Natural thermogenic gas seeps at the front of the Richardson Mountains: Indications for a petroleum system in Peel Plateau, Yukon, Canada

Abstract

Abstract Peel Plateau Yukon Territory is an under-explored potential petroleum region, where drilling during the 1960–70s identified natural gas shows in the upper Paleozoic succession. New fieldwork has identified natural gas seeps near the Trevor Fault, a major mapped structural element near the eastern Richardson Mountains front. One seep occurs where Trail River crosses the deformation front (66°28′, 135°08.3′) and the other, 45 km along strike of the deformation front, is associated with anomalously unfrozen holes in ice on Turner Lake (66°10.3′, 134°18.5′). Both samples contain methane (CH4), carbon dioxide (CO2) and dihydrogen sulphide (H2S). The Trail River sample has methane and carbon dioxide isotopic compositions (δ13CCH4 = −42.8‰ and δ13CCO2 = −14.9‰) generally similar to thermogenic gases and specifically like Lower Cretaceous Mannville Group gases in the Western Canadian Sedimentary Basin. The Turner Lake sample isotopic composition (δ13CCH4 = −35.2‰ and δ13CCO2 = −31.7‰) is inferred originally similar to the Trail River gas, but we also infer it was subsequently mildly oxidized microbially. We speculate that trace amounts of H2S are attributable to thermochemical sulphate reduction, consistent with the stratigraphic occurrence of natural gas and patterns of thermal maturity in nearby wells. The presence of H2S suggests both gases have Paleozoic petroleum source rocks, which is a general association in the Western Canadian Sedimentary Basin. The results indicate an effective thermogenic petroleum system, with probable Paleozoic petroleum source rocks for Peel Plateau. The association with faulting on the margin of the Richardson Mountains is reminiscent of petroleum seeps associated with major petroleum fields at Turner Valley, Waterton and Norman Wells. Recent work by others shows untested prospective structures in Peel Plateau, including one below the Turner Lake seep. These seeps, combined with new structural interpretations provide incentive to evaluate further the inferred petroleum potential of Peel Plateau.