Rio Blanco Oil Shale Company Tract C-a, Rio Blanco County, Colorado: Summary of geology and current development
— E. A. Ziemba

A generalized stratigraphic column of Tertiary Eocene rocks in the Piceance Creek basin is shown in Figure 1. The column makes no attempt to maintain vertical scale representative of individual stratigraphic unit thicknesses. Its primary function is to portray the relative stratigraphic positions of the basin's major and minor Eocene units, the position of the basin's main oil-shale interval within the Parachute Creek Member, Green River Formation, and the oil-shale interval as defined at Tract C-a. Also shown are key markers, both lithologic and electric log, which are important in oil-shale stratigraphic correlations.

 

Figure 2 is a southwest-northeast stratigraphic cross section across Tract C-a utilizing oil-grade logs of three of the 27 coreholes drilled on the tract. It is oriented approximately normal to depositional or isopach strike of the main oil-shale interval of the Parachute Creek Member. The section portrays the tract's principal stratigraphic oil-shale characteristics, including the positions of four key electric log markers (A- and B- grooves, Blue and Orange markers) and two key lithologic markers (Mahogany marker and Mahogany bed). These six stratigraphic markers are not only areally persistent within Tract C-a but also throughout most of the Piceance Creek basin.

 

The oil-shale zonation established by Rio Blanco Oil Shale Company within Tract C-a also is shown on the cross section of Figure 2. It is based on a zonation first established by the U.S. Geological Survey within the main oil-shale interval (A groove to Blue marker) on a regional scale. The Rio Blanco zonation expands that of the U.S. Geological Survey stratigraphically above and below the main oil-shale interval resulting in 19 discrete oil-shale zones; 9 relatively rich and 10 relatively lean. These zones are designated L-00 through L-8 in stratigraphically ascending order with the alternating rich and lean zones identified "R" and "L", respectively. However, the Mahogany and A-groove (AG) zonal nomenclature is retained because of their well-established usage.