This paper describes the reservoirs of three oil fields, Ain Zalah and Kirkuk in Iraq, and Dukhan in Qatar, Persian Gulf. They were chosen because they show fractures playing entirely different roles in the holding and producing of oil.

Ain Zalah is a field where the reservoir rock is extremely tight and, though it has a little porosity, has practically no permeability; fractures are all-important and without them there would be no production; they both hold and yield the oil, and large productions can be drawn from individual wells for limited periods.

Kirkuk is a field of very high average porosity but of much more varied permeability, this depending directly on stratigraphy. But the reservoir limestone, exceedingly complex stratigraphically and lithologically, is faulted and intensively fractured and jointed, and thus extreme freedom of movement of reservoir liquids is permitted, giving enormously high continuously sustained production from wells and almost instantaneous transmittal of pressure drop over great distances.

Dukhan is a field where the limestones are moderately to highly porous and permeable with the exception of a few bands. Some jointing exists and plays a part in top-to-bottom drainage, but its effect is rather minor, and maximum flow rate of individual wells is less than that of the best Ain Zalah wells (though much better maintained), and perhaps one fifth that of good Kirkuk wells.