Pennsylvania| Carboniferous | | | Ohio, East |Ordovician- |Paraffin | 7,285,005| 463,367,386 and West | Carboniferous | | | Oklahoma |Pennsylvanian |Paraffin |103,347,070| 851,320,457 Texas |Pennsylvanian, |Asph.-Par.| 38,750,031| 327,550,005 | Cretaceous-Quat.| | | Utah | -- | -- | (b) | (b) West |Devonian- | -- | 7,866,628| 294,474,710 Virginia | Carboniferous | | | Wyoming |Carboniferous- |Asph.-Par.| 12,596,287| 40,019,573 | Cretaceous | | | Other | -- | -- | 7,943| 112,925 | | |-----------| ------------- | | |355,927,716| 4,608,571,719 -------------+-----------------+----------+-----------+--------------- (a) Included in "Other." (b) Included in Wyoming. [Illustration: FIG. 7. Chart showing the present tendency of the United States in respect to its unmined reserve of petroleum. Data from U.S. Geological Survey. After Gilbert and Pogue.] =Methods of estimating reserves.= It may be of interest to inquire into the basis on which predictions are made of the life of an oil pool. The process is essentially a matter of platting curves of production, and of projecting them into the future with the approximate slopes exhibited in districts which are already approaching exhaustion.[21] While no two wells or two districts act exactly alike, these curves have group characteristics which are used as a rough basis for interpreting the future. [Illustration: FIG. 8. The annual output of the principal oil fields of the United States for the last twenty years. Data from U.S. Geological Survey.] A less reliable method is to calculate from geologic data the volume and porosity of the oil-bearing reservoirs, and to estimate the percentage of recovery on the basis of current practices and conditions. Complete data for this method are often not available; but in the early years of a field, before production curves are established, this method may serve for a rough approximation. [Illustration: FIG. 9. Curve showing the usual decline in oil field production after the period of maximum output is reached. After Ralph Arnold. The Petroleum Resources of the United States, Smithsonian report for 1916, p. 283. Compare this theoretical curve of final decrease with the production curve shown in Fig. 8.]