OIL AND GAS LANDS. 115 



Where the migration of oil is due to the pressure of dissolved or 

 occluded gas in the absence of water saturation the oil will move in 

 all directions until it is stopped by some impervious stratum, where 

 accumulation takes place in apparent disregard of structure. 



The simplest structure favorable to the accumulation of oil and gas 

 is that of a symmetrical anticline having little or no pitch of the axis 

 and moderately dipping flanks. If the requisite condition of porous 

 oil-bearing rock adequately sealed by impervious beds is fulfilled and 

 the strata are impregnated with water under moderate hydrostatic 

 pressure, the hydrocarbons will, under ideal conditions, segregate in 

 the axis of the fold and extend down the flanks a distance dependent 

 on the quantity present. Farther down the flanks and in the troughs 

 of the corresponding synclines water will as a rule be found. It is 

 evident that, other things being equal, the extent of the productive 

 area controlled by anticlinal structure is greater where the fold is 

 broad and the dip of the strata on the flanks relatively low than where 

 the fold is narrow and has steep flanks, for in the former case the 

 gathering ground for oil and gas is much greater than in the latter. 

 From the simple symmetrical anticline there are gradations on the 

 one hand into domes pitching away from a central point and on the 

 other hand through unsymmetrical folds to an extreme type in which 

 one flank is vertical or overturned. In every symmetrical fold the 

 boundary between an oil pool lying at the top of a fold and extending 

 part way down the sides of the fold and the water saturating the 

 rocks farther down is an approximately horizontal line, because as 

 long as the fold is regular the water tends to rise to the same level 

 all along it. If, however, there are minor irregularities on the 

 sides of the fold these have their effect on the distribution of the oil, 

 making the margin of the pool irregular or causing small pools to 

 collect along the slope. 



Structural features of other types are under certain conditions 

 favorable for the accumulation of petroleum and natural gas. Among 

 these may be mentioned monoclines, which present conditions favor- 

 able for the concentration of oil wherever there is a change in the rate 

 of dip or an abrupt change in the strike of the rocks, shallow syn- 

 clines where water is absent from the oil-bearing zone, and synclines 

 where the oil and water are of nearly the same gravity. Unconformi- 

 ties where steeply dipping petroliferous strata are overlain by rela- 

 tively impervious horizontal or nearly horizontal beds are also favor- 

 able. Faults are usually considered wholly unfavorable to the accu- 

 mulation of oil and gas, and for areas where the dislocations are many 

 and extensive this view is undoubtedly correct. However, in many 

 places faults have quite the contrary effect. For instance, strike 

 faults may cause a greater concentration of petroleum toward the 

 crest of a fold, and dip faults in a series where there are many oil 



