Diagenetic Studies of the Mannville Group with Reference to Petroleum Charging and Biodegradation in the Lloydminster Area, Saskatchewan

Diagenesis of the Mannville Group was investigated with 79 samples from the Lloydminster area in Saskatchewan based on combined studies of petrography, isotope geochemistry and petroleum inclusion microthermometry, with comparison to 32 samples from the Athabasca and Peace River areas in Alberta. The objective of this study is to characterize the diagenetic history of the Mannville Group and to determine the mechanisms that are responsible for the differences in oil charging and biodegradation between Lloydminster and the Athabasca and Peace River areas.;In the Athabasca and Peace River areas, both oil-saturated and oil-stained samples contain limited amounts of diagenetic minerals including pyrite, siderite, calcite, kaolinite and illite, with no micro-quartz and marcasite. Since bitumen in these areas has similar biomarkers as most of the heavy oil in the Lloydminster area, the oil-charging event in the Athabasca and Peace River areas is probably the same as hydrocarbon emplacement (I) in the Lloydminster area.;Based on the diagenetic studies and data from the organic matter maturation and AFT methods, the timing of hydrocarbon emplacement (I) is inferred to be from 75 Ma to 52 Ma in the Lloydminster area and from 75 Ma to 70 Ma in the Athabasca and Peace River areas. The maximum burial temperatures in the Athabasca and Peace River areas were below the reservoir pasteurization temperature of 80°C, therefore the oil experienced intense biodegradation and turned into the present state as solid bitumen. On the other hand, the maximum burial temperature in Lloydminster was above 80°C, and the time for biodegradation (between initial charging and maximum burial) is reduced and the degree of biodegradation is relatively limited.;Hydrocarbon emplacement (II), exclusively found in the Lloydminster area, postdates reservoir pasteurization. It is related to heat anomaly caused by the Eocene magmatism (52 - 43 Ma) in southwestern Saskatchewan. The relatively late timing (after maximum burial) and high temperature of this second phase of oil charging prevented significant biodegradation of the newly charged oil.;In the Lloydminster area, the oil-saturated sandstones display a low degree of compaction, and contain limited early diagenetic minerals such as glauconite, pyrite and siderite. In contrast, the oil-stained and oil-free sandstones contain micro-quartz, marcasite, kaolinite and illite formed during intermediate diagenesis. It is proposed that oil charging in the oil-saturated sandstones (hydrocarbon emplacement (I)) occurred very early in the diagenetic history and that further diagenesis was prevented by oil in the pore space. The oil in the oil-stained samples represents a second charging event (hydrocarbon emplacement (II)) when the reservoirs had experienced deeper burial diagenesis.