Subsurface Geologic Methods 



permeability, composition, texture) change rapidly both vertic- 

 ally and horizontally. Reef trends and development are not con- 

 stant; thus, it is necessary cautiously to outline drilling and leas- 

 ing programs. Acidizing, shooting, and formation testing are 

 critical subsurface problems in reef production problems. 



3. Secondary Recovery: Oil companies today are concerned more 

 than ever before with the problem of obtaining greater yield of 

 oil and gas per acre. Repressuring, water flooding, acidizing 

 and shooting of wells, directional drilling, proper water shut- 

 offs, systematization of drilling, proper well spacing and con- 

 trolled production are of major concern in secondary recovery 

 programs. The subsurface geologist and engineer must com- 

 bine their efforts to secure best results in secondary recovery 

 problems. 



4. Interpretation of Well-logging Data: Since 1930 the electrical 

 log has been very successful in evaluating features of the pene- 

 trated strata. Since this date radioactive, caliper, thermal, drill- 

 time, induction and micrologging have been introduced into 

 subsurface investigations. Information obtained from these 

 logs is based on the characteristics of the rocks — their composi- 

 tion, texture, and fluid and gas content. Many profile anomalies 

 cannot be adequately explained; thus, more attempt should be 

 made to evaluate these idiosyncrasies by detailed analysis. 



5. Acidizing and Shooting: What interval to acidize and to shoot 

 is a problem of major concern to operators drilling in carbonate 

 sections. Before either or both methods are initiated, the lith- 

 ologic and structural aspects of the strata should be adequately 

 known and this information integrated into other logging data. 



6. Prediction of Drilling Difficulties: The oil operator and con- 

 tractor are vitally interested in knowing the difficulties and 

 magnitude of difficulties prior to commencement of drilling. A 

 sandstone-shale section would present different problems than 

 a carbonate section or a section containing numerous beds of 

 salinifereous material. A rock sequence containing considerable 

 bentonite might alter an entire drilling and casing program. 

 Other problems may be encountered during penetration of fault 

 surfaces, unconformities and vuggulated strata in which circula- 

 tion could not be maintained. 



7. Improved Subsurface Mapping: Subsurface data may be con- 

 veniently represented by contour-type maps. These maps are 

 based on structural, isopachous, isochor, isothermal, isosperm, 

 isochron, isopotential, lithofacies, and depth pressure informa- 

 tion. Viscosity, fluid and gas density maps may also be pre- 

 pared. Paleogeologic data in many areas are commonly plotted 



