Geology and Geochemistry of Santa Rosa Tar Sands
Published:January 01, 1987
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 man-made 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.
Figures & Tables
Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.