Organic matter characterization in mixed hydrocarbon producing areas within the Duvernay Formation, Western Canada Sedimentary Basin, Alberta

This paper characterizes organic matter in the mixed hydrocarbon (oil-dry gas, oil-condensate) producing areas of the Late Devonian Duvernay Formation in Alberta (Canada) by high-resolution RockEval data (n = 246) and petrography (n = 35) at three drillhole locations (Locations A and C distal, Location B proximal to Leduc Reef, respectively). Total organic carbon (TOC) ranges from 0.15-6.22 wt.% at Location A, 0.43-8.20 wt.% at Location B, and 1.84-5.23 wt.% at Location C with an overall median value of 3.42 wt.%. S2 values range from 0.20-7.42 mg HC/g of rock at Location A, 0.22-4.23 mg HC/g of rock at Location B, and 1.68-5.04 mg HC/g of rock at Location C. These results for samples from Locations A, B and C indicate that kerogen is Type I to Type II. The hydrogen index (HI) of these rocks is significantly lower (< 160 mg HC/g TOC) than that of traditional oil-producing source rocks of this age (~ 400-700 mg HC/g TOC). At Locations A and C, Tmax lies within the oil window with a mean value of 459 °C; at Location B, Tmax varies greatly between 434 and 488 °C, between the oil and gas window, with no observable association with sample depth. Vitrinite reflectance of samples ranges from 0.99-1.32%. Tmax values do not correlate with vitrinite reflectance of the samples (R2 = 0.04), nor with depth (R2 = 0.4), suggesting that Tmax is not a reliable indicator of thermal maturity in this system. Low TOC/S2, high mineral carbon is associated with intervals of intergrown sparry calcite and pyrite in petrographic samples at Location B. Organic matter within the Duvernay Formation was characterized as almost entirely solid bitumens, with very minor inertinite, < 1% by volume). The RockEval and petrographic observations suggest that secondary diagenetic effects are the cause for the mixed hydrocarbon signature, rather than any spatial variation in depositional environment or thermal maturation regimes.