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AAPG Bulletin


Geology, Thermal Maturation, and Source Rock Geochemistry in a Volcanic Covered Basin: San Juan Sag, South-Central Colorado1

Robbie Rice Gries, J. L. Clayton, and China Leonard4


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 from the 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

1Manuscript received August 23, 1995; revised manuscript received September 4, 1996; final acceptance February 4, 1997.

2Priority Oil & Gas, LLC, 633 17th Street, Suite 1520, Denver, Colorado 80202.

3U. S. Geological Survey, P.O. Box 25046, MS 977, Denver Federal Center, Denver, Colorado 80225.

4Platte River Associates, Inc., 2000 W. 120th Avenue, Suite 10, Denver, Colorado 80234.

Use of product names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.

We 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 are appreciated.

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