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Molecular and Isotopic Compositions and Origins of Sweet and Sour Gases From the Montney and Doig Phosphate Formations, Northeast British Columbia, Canada

Abstract

The Lower Triassic Montney Formation is regarded as one of the most prolific unconventional resource systems in the Western Canada Sedimentary Basin, with recent government estimates of ~450TCF of marketable natural gas and over a billion barrels of marketable oil and associated liquids. The Montney Formation is an organic rich, hybrid/interbedded, fine-grained reservoir system. Reservoir lithofacies within the study area are interpreted to be in the distal portion of the basin, with variably silty and organic-rich mudstones, some of which are inferred to be source rocks. The basal phosphatic facies of the Middle Triassic Doig Formation is a prolific oil-prone source rock. Most of the natural gas produced from the Montney and Doig formations in northeast British Columbia is sweet, with BTU values between 987 and 1225. The hydrocarbons range from medium-density gases associated with medium-gravity oil through light- to-medium-density gases associated with light oil or condensate to dry thermogenic shale-gas. Thermal maturity, interpreted from carbon isotope data calibrated to source rock maturation parameters, ranges from early mature (VRe ~ 0.6%) to overmature (VRe > 2.0%). The gas isotope data further reveal mixing of residual microbial gases and thermogenic gases in portions of play, and mixing of primary hydrocarbons, generated from source organic matter, with secondary thermogenic gases cracked from residual oil. Montney gases produced in some areas of the play are sour, with H2S concentrations between 0.03 and 2.0 volume percent. Co-produced hydrocarbon gases are thermogenic, and were generated in the peak- to late-oil and wet gas/condensate windows at VRe of 0.8 to ~1.4%. Sulfur isotope and gas compositional data support a thermochemical sulfate reduction (TSR) interpretation for the H2S in these reservoirs. Although burial temperatures were adequate for TSR reactions to occur, petrographic data show that the Montney reservoirs do not contain a sulfate source. Deeper Paleozoic carbonate rocks are the likely source of the H2S. Other plausible sources of H2S include the Triassic Charlie Lake and Halfway formations, carbonate-associated sulfate released by dissolution of carbonate in subjacent strata, and in-situ formation water.