OIL AND GAS LANDS. 113 



such as compact shales, clays, or fine-grained sandy beds saturated 

 with water. 



The accumulations within the United States may be divided 

 roughly into three classes, as indicated below. 



1. Those occurring in strata of sandstone or limestone bounded 

 above and below by rocks comparatively impervious to oil. The 

 sandstone or limestone may be of broad or of very narrow extent, 

 in some places comprising merely small lenses of porous material 

 embedded in relatively impervious rocks, in others underlying hun- 

 dreds of square miles of territory in comparatively regular beds. 

 To this class belong the greater number of oil accumulations of this 

 country. 



2. Those occurring in porous strata, apparently lenticular, asso- 

 ciated with the " salt domes " of the Gulf Coastal Plain. 



3. Those occurring in fissures in shale, as in the Florence field of 

 Colorado. The fracturing of other rocks, such as limestone and 

 sandstone, affords favorable conditions for the accumulation of oil; 

 but sandstone and, under certain conditions, limestone are capable 

 of storing oil without fracturing, the fracturing merely increasing 

 their capacity. A fine-grained shale, on the contrary, although 

 capable of containing oil, does not permit its migration through the 

 rock mass with sufficient rapidity for collection in wells unless the 

 shale is broken by fissures, which serve as channels or reservoirs for 

 the slowly migrating oil. 



It may be stated as a fundamental principle that important accu- 

 mulations of petroleum and natural gas are to be found only in 

 stratified or sedimentary rocks. Eegions in which the strata have 

 been greatly disturbed or altered by intrusions of igneous rock are, 

 as a rule, unfavorable to the accumulation of petroleum, because the 

 attendant heat and fracturing would as a rule have had disastrous 

 effects on volatile substances of this character. An interesting appar- 

 ent exception has been noted in Ventura County, Cal., where oil to the 

 extent of 5 or 6 barrels a day has been obtained from wells drilled 

 in close-textured crystalline schist. Although the schist is underlain 

 by granite, it is overlain at a distance of only a few hundred feet 

 from the weUs by Tertiary rocks which in the same general region 

 are petroleum bearing. These relations suggest that the presence of 

 the oil in the schist is due to infiltration from the Tertiary sediments 

 through fractured zones rather than to origin in the sediments that 

 were metamorphosed to form the schist. 



In general, then, oil is found in sedimentary strata of greater or 

 less extent and regularity. These strata were originally deposited 

 by water in the ocean, in fresh-water lakes, or on great deltas prac- 

 tically at sea level. The beds were therefore horizontal, or nearly 

 78894°— BuU. 537—13 § 



