The Garbutt Formation of Liard Basin, British Columbia: a potential liquids-rich play

Abstract In Liard Basin of northern British Columbia, syn-depositional motion on the Bovie structure resulted in greater thicknesses of Cretaceous sediments. The Early Cretaceous Garbutt Formation was deposited during the initiation of a major marine transgression. Organic-rich shales and siltstones in its lower part record maximum flooding and have been informally termed the Radioactive Zone (RZ). The Garbutt Formation is dominated by shales and siltstones and is here subdivided into three informal units; the lower Garbutt Formation (LGF), the succeeding RZ and the upper Garbutt Formation (UGF). The combined thickness of the Radioactive Zone and lower Garbutt Formation is 40 to over 120 m thick and can be at greater than 2000 m depth in Liard Basin. The lower Garbutt Formation is not recognized east of the Bovie structure and the RZ is considerably thinner or absent locally, suggesting this area remained high relative to Liard Basin. Total organic carbon and Rock-Eval data suggest that the RZ is a good to very good source rock and that the LGF is also a good source rock. Organic matter is dominantly Type II kerogen, although more terrestrial Type III input becomes prevalent in western parts of the basin. Thermal maturity inferred from Tmax, in addition to hydrogen index levels, indicate that these rocks are in the oil window in northern Liard Basin and in the wet gas window within central Liard Basin. Porosities average 8% and matrix compositions average 39 wt.% quartz, 4 wt.% feldspar and calcite and 53 wt.% clays (illite/mica, kaolinite and chlorite). The properties of the RZ and LGF indicate that a potential liquids-rich shale gas play may occur within central Liard Basin.