704 J. 11. GAfeDNER I'HiS Mlb-COJ^TINENT OIL FlELt)S 



Feet 

 of Baylor County. Correlates with limestone and 

 shale series known as the "Albany formation," in Bay- 

 lor County, Texas 1,250-2,000 



PcMnsylvanian series. 



1. Cisco group. Sandstone, limestone, beds of gray sandy 



shale, dark colored graj^ shale and conglomerate. Con- 

 tains coal 7. Includes upper oil and gas sands in Pe- 

 trolia and Electra fields. . .^ 800- 900 



2. Canyon group. Sandstone, dark-blue shale, conglomerate, 



coal, and beds of massive escarpment forming lime- 

 stone. Includes lower oil and gas sands in Electra and 

 Petrolia fields 800- 950 



3. Strawn group. Sandstone, clay, carbonaceous shale, and 



chert conglomerate. Includes Millsap formations, or 

 beds for 1,000 feet below coal number 1. Contains oil 

 and gas sands in Strawn, Moran, and Brownwood fields 950-3,700 

 ( Unconformity. ) 



4. Bend group. Mississippian V Hard blue limestone with 



blue and black shale. Occurs only in central coal field . 350- 375 

 ( Unconformity. ) 

 Silurian, Ordovician, and Camhrian limestone. 



As a rule the oil and gas sands in the Pennsylvanian series of the north- 

 ern Texas fields are similar in nature to those of the main fields in Kansas 

 and Oklahoma; they are sandstones of a light to dark gray color, with a 

 porosity varying from 10 to 25 per cent. They lie interbedded with more 

 or less carbonaceous shale, as do corresponding sandstones in the fields of 

 the Mid-Continent region farther north. The N"acatoch, Blossom, and 

 Woodbine sands of the Caddo field (Texas-Louisiana) lie in the Upper 

 Cretaceous series, as do the sands at Corsicana. The Lower Cretaceous, 

 or Comanche series, includes at its base the Trinity sand which furnishes 

 the oil in the South Bosque field. These sands of the Cretaceous are nat- 

 urally not so hard as the strata of the older Pennsylvanian and, together 

 with all the intermediate beds, are drilled by the rotary system. 



Certain Texas fields have been well described by E. W. Shaw and Oeorge 

 C. Matson in Bulletin Number 609 of the United States Geological Sur- 

 vey, entitled "Natural Gas Eesources of Parts of Northern Texas." Mat- 

 son states that the Nacatoch sand in the Mexia-Groesbeck field is a light 

 gray, fine quartz sand, carrying many dark grains of glauconite. Tests 

 by C. E. Van Orstrand, of the Geological Survey, from examples sub- 

 mitted by Matson, showed an average porosity of 25.5 per cent. This 

 figure is probably a fair average for the Cretaceous sands of this region. 

 The oil-bearing formation in the Thrall field occurs in a vesicular, soft, 

 ffreen serpentine, found in the Taylor marls at a depth of 850 feet. From 



