GEOCHEMICAL METHODS 

Carbon Analyses

	Concentrations of total carbon and carbonate carbon were determined by coulometry 
(Engleman and, others, 1985).  Carbonate in the untreated sample is reacted with perchloric 
acid to liberate CO2, which is then titrated in a coulometer cell to measure carbonate carbon.  
Total carbon is measured by liberating CO2 by combustion of an untreated sample and titrating 
the CO2.  Values of total organic carbon (TOC) were determined by difference between total 
carbon and carbonate carbon.  Replicate analyses demonstrate the coulometer technique has a 
precision of better than ±1% for both carbonate and total carbon.  Percent CaCO3 was 
calculated by dividing percent carbonate carbon by 0.12, the fraction of carbon in CaCO3.  

Reference:

Engleman, E. E., Jackson, L. L., Norton, D. R., and Fischer, A. G., 1985, 
Determination of carbonate carbon in geological materials by coulometric titration:  
Chemical Geology, v. 53, p. 125-128.

Inorganic Geochemical Analyses

	Concentrations of 10 major elements (Si, Al, Fe, Mg, Ca, Na, K, Ti, P, and Mn) 
were measured by wavelength-dispersive X-ray fluorescence spectrometry at the USGS, Denver 
(XRF; Baedecker, 1987).  Concentrations of 30 major and trace elements 
(Al, Fe, Mg, Ca, Na, K, Ti, P, Mn, As, Ba, Cd, Ce, Co, Cr, Cu, Ga, La, Li, Mo, Nb, 
Nd, Ni, Sc, Sr, Th, V, Y, Yb, and Zn) were determined by inductively coupled, argon-plasma, 
emission spectrometry (ICP; Baedecker, 1987).  For many samples (usually those with high 
concentrations of CaCO3) concentrations of As, Cd, Mo, Nb, Nd, Pb, Sc, Th, and (or) Yb 
were below the limits of detection.  
	
Reference:

Bedecker, P.A. (Editor), 1987, Geochemical methods of analysis.  U.S. Geological Survey 
Bulletin 1770, 129 pp.

Carbon and Oxygen Isotope Analyses

Stable-carbon isotope ratios were determined on splits of the carbon samples from the 
Bridge Creek Limestone Member of the Greenhorn Formation using standard techniques 
(Pratt and Threlkeld, 1984).  Powdered whole-rock samples for determination of carbon 
and oxygen isotope ratios in carbonate were reacted at 90°C with 100% phosphoric acid, 
and the evolved CO2 was dehydrated and purified in a high-vacuum gas-transfer system.  
All isotope ratios were determined using an isotope-ratio mass spectrometer.  
Results are reported in the standard per mil (‰) ?-notation relative to the 
University of Chicago Pee Dee belemnite (PDB) marine-carbonate standard,
     		?‰ =[(Rsample/RPDB)-1] x 103,
where R is the ratio (13C:12C) or (18O:16O).
Reference:

Pratt, L. M., and Threlkeld, C. N., 1984, Stratigraphic significance of  12C/13C ratios in 
mid-Cretaceous rocks of the western interior, U. S. A., in Stott, D. F., and Glass, D. J., 
eds., The Mesozoic of middle North America:  Canadian Society of Petroleum Geology, 
v. 9, p. 305-312.

Rock-Eval Pyrolysis

	Rock-Eval pyrolysis was used to determine the type of organic matter in samples.  
The Rock-Eval method provides a rapid determination of the hydrogen and oxygen richness 
and degree of preservation of sedimentary organic matter (Tissot and Welte, 1984; Peters, 1986).
Concentrations of free and adsorbed hydrocarbons (HC) released by programmed heating of the 
sample in a stream of helium at a relatively low temperature (250°C) for 5 min are recorded 
as the area under the first peak on a pyrogram (S1) (milligrams of HC per gram of sample).  
The second peak on a pyrogram is composed of pyrolytic hydrocarbons generated by thermal 
breakdown of kerogen as the sample is heated from 250° to 550°C (S2) (milligrams of HC per 
gram of sample).  CO2 also is generated by kerogen degradation and is retained during the 
heating interval from 250° to 390°C, and it was analyzed as the third peak on the pyrogram 
(S3) (milligrams of CO2  per grams of sample).  The Rock-Eval instrument also records the 
temperature of maximum hydrocarbon yield (Tmax).  The S2 and S3 peak areas, when calibrated 
and normalized to percent total organic carbon (TOC), yield a hydrogen index (HI) and an 
oxygen index (OI) expressed as milligrams of HC and CO2 , respectively, per gram of TOC.  
Values of HI and OI correlate well with atomic H:C and O:C ratios determined in the same 
samples by other methods (e.g., Tissot and Welte, 1984).  

References:

Peters, K. E., 1986, Guidelines for evaluating petroleum source rock using programmed 
pyrolysis, American Association of Petroleum Geologists Bulletin, v. 70, p. 318-329.
Tissot, B. P., and D. H. Welte, 1984, Petroleum Formation and Occurrence, 2nd ed., 
Springer-Verlag, New York, 538 p.

Amoco Geochemical Data

	Geochemical analyses of 55 samples from the lower 26 m of the Greenhorn Formation 
(Bridge Creek Limestone, Hartland Shale, and Lincoln Shale Members) and upper 11 m of the 
Graneros Shale in the Bounds core.  Values of ?13C of isolated kerogen in the standard per 
mil (‰) ?-notation relative to the PDB marine-carbonate standard, percentages of N, C, H, 
and O in isolated kerogen, % TOC, and the Rock-Eval parameters S1, S2, H-index, and Tmax 
were measured using standard methods at Amoco's Research Center in Tulsa.  
Concentrations of major-element oxides and sulfur (in percent), and 13  trace elements 
(Ba, Cr, Cu, Nb, Ni, Pb, Rb, Sr, Th, U, Y, Zn, and Zr in parts per million, ppm) were 
determined by XRF at X-ray Assay Laboratories (XRAL), Toronto, Canada.