Subsurface Laboratory Methods 313 



acteristics for the Edwards limestone along the Balcones fault trend in 

 south Texas vary from 15 to 35 percent. 



Where the oil exists in fractured limestone, core analyses have very 

 little value. 



Chalk and Serpentine Production 



Production from fractured chalk or serpentine occurs from the frac- 

 ture planes and not from the dense, impermeable portions of the forma- 

 tion. Analyses made on the Austin chalk in Frio County, Texas, have 

 proved of little value to the oil operator because the saturation exists in 

 fracture planes. Where the saturation occurs in solution porosity in these 

 formations, the same techniques used in limestone analyses apply. 



Conglomerate Production 

 Analyses made by the writer on conglomerate cores from the north 

 and north-central Texas areas show very high permeabilities. The porosity 

 varies with the sorting and shape of the materials making up the conglom- 

 erate. Where the pebbles are of uniform size and cementation is at a 

 minimum, the porosity approaches a maximum. Usually, conglomerate 

 cores are thoroughly flushed by drilling water, and the estimation of the 

 connate-water saturation becomes a problem. Minimum-reserve calcula- 

 tions based on analyses of consolidated conglomerate may be made using 

 total water saturations as the connate water. 



Water Sands 



Water-productive sands associated with oil sands may be recognized 

 by low and irregular oil saturations in the transitional zones (fig. 127). 

 Hydrocarbons may not be present below the oil-water contact. The oil- 

 water contact of a uniformly permeable sand is defined as the level in 

 the sand column below which water alone will be produced. The total 

 water saturations of water sands are usually higher than those for com- 

 mercially productive sands and will approach 100 percent of the pore 

 space when hydrocarbons are absent. Sands that produce water contain 

 large amounts of "free" or noncapillary water. 



Transitional Zones 



Where a gas cap or bottom water is present in a thick sand column, 

 assuming an oil column exists, there are transitional zones from the gas 

 to the oil zones and from the oil to the water-productive zones. These 

 zones vary in thickness within the same reservoir. 



The gas-to-oil transitional zone may be recognized by an increase 

 in residual-oil content as the gas-oil contact is approached. It is advisable 

 to confine the completion interval to the oil zone and several feet below 

 the designated gas-oil contact in order to minimize the possibility of gas 

 channeling and high gas-oil-ratio production. 



The transitional zone from oil to water is usually characterized by 



