Geology and geochemistry of Santa Rosa tar sands

Heavy oil deposits 11 km (7 mi) north of the town of Santa Rosa, Guadalupe County, New Mexico, in the Triassic Santa Rosa Sandstone, were mined during the 1930s. Renewed interest in this deposit in the late 1970s so far has not resulted in exploitation because of unfavorable economics and the encroachment of manmade Santa Rosa Lake. The Santa Rosa Sandstone in east-central New Mexico is from 35 to 108 m (115-355 ft) in thickness and consists of light-brown to gray, cross-bedded, medium- to coarse-grained sandstone, and subordinate brown and red mudstone. The sedimentary section below the Triassic includes strata of Permian, Pennsylvanian, and Mississippian age, with an aggregate thickness of about 1830 m (6000 ft) resting on Proterozoic basement. These rocks have a regional dip of a few degrees to the southeast. Organic geochemical analyses characterize the oil as a moderately immature, naphthenic oil that has undergone extensive bacterial degradation. Several geochemical properties point to the Permian San Andres Formation or Pennsylvanian strata as the most likely source rock. Migration of the oil may have taken place along sinkholes or faults, whereas distribution of the bituminous material within the sandstone has been controlled by local variation in porosity and permeability. Updip migration and bacterial action led to the accumulation of extra-heavy oil near the surface.