Basin-wide unconventional potential of the Doig Formation, Triassic of the Western Canada Sedimentary Basin

Abstract

The Doig Formation is a Lower to Middle Triassic fine-grained unit of the Western Canada Sedimentary Basin, which includes a phosphate-bearing basal unit informally defined as the Doig Phosphate Zone (DPZ). The DPZ is recognized as a significant source of hydrocarbons hosted in conventional reservoirs in the basin. Phosphatic mudstones are uncommon and studies of their source-rock potential are scarce. The regional distribution of the Doig Formation source-rock properties across its entire stratigraphic and spatial range is poorly understood. In this study, we characterize the source-rock potential of the Doig Formation through an integrated assessment of thickness, organic content, kerogen type and thermal maturity. A new method for estimating the relative contribution of kerogen type and its original hydrogen index pseudo-van Krevelen, is introduced. The method assumes a binary mixture of kerogen types and requires an assumption for the initial value of immature kerogen from each end-member. The Doig Formation thickens southwestwards from the eastern erosional edge towards the deformation front to a maximum thickness of 229 m. The upper Doig and DPZ have distinct thickness distribution patterns, which suggests a transition from a tectonically-influenced sedimentation marked by uplifting, faulting and erosion, to the start of physiographic healing and a return to large-scale subsidence. The source-rock generative potential of the Doig Formation is fair to good, containing kerogen Types II and III. The DPZ has higher total organic carbon (TOC) content, of up to 12.2% and a higher proportion of Type II kerogen than that the upper Doig, where the maximum TOC content is 8.4 wt%. Original TOC was higher in the northern part of the lateral extent, which is likely related to a connection with open ocean; and in a linear trend through the center of the basin, which represents the trough of a marine fore-arc basin. Thermal maturity increases to the southwest, from early oil to dry gas windows, in a trend that approximately correlates to gas wetness and liquid yield. A zone of dry gas occurs near the eastern edge, however, which may be explained by preferential updip migration of thermogenic methane generated at greater depths, or mixing of thermogenic and biogenic gas.