Subsurface Laboratory Methods 283 



Jurassic 



Marine Jurassic beds include the important Sundance sandstone of 

 Colorado and central and eastern Wyoming, the Nugget of western Wyo- 

 ming and western Colorado, and the Ellis formation of Montana. The 

 Sundance is an important oil producer in Colorado at lies, Moffat, and 

 Wilson Creek and in Wyoming at Salt Creek, Lance Creek, Big Medi- 

 cine Bow, Rock Creek, and other localities. Oil production at Kevin- 

 Sunburst, Montana, comes from the zone at the contact of the Ellis for- 

 mation and Madison limestone, much of which is from the reworked 

 Madison limestone. 



The Morrison formation overlies the Sundance in Colorado and Wyo- 

 ming and represents the transition zone from Lower Cretaceous to the 

 marine Jurassic. Its waters are included in this section. The Morrison 

 produces oil at Wilson Creek, Colorado, and in extent and productivity 

 outranks the Sundance. Scattered showings of oil and gas have been en- 

 countered elsewhere in the Morrison formation. 



Morrison waters in Colorado range from soft, alkaline types to saline 

 waters containing appreciable hardness. They vary in concentration from 

 about 3,000 to 15,000 parts per million total solids and usually contain 

 appreciable amounts of sulphate. Most of the Sundance waters of Colo- 

 rado do not contain sulphate — by which they can be distinguished from 

 Morrison waters — and the alkaline and moderately dilute waters are 

 usually soft. The saline waters generally contain appreciable hardness 

 and in many respects resemble Morrison waters. 



The Ellis waters of Montana are quite uniform over the northern 

 portion of the plains and consist principally of sodium chloride and 

 sodium bicarbonate with minor but persistent secondary characteristics. 

 These waters almost invariably contain hydrogen, sulphide, and, with the 

 exception of Cosmos-Vanalto waters at Border-Red Coulee, are the young- 

 est waters of the state to carry hydrogen sulphides. They average about 

 3,500 parts per million total solids. 



The Sundance waters of Wyoming are rather variable in concentration 

 and composition. They range from a low of about 1,200 parts per million 

 total solids at Big Medicine Bow to as high as 40,000 parts per million at 

 Steamboat Butte, where the water takes on evaporite characteristics similar 

 to Triassic waters. The Sundance is the youngest formation in Wyoming 

 in which secondary salinity becomes an appreciable and persistent part of 

 the chemical system, yet secondary salinity is present only in a few fields 

 such as Alkali Butte, the third sand at Salt Creek, the basal sand at Lance 

 Creek, and other scattered localities. It is believed that secondary salinity 

 is persistently present in Sundance waters in those beds in which limestone 

 predominates; where sandstone predominates, the water is of the same 

 general character — relatively soft, with little or no sulphate — as Cretaceous 

 waters. Thus, it is inferred that the characteristics and composition of for- 



