The San Juan sag, concealed
by the vast San Juan volcanic field of south-central Colorado, has only
recently benefited from oil and gas wildcat drilling and evaluations. Sound
geochemical analyses and maturation modeling are essential elements for
successful exploration and development. Oil has been produced in minor
quantities from an Oligocene sill in the Mancos Shale within the sag, and
major oil and gas production occurs from stratigraphically equivalent rocks
in the San Juan basin to the southwest and in the Denver basin to the northeast.
The objectives of this study were to identify
potential source rocks, assess thermal maturity, and determine hydrocarbon-source
bed relationships. Source rocks are present in the San Juan sag in the
upper and lower Mancos Shale (including the Niobrara Member), which consists
of about 666 m (2184 ft) of marine shale with from 0.5 to 3.1 wt. % organic
carbon. Pyrolysis yields (S1 + S2 = 2000-6000 ppm)
and solvent extraction yields (1000-4000 ppm) indicate that some intervals
within the Mancos Shale are good potential source rocks for oil, containing
type II organic matter, according to Rock-Eval pyrolysis assay. Oils produced
1997. The American Association of Petroleum Geologists. All rights reserved.
received August 23, 1995; revised manuscript received September 4, 1996;
final acceptance February 4, 1997.
Oil & Gas, LLC, 633 17th Street, Suite 1520, Denver, Colorado 80202.
S. Geological Survey, P.O. Box 25046, MS 977, Denver Federal Center, Denver,
River Associates, Inc., 2000 W. 120th Avenue, Suite 10, Denver, Colorado
Use of product
names is for descriptive purposes only and does not constitute endorsement
by the U.S. Geological Survey.
are grateful to Waggoner-Baldridge Energy Co., Meridian Oil, and UP Resources
for supplying samples and data for this study. Lynn Gries provided hours
of picking clean shale samples and manuscript typing. T. A. Daws and A.
Warden, U.S. Geological Survey, are gratefully acknowledged for performing
Rock-Eval and carbon isotope analyses, respectively. We thank R. Walker,
U.S. Geological Survey, and Ann Priestman, GeoGraphics, for drafting the
figures. Reviews of earlier versions of the manuscript by Vito Nuccio,
Chuck Chapin, P. Lillis, N. Bostick, P. Wagner, and an anonymous reviewer