Pyrobitumens are common within deep basin, Devonian carbonate gas reservoirs of the Western Canada Sedimentary Basin. Incident light microscopy and SEM-EDS have been used to study the origin of pyrobitumens from Upper Devonian isolated reefs of the Leduc Formation, west central Alberta. Reflectance properties (%Ro max and %Ro min), coke microtextures and relative S/C ratios are used to classify the pyrobitumens. In portions of reservoir units where NSO-enriched, low aromaticity crudes may have been dominant, oil to gas transformations produced mainly low-reflecting isotropic to fine-grained mosaic coke microtextures within the reservoir pyrobitumens (Type A). In contrast, where ‘normal’ crudes were most likely dominant in the reservoir units prior to thermal cracking, pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate (Type B). Oil to gas cracking within some of the reservoir units resulted in overpressuring and is represented by highly deformed pyrobitumens with fibrous coke textures and high bireflectance. Relative concentrations of Type A and B pyrobitumens vary vertically and temporally in some of the reefal buildups. The variations in the dominant pyrobitumen types are attributed to crude oil alteration which preceded crude oil to gas transformation (e.g. tar mats) in some zones whereas other zones may have been altered contemporaneously with crude oil cracking. The later may have resulted from the oxidation and sulphur enrichment of the crude oil during thermochemical sulphate reduction. This reaction is prevalent within Devonian evaporite-carbonate strata of the Western Canada Sedimentary Basin, generating copious volumes of H 2S. The interaction between crude oil and sulphur-enriched, highly alkaline fluids is suggested by the presence of high S/C, very fine-grained, late stage calcined needle cokes within brecciated pyrobitumens.