--> Significance of Optical and Physical Maturity Anomalies to Exploration in the Paradox Basin, by J. R. Castano and W. G. Dow; #90986 (1994).

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Abstract: Significance of Optical and Physical Maturity Anomalies to Exploration in the Paradox Basin

John R. Castano, Wallace G. Dow

The primary thermal maturity tools used by most explorationists are vitrinite reflectance (VR) and Rock-Eval pyrolysis Tmax. In the Paradox Basin of Utah and Colorado the typical relationship of VR to Tmax does not hold. Consequently, significant errors in exploration strategy could result if conventional wisdom were followed.

Oil and gas in the Paradox Basin are present almost exclusively in lenticular stratigraphic traps in Pennsylvanian carbonates sealed by dense carbonates or anhydrite. The reservoir rocks are closely associated with organic-rich black shales; very short range migration prevails. Although the black shales contain mainly type II kerogen, sufficient vitrinite is present for reliable maturity analysis and calculated VR agrees with kerogen color in transmitted light (TAI). There is a distinct disagreement between VR and pyrolysis Tmax maturities, with Tmax being consistently low compared to published Tmax versus VR calibrations. For example, at 1% VR Tmax averages 442°C instead of the expected 450°C. A very slow heating rate in the Paradox Basin may have had an effect on the kinet cs involved in maturation of organic matter (expressed by VR) and generation of hydrocarbons (expressed by Tmax). The maturity pattern is controlled by regional heat flow and burial history; locally intrusions interrupt regional trends. Most oil production lies between 1.0-1.5% VR, with 50 degree API oil occurring around the 1.5% isoreflectance contour; gas is found at higher maturities. Minor oil production is obtained between 0.8-1.0% VR (Tmax 437-442°C), while only the small Mexican Hat field and oil shows are noted between 0.6-0.8% VR (Tmax 433-437°C). These relationships have proven valuable in locating the fairway for Cane Creek production.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994