anticipated for each horizon. 

 VIII. Production statistics and reserves, using latest data available. 



IX. Bibliography. 



Questions 



1. What are some of the responsibilities of the subsurface geologist? 



2. Discuss ditch sampling, coring, and preparation of core samples for 

 analysis and shipment. 



3. Give an example of organizing a core description. 



4. What is meant by formation testing and how is the information 

 recorded? 



5. What solvents are used for testing the presence of oil in a sample? 



