Straticraphic, Structural, and Correlation Considerations 63 



3 





D H 



Figure 29. A, B — Primary and secondary porosity and permeability frequently found 

 in limestones and dolostones. Such conditions favor oil and gas concentrations. 

 These accumulations are difiBcult to discover and develop and their reserves diffi- 

 cult to predict. C-F — Typical stratigraphic traps. C — progressive overlap. D — 

 unconformity. E — updip lensing. F — sand lentils. G — Accumulation in fold frac- 

 tures of relatively impermeable strata. H — Updip tar seal. 



