"The growing importance of the Sundance (Jurassic) formation as a source of oil in the Rocky Mountain area has prompted the making of a series of porosity determinations of core samples collected from t he reservoir sands of the Lance Creek oil field ,Niobrara County, Wyoming. These analyses are preliminary to a study of fluid and energy relations in the Basal Sundance reservoir of that field, and the determinations of porosity will be used in subsequent studies requiring calculations of the volumetric capacity of the reservoir and estimation of the reserves of oil and gas."