The origin and migration of petroleum in the western canadian sedimentary basin, Alberta, a geochemical and thermal maturation study
Authors: Deroo G , Powell T G , Tissot B , McCrossan R G 
Published: March 2013

This report gives the results of organic geochemical and maturation studies of oil- and gas-bearing sedimentary rocks in the Albertan part of the Western Canadian Sedimentary Basin. The geochemical work comprises a study of 106 conventional oils, 63 heavy oil
extracts and 189 extracts from Paleozoic and Mesozoic sediments. Samples were examined successively from two areas; the central part of the Alberta Syncline adjacent to the Foothills including the Leduc Woodbend-Homeglen Rimbey reefal trend, and the northeastern and eastern parts of the Alberta
Syncline including the Athabasca Tar Sands and Lloydminster regions. The maturation study has determined the degree of organic metamorphism ( DOM) of producing oil and gas pools in Alberta based on rank variations of subsurface and near-surface coals. The rank of subsurface coals (Mannville
Formation, Lower Cretaceous) was determined from vitrinite reflectance measurements of thirty samples collected from coreholes along a 338 km ( 210 mile) section across the eastern flank of the Alberta Syncline. The coals lie at depths of from 600 to 3300 m ( 1968-10 826ft.) and increase in rank
from 0. 4 to 1. 6 per cent Ro from east to west. The near-surface coals, of younger formations, show a similar increase in rank, which is illustrated with an isomoisture contour map of Alberta. The east-west changes are from 30 to 5 per cent in moisture content. The maximum depth of burial of the
near-surface coals has been obtained from moisture overburden correlations. By adding this depth to the present subsurface position of the Mannville coals, the original overburden of these coals is obtained also. When this depth is plotted against the reflectance data of the coals, a smooth curve
results. Isoreflectance contours have been derived which permit rank assignments at various depth levels above and below the Mannville curve. Thus, by plotting the depth of petroleum zones at the moisture position of the overlying near-surface coals (obtained from isomoisture contour map). the Ro
(or degree of organic metamorphism) of these zones has been determined. The corresponding paleotemperatures were derived from Karweil's ( 1956) diagram by using 35 m. y. as coalification time for the Mannville coals. Results show that, in Alberta, 88 per cent of initial oil reserves have a DOM
comparable with a coal rank of 0. 5 to 0. 9 per centRo (reflectance), or with high volatile C to middle of high volatile A bituminous coal; the corresponding paleotemperatures range from 68° to 116°C. No oil was found above 1. 3 per centRo (medium volatile bituminous coal), or above
143°C. With regard to the DOM of initial gas reserves, biogenic methane occurs at a coal rank of 0. 4 to 0. 5 per cent Ro (sub bituminous coal) or between 52° to 68°C; thermal gas ranges from 0. 8 to 2. 0 per centRo (middle of high volatile A bituminous coal to semianthracite), or
between temperatures of 106° and 177°C. A post-Paleozoic regional unconformity separates the detrital deposits of Cretaceous and Jurassic ages from the predominantly carbonate deposits of Mississippian and Devonian ages. In the central part of the basin, the oils occurring near the
unconformity can be separated from the others on the basis of their gross properties. For the most part, they are derived from source rocks within the Mannville Formation. This formation was the source of the oil found in the Lower and Upper Mannville reservoirs throughout the region and the
Jurassic, Mississippian and Upper Devonian (Wabamun Group) deposits adjacent to the unconformity. In these Paleozoic reservoirs, certain oils have lost the lightest fractions with a resulting increase in the percentage of sulphur and asphalts and an increase in viscosity and specific gravity. The
reservoirs associated with the Cretaceous transgression, and occurring to the northeast of Leduc-Woodbend, have been partly supplied with oils from source rocks in the Devonian (Winterburn Group). Their affinity with the Devonian oils is evident from the isoprenoid distribution and is confirmed by
detailed analysis of the thiophenic and aromatic compounds. The source for the oils in the remaining Cretaceous and Devonian reservoirs is in the contemporaneous fine -grained sediments. The source rocks for these two groups of oils have undergone similar maturation histories and their organic
content is similar in composition. Consequently, a clear distinction between the Cretaceous and Devonian oils was not apparent xvi until an advanced stage of analysis, namely the analysis of the aromatic fraction. At this stage, it is possible also to separate two groups of Devonian oils, those
occurring in the Beaverhill Lake Formation from those found in the Woodbend Group. In the northeastern and eastern part of the basin, the heavy oil extracts from Lower Cretaceous sandstones and some Devonian carbonates show a gradational change in composition northeastward toward the shallower part
of the basin. Conventional pooled oils from the same general area fit into the same picture. Three stages of degradation are recognized departing from a normal oil: type "a", with a decrease in normal alkanes and an apparent increase in phytane and pristane; type "b", with the disappearance of the
normal alkanes leaving only pristane and phytane; type "c", in which even the isoprenoids have disappeared leaving only other isoalkanes and cyclo-alkanes. The heavy oils, including those of the Athabasca deposits, are very similar in their cyclo-alkane content to the conventional Lower Cretaceous
oils from the central part of the Alberta Basin. A study of the aromatic and thiophenic compounds reinforces that interpretation. Examination of rock extracts from the margin of the basin indicates that they have undergone little diagenesis. Thus, it is concluded that biodegradation and water
washing with some possible inorganic oxidation are responsible for the progressive alteration of, primarily, normal Lower Cretaceous oil migrating up dip where it encounters fresh water invading the basin from the outcrop area along the shield edge. Because of the huge quantities of bitumen in the
heavy oil deposits, it is necessary to admit some contribution from the pre-Cretaceous sediments in the centre of the basin. Comparative analysis of the aromatic fractions of oils from throughout the basin allows the relative effects of diagenesis, secondary migration, or alteration processes to be
established.