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Applying Gas Geochemistry to Reservoir Filling Models: Tight Gas Sandstones, Rocky Mountains, USA

Harris, Nicholas B.*1; Philp, Paul 2; Zhou, Zheng 3; Ballentine, Chris 3
(1) Earth & Atmospheric Sciences, Univ of Alberta, Edmonton, AB, Canada.
(2) School of Geology and Geophysics, University of Oklahoma, Norman, OK.
(3) School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom.

The compositions of natural gases from large tight sand fields in the Rocky Mountains show significant spatial and stratigraphic variability, both within and between fields. This may result from multiple source rocks, varying thermal maturity, migration effects and post-emplacement alteration of gases. To deconvolve these effects, we have developed a large geochemical data set - bulk hydrocarbon gases, CO2, hydrogen and carbon isotopes and noble gases from production and mud gas samples - from Jonah Field (Green River Basin), Greater Natural Buttes (GNB) (Uinta Basin) and the Mamm Creek - Rulison - Parachute - Grand Valley complex (Piceance Basin). Reservoirs in these fields are Upper Cretaceous discontinuous fluvial sandstones. Possible gas sources include Upper Cretaceous coals and carbonaceous shales, the marine Upper Cretaceous Mancos Shale and equivalents, and the marine Lower Cretaceous Mowry Shale.

Production gases generally show overlap between the fields in bulk hydrocarbon compositions, with considerable intra-field variability. Piceance gases are characterized by relatively positive d13C ethane, possibly reflecting greater contribution from coaly source rocks. The d13C of produced CO2 also distinguishes the fields, which may reflect differing contributions of gas from marine sources or biogenic oxidation of hydrocarbon gases. Vertical trends in gas composition differ significantly between fields. Jonah gases become wetter and isotopically lighter with increasing depth, interpreted as resulting from bacterial oxidation of gases in shallow sections of the field. Isotopically light CO2 is restricted to the margins of the field, suggesting that oxidation was associated with pressure bleed-off near field-bounding faults. In the Piceance Basin fields, gases first become wetter with depth, then significantly drier and isotopically light. Shallow dry gas reflect bacterial oxidation of gases, while dry gas at greater depth results from migration of late gas from secondary cracking.

Our data suggest that gas migration was continuous and more or less vertical, reflecting diffusion or seepage through a pervasive network of small-scale fractures; gas was not channeled by major fracture systems or faults. Noble gases and secondary oxidation of hydrocarbon gases shows strong spatial control in both the Piceance and Jonah systems, however, suggesting that formation water interactions with gas columns were substantially affected by fractures or faults.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California