Correlation of Rocky Mountain Produced Natural Gases to Sources by Application of Gas Isotope Kinetic Modeling

Abstract

Due to variation in maceral types and concentrations, source rocks have different gas formation kinetic characteristics which result in isotopically different gases. Additionally, cracking of crude oils to gases varies with oil composition. The identification of gases from oil cracking is possible as resulting gases have very different carbon isotopic signatures compared to gases from primary cracking of kerogen (and bitumen). Therefore, experimental calibration of oil cracking allows a differentiation of gas from primary cracking of kerogen. Gas to source correlations can not be completely successful by application of theoretical concepts, but are enhanced by gas formation simulation experiments where gas evolution is monitored and quantified during laboratory thermal maturation of source rocks and cracking of oils. Such experiments allow for the correlation of gas samples back to a more specific source unit. Gas isotope kinetic modeling as described in Tang et al., 2000 (Geochimica et Cosmochimica Acta 64, p. 2673–2687), and Tang and Schoell et al., 2005 (Abstract 96191, AAPG Annual Convention and Exhibition, Calgary, AB, Canada) was employed. Experimental data were available for three Greater Green River Basin samples: a Rock Springs coal, a Mancos Shale, and the asphaltene fraction of a Mowry-sourced oil. These three samples account for much of the types of gases generated in the Greater Green River Basin. A sample set of >400 produced gases with molecular and isotopic compositional data allowed differentiation of e.g., multiple gas sources in Pinedale and Jonah fields of Wyoming which were geologically reasonable. Using the initial laboratory Greater Green River Basin results, this approach has been expanded to include additional carbon isotopic data to evaluate produced gases from five additional Rocky Mountain basins: Denver, Piceance, Powder, San Juan and Uinta. The data commonly show carbon isotopic evolution trends that can be interpreted in terms of source type(s) and generation depths, complementing more traditional gas – source correlation techniques.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015