AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Characterization of Gas Generated by Sequential Hydrous Pyrolysis of Potential Gas-Prone Source Rocks for Tight-Gas Reservoirs in the Rocky Mountain Area
(1) Geology, Colorado School of Mines, Golden, CO.
(2) U.S. Geological Survey, Denver, CO.
(3) Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
The source of unconventional gas in tight-gas reservoirs of the Rocky Mountain area is uncertain, but possible Cretaceous gas-prone source rocks include the Cameo coal zone, Mowry, Mancos, and Baxter/Hilliard Shales. Hydrous pyrolysis experiments were conducted on immature samples of these source rocks to characterize their generated gases and evaluate their potential as sources for gas accumulations in the Green River and Piceance Basins. The experiments were conducted sequentially for 72 h at 300, 330, and 360°C, equivalent to measured thermal maturities of 1.0, 1.3, and 1.6 %Ro, respectively. After each 72 h experiment, the generated gas and expelled oil were removed from the reactor. Gases generated from each sequential experiment were analyzed for molecular composition and stable carbon and hydrogen isotopes.
All source rocks generated significant hydrocarbon and non-hydrocarbon gases (N2, H2S, H2, and CO2). Cumulative yields of methane to butane increased with increasing thermal maturation. On a per gram of total-organic-carbon (TOC) basis, the methane yield from the Cameo coal at 360°C exceeded that at 300°C by almost eightfold. The cumulative methane yields on a TOC basis, from highest to lowest, are Cameo coal, Baxter/Hilliard, Mowry, and Mancos Shales. Cumulative gas wetness from both Mancos and Mowry Shales were high. With increasing thermal maturity, δ13C of methane from all four source rocks became lighter, contrary to conventional models.
The Baxter/Hilliard Shale generated the greatest amount of H2, H2S, and CO2, on a TOC basis, whereas Cameo coal generated the least. The relatively positive δ13C values for CO2 from the Mancos Shale and Baxter Shale suggests it was sourced from thermal decomposition of carbonate minerals in the original rock samples. More negative δ13C values for CO2 from the Cameo coal indicate that gas is from an organic source, while intermediate δ13C of CO2 from the Mowry Shale indicates contributions from both organic and inorganic sources.
Gases generated from laboratory experiments are isotopically lighter than gas sampled from the Jonah and Piceance Basin fields. Two explanations are proposed: 1) Migration effects from the source to the reservoir and escaping gas from reservoirs may significantly alter final gas compositions; 2) Gases from experiments only reach primary cracking stage of gas generation, whereas field gas may represent gas from secondary cracking of oils deeper in the basins.