60 Subsurface Geologic Methods 



reservoirs into five major groups: (1) convex-trap reservoirs, (2) per- 

 meability-trap reservoirs, (3) pinch-out traps reservoirs, (4) fault-trap 

 reservoirs and (5) piercement-trap reservoirs. Examples for these types 

 are given in figures 31 and 32. 



In exploiting a potential area for oil and gas the geologist is expected 

 to consider the factors suggested as outlined below: 



I. Source rock 



(1) What rock types would probably serve as the most favorable source, 

 and what percentage of these types constitutes the total section? 



(2) What are the sedimentational and structural relationships between 

 possible source strata, reservoir strata, and entrapment conditions? 



(3) Is permeability sufficient to permit migration from the source strata 

 to associated porous types? 



(4) What is the thickness, organic variation, and distribution? 



II. Reservoir rock 



(1) What rock types are most favorable (sandstone, fractured limestone 

 or shale, cavernous limestone and dolomite, etc.) ? 



(2) What relationships exist between the reservoir rock and the general 

 and local structural and stratigraphic fabric? 



(3) Could the strata qualify as both a source and a reservoir rock? 



(4) What about thickness, distribution, and uniformity of lithology and 

 permeability? 



(5) What are the time and accumulation relationships to erosional sur- 

 faces ? 



(6) Are the strata within reach of economic drilling? 



III. Entrapment conditions 



(1) Are the entrapments controlled by sedimentational or structural irreg- 

 ularities or both? If go, to what extent? 



(2) What are the relationships between the trap feature, the source rock, 

 and the reservoir? 



(3) What is the extent of the trap? 



IV. Geologic history 



(1) What is the sedimentational history of the area? 

 (a) Period and extent of oscillations. 



(6) Sedimentational trends developed during oscillation stages. 



(c) Source direction of sediments. 



(d) Diagenetic changes. 



(e) Ratio and variations of lithologies in the section. 



(2) What is the structural history of the area? 



(a) What effect did structural adjustment during sedimentation have 



on the development of the stratigraphic pattern? 

 (6) When did the major periods of folding and erosion occur? 



Evaluation of the preceding questions may not be based entirely on 

 surface data. In this case, it becomes the responsibility of the subsurface 

 geologist to integrate information obtained from drilling and from geo- 

 physical results. 



Correlation Considerations 



There is constant demand from the stratigraphic geologist for ac- 

 curate evaluation of sedimentary units, their lithologic, paleontologic, 



