Synthesis of organic matter composition and maturation and gas data from selected deep source rock units for some wells in the Fox Creek area
Authors: Lavoie D , Ardakani O H , Rivard C 
Published: April 2021

An analysis of historical and new organic matter data, as well as gas composition and isotopic ratios for deep hydrocarbon source rocks, is part of a holistic study of the potential environmental impact of unconventional hydrocarbon development in the Fox Creek
area in Alberta. The goal of this activity is to use thermal maturation and gas data from deep source rocks to generate a database for the interpretation of source rocks for eventual dissolved hydrocarbons in shallow groundwater.
The organic matter study consists of new petrographic observations on pellets from the Duvernay, Montney, Nordegg, and Mannville organic-rich samples. The petrography indicates that the organic matter is of marine Type II origin with one coal (Type III) sample in the Mannville Group. The organic
matter reflectance measurements were taken on bitumen and converted to equivalent vitrinite reflectance values; vitrinite reflectance has been directly measured from the coal sample. The data indicate that all units reached thermal conditions for the production of hydrocarbons. The deep Duvernay
Formation has the highest thermal maturity data that suggest the end of the oil window in the eastern end of the study area and the onset of the condensate zone in the western part of the study area.
Rock-Eval data from publicly available wells in the GSC database were evaluated for the Duvernay, Montney, and Nordegg units. The Tmax values indicate that these units are within the hydrocarbon production zones. As with the vitrinite-equivalent data, the Duvernay Formation has reached the highest
thermal maturity in the study area with values indicative of the condensate zone, the other units belong to the oil zone.
Gas samples were collected in isotubes at the time of drilling. Samples were collected at various depths with some wells having only a short interval with data, while others have samples from a much longer interval. Data from 7 wells were available for our study, 6 of the wells are located at the
eastern end of the study area while the last one is much further north. From gas composition (wetness ratio: C1/(C2+C3)) and delta-13C ratios of methane, the origin of the gas is thermogenic. The vertical trends of delta-13C ratios of ethane versus depth and wetness, as well as butane ratios
(iC4/nC4) versus depth and wetness for wells with a longer distribution of samples, suggest partial isotopic rollover. The roll-over was not fully developed as nowhere do we see delta-13C ratios of ethane being lighter (more negative) than that delta-13C ratio of methane. This suggests, as supported
by the organic matter reflectance and Rock-Eval data, that the study area never reached very high thermal conditions. The fact that significant statistical overlap of delta-13C ratios of methane and the inverted trend of delta-13C ratios for ethane challenges the use of these isotopes data for
source interpretation of eventual dissolved hydrocarbons in groundwater. However, some limited data for the shallowest sampling intervals (&amp;lt;1500 m) in some wells suggest that more significantly different data might be present in shallower sections in the subsurface. Data from eventual
shallow coals intercepted by monitoring wells to be drilled are considered critical for the interpretation of the source for eventual dissolved hydrocarbons in groundwater.