48 



Subsurface Geoiogic Methods 



Figure 19. "Shoestring-sand" type oil field; Bush City field, Anderson County, 

 Kansas. Cross section in A shows flat top and bottom convex downward of a 

 typical stream-channel deposit. Plan in B shows windings of an ancient stream 

 channel. 



uplift are responsible for the vast quantities of oil in the East Texas 

 field. The Woodbine formation of Upper Cretaceous age has an uncon- 

 formity at the top of the formation and is overlain nonconformably by the 

 Eagle Ford in the Mexia and Balcones fault zones and by the Austin chalk 

 in east Texas (fig. 21). The base of the Woodbine is marked by another 

 unconformity, below which are beds of the Comanche group. These two 

 unconformities are of regional extent and intersect on the flank of the 

 Sabine uplift in east Texas. Oil from the Woodbine is found in the fault 

 zone, where it is trapped at a fault surface and below the upper uncon- 

 formity. Figure 21 is an idealized west-east section through the Tyler 



Figure 20. Idealized diagram of Urania field, Louisiana, where production comes 

 from oil trapped in the highs of old erosion surface disconformably overlain by 

 impervious shales. Both the Cane River and Wilcox formations are Eocene. 



